ANESTHETIC GASES:
|
Table
1: Inhaled Anesthetic Agents |
|||
Generic
or |
Commercial
name |
Year
of |
Currently |
Diethyl
ether |
Ether |
1842 |
No |
Nitrous
oxide |
Nitrous
oxide |
1844 |
Yes |
Chloroform |
Chloroform |
1847 |
No |
Cyclopropane |
Cyclopropane |
1933 |
No |
Trichloroethylene |
Trilene® |
1934 |
No |
Fluroxene |
Fluoromar® |
1954 |
No |
Halothane |
Fluothane® |
1956 |
Yes |
Methoxyflurane |
Penthrane® |
1960 |
Infrequently |
Enflurane |
Ethrane® |
1974 |
Yes |
Isoflurane |
Forane® |
1980 |
Yes |
Desflurane |
Suprane® |
1992 |
Yes |
Sevoflurane |
Ultane® |
1995 |
Yes |
It is estimated that more than
200,000 health care professionals --including anesthesiologists, nurse
anesthetists, surgical and obstetric nurses, operating room (OR) technicians,
nurses aides, surgeons, anesthesia technicians, postanesthesia care nurses,
dentists, dental assistants, dental hygienists, veterinarians and their
assistants, emergency room staff, and radiology department personnel --are
potentially exposed to waste anesthetic gases and are at risk of occupational
illness. Over the years there have been significant improvements in the
control of anesthetic gas pollution in health-care facilities. These have
been accomplished through the use and improved design of scavenging systems,
installation of more effective general ventilation systems, and increased
attention to equipment maintenance and leak detection as well as to careful
anesthetic practice. However, occupational exposure to waste gases still
occurs.
Exposure measurements taken in ORs
during the clinical administration of inhaled anesthetics indicate that waste
gases can escape into the room air from various components of the anesthesia
delivery system. Potential leak sources include tank valves, high- and
low-pressure machine connections; connections in the breathing circuit,
defects in rubber and plastic tubing, hoses, reservoir bags, and ventilator
bellows, and the Y-connector. In addition, selected anesthesia techniques and
improper practices such as leaving gas flow control valves open and
vaporizers on after use, spillage of liquid inhaled anesthetics, and poorly
fitting face masks or improperly inflated tracheal tube and laryngeal mask
airway cuffs also can contribute to the escape of waste anesthetic gases into
the OR atmosphere.
Studies of the effects of these
agents in the health-care setting have been made more difficult due to high
job turnover of affected employees. Publications report a wide range of
exposure levels in hospital, medical, dental, and veterinary facilities
(Askrog and Petersen 1970; American Society of Anesthesiologists 1974;
Sweeney et al. 1985; Jastak 1989; Burkhart and Stobbe 1990; Henry and Jerrell
1990; Rowland et al. 1992; NIOSH 1977, 1994).
Unlike the situation in the OR,
health-care workers in the recovery room (also known as the postanesthesia
care unit or PACU) encounter occupational exposure to waste anesthetic gases
from the patients instead of the anesthesia delivery system. While in the OR,
patients anesthetized with inhaled anesthetic agents take-up varying
quantities of these agents depending on the specific agent and its
solubility, the duration of anesthesia, and the physiological make-up of the
patient. In the PACU, these gases are eliminated by the patient’s respiratory
system into the ambient environment. In contrast to the OR, the ambient air
in the PACU may contain multiple anesthetic gases, which include but are not
limited to nitrous oxide, halothane, enflurane, isoflurane, desflurane, and
sevoflurane.
Because PACU nurses must monitor
vital functions in close physical proximity to the patient, they can be
exposed to measurable concentrations of waste anesthetic gases. While random
room samples may indicate relatively low levels of waste gases, the breathing
zone of the nurses may contain higher levels.Consequently, air samples
obtained within the breathing zone of a nurse providing bedside care are most
likely to represent the gas concentrations actually inhaled.
In general, the detection of
halogenated anesthetic agents by their odor would indicate the existence of
very high levels, as these agents do not have a strong odor at low
concentrations. For example, detection of high levels of halothane may be
difficult for PACU nurses because one study (Hallen et al. 1970) found that
fewer than 50% of the population can detect the presence of halothane until
concentrations are 125 times the NIOSH REL.
In anesthetizing locations and
PACUs where exposure to waste gases is known to occur, it is important for
health-care workers and their employers to understand the potential risks of
excess exposure to waste anesthetic gases and to implement the appropriate
controls to minimize these risks. During the past 25 years multiple studies
have attempted to elucidate the risk of exposure to anesthetic agents. Animal
and human studies have assessed hematopoietic, central nervous system, and behavioral
effects and the effects of anesthetic agents on fertility, carcinogenicity,
teratogenicity, and reproduction. Epidemiological studies have generally
focused on OR and dental workers, the two occupational groups most frequently
exposed to anesthetics. The following discussion highlights these findings.
1.
Nitrous
Oxide
While mutagenicity testing of nitrous oxide
(N2O) has demonstrated negative
results (Baden 1980), reproductive and teratogenic studies in several animal
species have raised concern about the possible effects of nitrous oxide
exposure in humans. In general, studies demonstrate reproductive and
developmental abnormalities in animals exposed to high concentrations ofN2O. In one study by Viera et al.
(1980), spontaneous abortion was observed in rats at 1000 ppm or more.
According to NIOSH (1994), similar concentrations of 1000 ppm have been found
in operating rooms and in dental operatories not equipped with scavenging
systems.
Smith, Gaub, and Moya (1965) reported fetal
resorption in rats exposed to nitrous oxide at high doses. Fink, Shepard, and
Blandau (1967) administered 45% to 50% nitrous oxide and 21% to 25% oxygen to
pregnant rats for 2, 4, and 6 days starting at day 8 of gestation. Surviving
fetuses from these rats demonstrated rib and vertebral defects. Corbett and
colleagues (1973) also reported an increase in fetal deaths and a smaller
number of offspring in rats exposed to levels ranging from 1,000 to 15,000
ppm of nitrous oxide.
There are also studies involving human
subjects. A recent retrospective study (Rowland et al. 1992) reported that
female dental assistants exposed to unscavenged N2O for 5 or more hours per week had
a significantly increased risk of reduced fertility compared with non-exposed
female dental assistants. The exposed assistants had a 59% decrease in
probability of conception for any given menstrual cycle compared with the
non-exposed assistants. For dental assistants who used scavenging systems
during N2O
administration, the probability of conception was not significantly different
from that of the non-exposed assistants. The Rowland study authors suggest
that"exposure to high levels of unscavenged N2O can impair fertility and
scavenging equipment is important in protecting the reproductive health of
women who work with the gas." The study revealed that the mean time to
conception among the women who worked with scavengedN2O was similar to that among the
non-exposed women, but it was much longer among the women who worked with
unscavenged N2O for 5
or more hours a week.
Rowland and colleagues (1995) examined the
relationship between occupational exposure to N2O and spontaneous abortion in
female dental assistants. Duration of exposure was a surrogate for exposure
data. Nitrous oxide exposure was divided into two separate variables:
scavenged hours (hours of exposure per week in the presence of scavenging
equipment) and unscavenged hours of exposure per week. Women who worked with
N2O at least 3 hours per week in
offices not using scavenging equipment had an increased risk of spontaneous
abortion (relative risk = 2.6, 95% confidence interval [CI] = 1.3-5.0)
adjusted for age, smoking, and number of amalgams prepared per week. This
finding was not observed among workers in offices where scavenging equipment
was in use. The authors concluded,"Scavenging equipment can make large
differences in exposure levels at moderate cost and appears to be important
in protecting the reproductive health of women who work with nitrous
oxide."
Several summaries of the epidemiologic
studies of exposure to N2O and reviews of the topic generally including animal and
retrospective studies (Purdham 1986; Kestenberg 1988; and NIOSH 1994) have
been published. They report a consistent excess of spontaneous abortion in
exposed women. Other summaries of the epidemiologic studies do not establish
a cause-effect relationship (Buring et al. 1985; Tannenbaum and Goldberg
1985). Evidence for congenital abnormalities is less strongly associated with
exposure.
2.
Halogenated
Agents
Halogenated agents are used with and without
N2O and have been linked to
reproductive problems in women and developmental defects in their offspring.
As early as 1967 there were reports from the Soviet Union, Denmark, and the
United States (Vaisman 1967; Askrog and Petersen 1970; Cohen, Bellville, and
Brown 1971) that exposure to anesthetic agents including halothane may cause
adverse pregnancy outcomes in health-care personnel. Several animal studies
in rats, mice and hamsters showed embryolethal and teratogenic effects and
supported the findings in humans (Basford and Fink 1968; Wharton 1979),
although often at quite high concentrations (3000-6000 ppm). One (Popova et
al. 1979) reported fetal resorptions in rats at 9 ppm.
A number of human epidemiologic studies have
been performed since the early 1970s to assess the potential harm to
reproductive health that exposure to anesthetics might cause. Generally,
these were mailed questionnaire surveys completed by persons (usually
anesthesiologists and nurses) identified through registries. As such, the
studies were retrospective and inquired about previous reproductive outcomes
for which validation was not available. In addition, no exposure data were
available and many of the early studies predated the use of scavenging
systems. Studies documenting a statistically significant excess of
spontaneous abortions in exposed female anesthesiologists include those of
Cohen and colleagues 1971, Knill-Jones and colleagues 1972, ASA 1974, and
Pharoah and colleagues 1977. Studies also documented increases in spontaneous
abortion among nonphysician female OR personnel (Cohen et al. 1971; Rosenberg
and Kirves 1973; ASA 1974; Knill-Jones et al. 1975; and Tomlin 1979). Also of
interest, one study documented increased incidence rates of spontaneous
abortion among wives of exposed males (ASA 1974). In some exposed
populations, studies failed to show that exposure to anesthetic agents caused
increased risk of spontaneous abortion (Rosenberg and Vanttinnen 1978;
Axelsson and Rylander 1982; Tannenbaum and Goldberg 1985; Buring et al.
1985).
The evidence for an association between
anesthetic exposure and congenital anomalies is less consistent. Only a few
studies in some subpopulations of exposed workers found a positive
association (Corbett et al. 1974; ASA 1974; Pharoah et al. 1977). Other
studies reported no association with congenital anomalies (Axelsson and
Rylander 1982; Lauwerys et. al. 1981; Cohen et. al. 1980; Rosenberg and
Vanttinnen 1978).
The retrospective study by Cohen and colleagues
(1980) reported that female dental chairside assistants who had experienced
heavy exposure (defined as more than eight hours per week) to waste
anesthetic gases reported a significant increase in the rate of spontaneous
abortions (19.1 per 100 pregnancies) compared with the rate in the
non-exposed pregnant control (8.1 per 100). For the wives of dentists who had
also experienced heavy exposure, a significant increase in the rate of
spontaneous abortions (10.2 per 100) was also reported compared with the rate
in the wives of dentists not exposed (6.7 per 100). The non-exposed group was
restricted to those who did not report anesthetic exposure in any of the
years before conception and including the year of conception.
Another study of reproductive outcomes
associated with exposure to anesthetic gases (also a questionnaire survey,
conducted between 1981 and 1985) documented both a statistically
significantly increased odds ratio for spontaneous abortion in exposed
females (odds ratio 1.98; CI = 1.53-2.56) and spouses of exposed male workers
(odds ratio 2.30; CI = 1.68-3.13), and for congenital abnormality in
offspring of exposed females \ (odds ratio 2.24; CI = 1.69-2.97) and
offspring of spouses of exposed male workers (odds ratio 1.46; CI =
1.04-2.05) (Guirgis et al. 1990).Duration of exposure as estimated by a
hygiene investigation was used as an exposure surrogate. These findings of a
positive association were surprising because scavenging systems were thought
to have been more likely in use during the study period compared to many of
the previously cited papers, almost a decade older.
In the mid 1970's, human studies testing the
cognitive and the motor skills of male subjects/volunteers, showed that
exposure to concentrations of anesthetic gas mixtures commonly found in the
unscavenged operating room, resulted in decreased ability to perform complex
tasks (Bruce et al. 1974, 1975, later invalidated by the author, 1983, 1991).
These volunteers exhibited decrements in performance following exposures at:
500 ppm N2O in
air; 500 ppm N2O plus
15 ppm halothane in air; and 500 ppm N2O plus 15 ppm enflurane in air.
However, studies that attempted to replicate the results of the human
performance studies that showed decrements failed to confirm these findings
(Smith and Shirley 1978).
Potential harmful effects due to desflurane
exposure have been addressed in a few recent studies, including those of
Holmes and colleagues (1990), an animal study; and Weiskopf and colleagues
(1992), a study conducted with human volunteers. However, desflurane’s
potential as a hazard to health-care personnel has not been thoroughly
evaluated. Sevoflurane (Ultane®), the newest anesthetic agent in clinical
practice, has also not been thoroughly evaluated. The levels of risk for
isoflurane, desflurane, and sevoflurane have not been established. Since
there are limited data, occupational exposure limits for these agents have
not been determined. Therefore, until more information is available, it is
prudent to attempt to minimize occupational exposure to these as with all
anesthetic agents.
Unlike N2O, there is evidence that
halothane is mutagenic in certain in vitro test systems (Garro and Phillips
1978) and that halothane is metabolized to reactive intermediates that
covalently bind to cellular macromolecules, suggesting potential mechanisms
of toxicity (Gandolfi et al. 1980).
3.
Summary
Despite questions about design issues or
selection bias in some studies, the weight of the evidence regarding
potential health risks from exposure to anesthetic agents in unscavenged environments
suggests that clinicians need to be concerned. Moreover, there is biological
plausibility that adds to the concern that high levels of unscavenged waste
anesthetic gases present a potential for adverse neurological effects or
reproductive risk to exposed workers or developmental anomalies in their
offspring (Cohen et al. 1980; Rowland 1992).
While the use of prospective studies and
carefully designed research protocols is encouraged to elucidate areas of
controversy, a responsible approach to worker health and safety dictates that
any exposure to waste and trace gases should be kept to the lowest practical
level.
An anesthesia machine is an
assembly of various components and devices that include medical gas cylinders
in machine hanger yokes, pressure regulating and measuring devices, valves,
flow controllers, flow meters, vaporizers, CO2 absorber canisters, and breathing
circuit assembly. The basic two-gas anesthesia machine has more than 700
individual components.
The anesthesia machine is a basic
tool of the anesthesiologist/anesthetist and serves as the primary work
station. It allows the anesthesia provider to select and mix measured flows
of gases, to vaporize controlled amounts of liquid anesthetic agents, and thereby
to administer safely controlled concentrations of oxygen and anesthetic gases
and vapors to the patient via a breathing circuit. The anesthesia machine
also provides a working surface for placement of drugs and devices for
immediate access and drawers for storage of small equipment, drugs, supplies,
and equipment instruction manuals. Finally, the machine serves as a frame and
source of pneumatic and electric power for various accessories such as a
ventilator, and monitors that observe or record vital patient functions or
that are critical to the safe administration of anesthesia.
1.
Gas Flow in the Anesthesia Machine and Breathing System
The internal piping of a basic two-gas
anesthesia machine is shown in Figure 1. The machine has many connections and
potential sites for leaks. Both oxygen and N2O may be supplied from two sources
(Figure 2): a pipeline supply source (central piping system from bulk
storage) and a compressed gas cylinder supply source. In hospitals, the
pipeline supply source is the primary gas source for the anesthesia machine.
Pipeline supplied gases are delivered through wall outlets at a pressure of
50-55 psig through diameter indexed safety system (DISS) fittings or through
quick-connect couplings that are gas-specific within each manufacturer's
patented system.
Because pipeline systems can fail
and because the machines may be used in locations where piped gases are not
available, anesthesia machines are fitted with reserve cylinders of oxygen
and N2O. The
oxygen cylinder source is regulated from approximately 2,200 psig in the
tanks to approximately 45 psig in the machine high-pressure system, and the N2O cylinder source is regulated
from 745 psig in the tanks to approximately 45 psig in the machine
high-pressure system.
Figure 1. The flow arrangement of a basic
two-gas anesthesia machine. A, The fail-safe valve in Ohmeda machines
is termed a pressure sensor shut-off valve; in Dräger machines it is the
oxygen failure protection device (OFPD). B, Second-stage oxygen
pressure regulator is used in Ohmeda (but not Dräger Narkomed) machines. C,
Second-stage nitrous oxide pressure regulator is used in Ohmeda Modulus
machines having the Link 25 Proportion Limiting System; not used in Dräger
machines. D, Pressure relief valve used in certain Ohmeda machines;
not used in Dräger machines. E, Outlet check valve used in Ohmeda
machines except Modulus II Plus and Modulus CD models; not used in Dräger
machines. The oxygen take-off for the anesthesia ventilator driving gas
circuit is downstream of the main on/off switch in Dräger machines, as
shown here. In Ohmeda machines, the take-off is upstream of the main
on/off switch. (Adapted from Check-out: a guide for preoperative inspection
of an anesthesia machine, ASA, 1987. Reproduced by permission of the American
Society of Anesthesiologists, 520 N. Northwest Highway, Park Ridge, Ill.)
Figure 2. The supply of nitrous oxide and
oxygen may come from two sources: the wall (pipeline) supply and the reserve
cylinder supply. (Reproduced by permission of Datex·Ohmeda, Madison,
Wisconsin).
Compressed gas cylinders of oxygen, N2O, and other medical gases are
attached to the anesthesia machine through the hanger yoke assembly. Each
hanger yoke is equipped with the pin index safety system, a safeguard
introduced to eliminate cylinder interchanging and the possibility of accidentally
placing the incorrect gas tank in a yoke designed for another gas tank.
Figure 3 shows the oxygen pathway through the flowmeter, the agent
vaporizer, and the machine piping, and into the breathing circuit. Oxygen
from the wall outlet or cylinder pressurizes the anesthesia delivery system.
Compressed oxygen provides the needed energy for a pneumatically powered
ventilator, if used, and it supplies the oxygen flush valve used to
supplement oxygen flow to the breathing circuit. Oxygen also"powers"
an in-line pressure-sensor shutoff valve ("fail-safe" valve) for
other gases to prevent their administration if the O2 supply pressure in the O2 high pressure system falls below
a threshold value.
Figure 3. Oxygen and N2O flow from their supply sources via their flow control valves,
flowmeters and common manifold to the concentration-calibrated vaporizer and
then via the machine common gas outlet to the breathing system. The high
pressure system of the anesthesia machine comprises those components from the
compressed gas supply source to the gas (O2 and N2O) flow
control valves. The low pressure system of the anesthesia machine comprises
those components downstream of the gas flow control valves. (Reproduced by
permission of Datex·Ohmeda, Madison, Wisconsin).
Once the flows of oxygen, N2O, and other medical gases (if
used) are turned on at their flow control valves, the gas mixture flows into
the common manifold and through a concentration-calibrated agent-specific
vaporizer where a potent inhaled volatile anesthetic agent is added. The
mixture of gases and vaporized anesthetic agent then exits the anesthesia
machine low pressure system through the common gas outlet and flows to the
breathing system.
The circle system shown in Figure 4 is the breathing system most
commonly used in operating rooms (ORs). It is so named because its components
are arranged in a circular manner. The essential components of a circle breathing
system (Figure 5) include a site for inflow of fresh gas (common [fresh] gas
inlet), a carbon dioxide absorber canister (containing soda lime or barium
hydroxide lime) where exhaled carbon dioxide is absorbed; a reservoir bag;
inspiratory and
Figure 4. Basic circle breathing system.
(Reproduced by permission of Datex·Ohmeda, Madison, Wisconsin).
expiratory unidirectional valves; flexible corrugated breathing
tubing; an adjustable pressure-limiting (APL) or "pop-off" valve
for venting excess gas; and a"Y" piece that connects to a face
mask, tracheal tube, laryngeal mask airway (LMA) or other airway management
device.
Figure 5. Essential components of a
circle breathing system. (Adapted from Principles of Anesthesiology: general
and regional anesthesia, Collins, Vincent J., M.D., Executive Editor: Cann,
Carroll C., 1993. Reproduced by permission of Lippincott Williams and
Wilkins, Malvern, Pennsylvania).
Once inside the breathing system, the mixture of gases and vapors
flows to the breathing system’s inspiratory unidirectional valve, then on
toward the patient. Exhaled gases pass through the expiratory unidirectional
valve and enter the reservoir bag. When the bag is full, excess gas flows
through the APL (or pop-off) valve and into the scavenging system that
removes the waste gases. On the next inspiration, gas from the reservoir bag
passes through the carbon dioxide absorber prior to joining the fresh gas
from the machine on its way to the patient. The general use of fresh gas flow
rates into anesthetic systems in excess of those required to compensate for
uptake, metabolism, leaks, or removal of exhaled carbon dioxide results in
variable volumes of anesthetic gases and vapors exiting the breathing system
through the APL valve.
When an anesthesia ventilator is used, the ventilator bellows
functionally replaces the circle system reservoir bag and becomes a part of
the breathing circuit. The APL valve in the breathing circuit is either
closed or excluded from the circuit using a manual
("bag")/automatic (ventilator) circuit selector switch. The
ventilator incorporates a pressure-relief valve, that permits release of
excess anesthetic gases from the circuit at end-exhalation. These gases
should also be scavenged.
2.
Sources of Leaks Within the Anesthesia Machine and Breathing System
No anesthesia machine system is totally
leak-free (Emergency Care Research Institute 1991). Leakage may originate
from both the high-pressure and low-pressure systems of the anesthesia or
analgesia machine.
The high-pressure system consists of all
piping and parts of the machine that receive gas at cylinder or pipeline
supply pressure. It extends from the high-pressure gas supply (i.e., wall
supply or gas cylinder) to the flow control valves. Leaks may occur from the
high-pressure connections where the supply hose connects to the wall outlet or
gas cylinder and where it connects to the machine inlet. Therefore,
gas-supply hoses should be positioned to prevent strain on the fittings (ASTM
Standard F1161-88; Dorsch and Dorsch 1994) and constructed from supply-hose
materials designed for high-pressure gas flow and minimal kinking (Bowie and
Huffman 1985). High-pressure leakage may also occur within the anesthesia
machine itself. Other potential sources of leaks include quick-connect
fittings, cylinder valves, absent or worn gaskets, missing or worn yoke plugs
in a dual yoke assembly, and worn hoses.
The low-pressure system of the anesthesia
machine (in which the pressure is slightly above atmospheric) consists of
components downstream of the flow-control valves. It therefore includes the
flow meter tubes, vaporizers, common gas outlet and breathing circuit, (i.e.,
from the common gas outlet to the patient). Low-pressure system leaks may
occur from the connections and components anywhere between the anesthesia gas
flow control valves and the airway. This leakage may occur from loose-fitting
connections, defective and worn seals and gaskets, worn or defective
breathing bags, hoses, and tubing, loosely assembled or deformed slip joints
and threaded connections, and the moisture drainage port of the CO2 absorber, which may be in
the"open" position.
Low-pressure system leaks also may occur at
the gas analysis sensor (i.e., circuit oxygen analyzer) and gas sampling
site(s), face mask, the tracheal tube (especially in pediatric patients where
a leak is required around the uncuffed tracheal tube), laryngeal mask airway
(over the larynx), and connection points for accessory devices such as a
humidifier, temperature probe, or positive end-expiratory pressure (PEEP)
valve. Inappropriate installation of a calibrated vaporizer(s) or
misalignment of a vaporizer on its manifold (ECRI 1991) can also contribute
to anesthetic gas leakage.
Minute absorbent particles that may have been
spilled on the rubber seal around the absorber canister(s) may also prevent a
gas-tight seal when the canister(s) in the carbon dioxide absorber is (are)
reassembled (Eichhorn 1993). The exhaust from a sidestream sampling
respiratory gas analyzer and/or capnograph should also be connected to the
waste gas scavenging system because the analyzed gas sample may contain N2O or halogenated vapors.
3.
Checking Anesthesia Machines
Prior to induction of anesthesia, the
anesthesia machine and its components/accessories should be made ready for
use. All parts of the machine should be in good working order with all
accessory equipment and necessary supplies on hand. The waste gas disposal
system should be connected, hoses visually inspected for obstructions or
kinks, and proper operation determined. Similarly, the anesthesia breathing
system should be tested to verify that it can maintain positive pressure.
Leaks should be identified and corrected before the system is used (Bowie and
Huffman 1985; Food and Drug Administration 1993; Dorsch and Dorsch 1994). The
ability of the anesthesia system to maintain constant pressure is tested not
only for the safety of the patient dependent on a generated positive pressure
ventilation but also to test for leaks and escape of anesthetic gases, which
may expose health-care personnel to waste anesthetic gases.
Several check-out procedures exist. The 1993
Food and Drug Administration (FDA) Anesthesia Apparatus Checkout
Recommendations Document which is shown in Appendix 2, is based on guidelines
developed by the FDA, as advised by anesthesiologists and manufacturers. This
checkout serves only as a generic guideline because the designs of different
machines and monitors vary considerably. The guideline encourages users to
modify the recommendations to accommodate differences in equipment design,
modifications, and variations in local clinical practice. The user must refer
to the machine manufacturer's operator’s manual for the manufacturer’s
specific procedures or precautions.
Occupational exposures can be
controlled by the application of a number of well-known principles including
engineering and work practice controls, administrative controls, personal
protective equipment, and monitoring. These principles may be applied at or
near the hazard source, to the general workplace environment, or at the point
of occupational exposure to individuals. Controls applied at the source of
the hazard, including engineering and work practice controls, are generally
the preferred and most effective means of control. In anesthetizing locations
and PACUs, where employees are at risk of exposure to waste anesthetic gases,
exposure may be controlled by some or all of the following: (1) effective
anesthetic gas scavenging systems that remove excess anesthetic gas at the
point of origin; (2) effective general or dilution ventilation; (3) good work
practices on the part of the health-care workers, including the proper use of
controls; (4) proper maintenance of equipment to prevent leaks; and (5)
periodic personnel exposure and environmental monitoring to determine the
effectiveness of the overall waste anesthetic gas control program.
The following is a general
discussion of engineering controls, work practices, administrative controls,
and personal protective equipment that can reduce worker exposure to waste
anesthetic gases. However, not every control listed in this section may be
feasible in all settings. Additional location-specific controls and
appropriate exceptions are addressed in section F.
1.
Engineering Controls
The collection and disposal of waste
anesthetic gases in operating rooms and non-operating room settings is
essential for reducing occupational exposures. Engineering controls such as
an appropriate anesthetic gas scavenging system are the first line of defense
and the preferred method of control to protect employees from exposure to
anesthetic gases. An effective anesthetic gas scavenging system traps waste
gases at the site of overflow from the breathing circuit and disposes of
these gases to the outside atmosphere. The heating, ventilating, and air
conditioning (HVAC) system also contributes to the dilution and removal of
waste gases not collected by the scavenging system or from other sources such
as leaks in the anesthetic apparatus or improper work practices.
The exhalation of residual gases by patients
in the PACU may result in significant levels of waste anesthetic gases when
appropriate work practices are not used at the conclusion of the anesthetic
or inadequate ventilation exists in the PACU. A nonrecirculating ventilation
system can reduce waste gas levels in this area. Waste gas emissions to the
outside atmosphere must meet local, state, and Environmental Protection
Agency (EPA) regulatory requirements.
A scavenging system consists of five basic
components (ASTM, F 1343 - 91):
·
A gas
collection assembly such as a collection manifold or a distensible bag
(i.e., Jackson-Rees pediatric circuit), which captures excess anesthetic
gases at the site of emission, and delivers it to the transfer tubing.
·
Transfer
tubing, which
conveys the excess anesthetic gases to the interface.
·
The interface,
which provides positive (and sometimes negative) pressure relief and may
provide reservoir capacity. It is designed to protect the patient's lungs
from excessive positive or negative scavenging system pressure.
·
Gas
disposal assembly tubing, which conducts the excess anesthetic gases from the interface to the
gas disposal assembly.
·
The gas
disposal assembly, which conveys the excess gases to a point where they
can be discharged safely into the atmosphere. Several methods in use include
a nonrecirculating or recirculating ventilation system, a central vacuum
system, a dedicated (single-purpose) waste gas exhaust system, a passive duct
system, and an adsorber.
In general, a machine-specific interface must
be integrated with a facility’s system for gas removal. The interface permits
excess gas to be collected in a reservoir (bag or canister) and limits the
pressure within the bag or canister. A facility’s gas disposal system
receives waste anesthetic gases from the interface and should vent the waste
gases outside the building and away from any return air ducts or open
windows, thus preventing the return of the waste gases back into the
facility. (Refer to Appendix 3 for a more detailed description of how the
scavenging interface works.)
Removal of excess anesthetic gases from the
anesthesia circuit can be accomplished by either active or passive
scavenging. When a vacuum or source of negative pressure is connected to the
scavenging interface, the system is described as an active system. When a
vacuum or negative pressure is not used, the system is described as a passive
system. With an active system there will be a negative pressure in the gas
disposal tubing. With a passive system, this pressure will be increased above
atmospheric (positive) by the patient exhaling passively, or manual
compression of the breathing system reservoir bag.
Use of a central vacuum system is an example
of an active system: The waste anesthetic gases are moved along by negative
pressure. Venting waste anesthetic gas via the exhaust grille or exhaust duct
of a nonrecirculating ventilation system is an example of a passive system:
The anesthetic gas is initially moved along by the positive pressure from the
breathing circuit until it reaches the gas disposal assembly.
Active Systems
Excess anesthetic gases may be removed by a
central vacuum system (servicing the ORs in general) or an exhaust system
dedicated to the disposal of excess gases. When the waste anesthetic gas
scavenging system is connected to the central vacuum system (which is shared
by other users, e.g., surgical suction), exposure levels may be effectively
controlled. The central vacuum system must be specifically designed to handle
the large volumes of continuous suction from OR scavenging units. If a
central vacuum system is used, a separate, dedicated gas disposal assembly
tubing should be used for the scavenging system, distinct from the tubing
used for patient suctioning (used for oral and nasal gastric sources as well
as surgical suctioning).
Similarly, when a dedicated exhaust system
(low velocity) is used, excess gases can also be collected from one or more
ORs and discharged to the outdoors. The exhaust fan must provide sufficient
negative pressure and air flow so that cross-contamination does not occur in
the other ORs connected to this system. Active systems are thought to be more
effective than passive systems at reducing excess waste anesthetic gas
concentrations because leaks in the scavenging system do not result in an
outward loss of gas.
Passive Systems
HVAC systems used in health-care facilities
are of two types: nonrecirculating and recirculating. Nonrecirculating
systems, also termed "one-pass" or "single-pass"
systems, take in fresh air from the outside and circulate filtered and
conditioned air (i.e., controlled for temperature and humidity) through the
room. Whatever volumes of fresh air are introduced into the room are
ultimately exhausted to the outside. Waste anesthetic gases can be
efficiently disposed of via this nonrecirculating system.
When a nonrecirculating ventilation system
serves through large-diameter tubing and terminating the tubing at the room’s
ventilation exhaust as the disposal route for excess anesthetic gases,
disposal involves directing the waste gases grille. The sweeping effect of
the air flowing into the grille carries the waste gases away. Because all of
the exhausted air is vented to the external atmosphere in this type of
system, the excess anesthetic gases can be deposited into the exhaust stream
either at the exhaust grille or further downstream in the exhaust duct.
Concern for fuel economy has increased the
use of systems that recirculate air. Recirculating HVAC/ventilation
systems return part of the exhaust air back into the air intake and
recirculate the mixture through the room. Thus, only a fraction of the
exhaust air is disposed of to the outside. To maintain minimal levels of
anesthetic exposure, air which is to be recirculated must not contain
anesthetic gases. Consequently, recirculating systems employed as a disposal
pathway for waste anesthetic gases must not be used for gas waste disposal.
The exception is an arrangement that transfers waste gases into the
ventilation system at a safe distance downstream from the point of
recirculation to ensure that the anesthetic gases will not be circulated
elsewhere within the building.
Under certain circumstances a separate duct
for venting anesthetic gases directly outside the building without the use of
a fan, may be an acceptable alternative. By this technique, excess anesthetic
gases may be vented through the wall, window, ceiling, or floor, relying only
on the slight positive pressure of the gases leaving the gas collection
assembly to provide the flow. However, several limitations are apparent. A
separate line would be required for each OR to prevent the
cross-contamination with anesthetic gases among the ORs. A safe disposal site
would be necessary. The possible effects of variations in wind velocity and
direction would require a means for preventing a reverse flow in the disposal
system. Occlusion of the outer portion of such a passive system by ice or by
insect or bird nests is also possible. The outside opening of a through-wall,
-window, -ceiling, or -floor disposal assembly should be directed downward,
shielded, and screened to prevent the entrance of foreign matter or ice
buildup. Despite these limitations, the separate duct without the use of a
fan may be ideal in older facilities constructed with windows that cannot be
opened and in the absence of nonrecirculating air conditioning.
Adsorbers can also trap most excess anesthetic gases.
Canisters of varying shapes and capacities filled with activated charcoal
have been used as waste gas disposal assemblies by directing the gases from
the gas disposal tubing through them. Activated charcoal canisters will
effectively adsorb the vapors of halogenated anesthetics but not N2O. The effectiveness of individual
canisters and various brands of charcoal vary widely. Different potent
inhaled volatile agents are adsorbed with varying efficiencies. The
efficiency of adsorption also depends on the rate of gas flow through the canister.
The canister is used where portability is necessary. The disadvantages are
that they are expensive and must be changed frequently. Canisters must be
used and discarded in the appropriate manner, as recommended by the
manufacturer.
General or Dilution Ventilation
An effective room HVAC system when used in
combination with an anesthetic gas scavenging system should reduce, although
not entirely eliminate, the contaminating anesthetic gases. If excessive
concentrations of anesthetic gases are present, then airflow should be
increased in the room to allow for more air mixing and further dilution of
the anesthetic gases. Supply register louvers located in the ceiling should
be designed to direct the fresh air toward the floor and toward the
health-care workers to provide dilution, and removal of the contaminated air
from the operatory or PACU. Exhaust register louvers should be properly
located (usually low on the wall near the floor level) in the room to provide
adequate air distribution. They should not be located near the supply air
vents because this will short-circuit the airflow and prevent proper air
mixing and flushing of the contaminants from the room.
2.
Work Practices
Work practices, as distinct from engineering
controls, involve the way in which a task is performed. OSHA has found that
appropriate work practices can be a vital aid in reducing the exposures of OR
personnel to waste anesthetic agents. In contrast, improper anesthetizing
techniques can contribute to increased waste gas levels. These techniques can
include an improperly selected and fitted face mask, an insufficiently
inflated tracheal tube cuff, an improperly positioned laryngeal mask, or
other airway, and careless filling of vaporizers and spillage of liquid
anesthetic agents.
General work practices recommended for
anesthetizing locations include the following:
·
A
complete anesthesia apparatus checkout procedure should be performed each day
before the first case. An abbreviated version should be performed before each
subsequent case. The FDA Anesthesia Apparatus Checkout Recommendations (Appendix
2) should be considered in developing inspection and testing procedures for
equipment checkout prior to administering an anesthetic.
·
If a
face mask is to be used for administration of inhaled anesthetics, it should
be available in a variety of sizes to fit each patient properly. The mask
should be pliable and provide as effective a seal as possible against leakage
into the surrounding air.
·
Tracheal
tubes, laryngeal masks, and other airway devices should be positioned
precisely and the cuffs inflated adequately.
·
Vaporizers
should be filled in a well-ventilated area and in a manner to minimize
spillage of the liquid agent. This can be accomplished by using a specialized
"key-fill" spout to pour the anesthetic into the vaporizer instead
of pouring from a bottle into a funnel-fill vaporizer. When feasible,
vaporizers should be filled at the location where the anesthetic will be
administered and, when filled electively, with the fewest possible personnel
present in the room. Vaporizers should be turned off when not in use.
·
Spills
of liquid anesthetic agents should be cleaned up promptly. (Refer to section
G - Clean-up and Disposal of Liquid Anesthetic Agent Spills.)
·
Before
extubating the patient's trachea or removing the mask or other airway
management device, one should administer non-anesthetic gases/agents so that
the washed-out anesthetic gases can be removed by the scavenging system. The
amount of time allowed for this should be based on clinical assessment and
may vary from patient to patient. When possible, flushing of the breathing
system should be achieved by exhausting into the scavenging system rather
than into the room air.
Work practices performed by biomedical
engineers and technicians also contribute significantly to the efficacy of
managing waste gas exposure. It is, therefore, important for this group of
workers to do the following:
·
Monitor
airborne concentrations of waste gases by sampling, measuring, and reporting
data to the institution's administration. Air monitoring for waste anesthetic
gases should include both personal sampling (i.e., in a health-care worker’s
breathing zone) and area sampling.
·
Assist
in identifying sources of waste/leaking gases and implementing corrective
action.
·
Determine
if the scavenging system is designed and functioning properly to remove the
waste anesthetic gases from the breathing circuit, and ensure that the gases
are vented from the workplace in such a manner that occupational re-exposure
does not occur (e.g., smoke trail tests of exhaust grilles used with passive
scavenging systems).
·
Ensure
that operatory and PACU ventilation systems provide sufficient room air
exchange to reduce ambient waste gas levels.
3.
Administrative
Controls
Administrative controls represent another
approach for reducing worker exposure to waste gases other than through the
use of engineering controls, work practices, or personal protective
equipment. Administrative controls may be thought of as any administrative
decision that results in decreased anesthetic-gas exposure. For workers
potentially exposed to waste anesthetic gases, the program administrator
should establish and implement policies and procedures to:
·
Institute
a program of routine inspection and regular maintenance of equipment in order
to reduce anesthetic gas leaks and to have the best performance of scavenging
equipment and room ventilation. Preventive maintenance should be performed by
trained individuals according to the manufacturer’s recommendations and at
intervals determined by equipment history and frequency of use. Preventive
maintenance includes inspection, testing, cleaning, lubrication, and
adjustment of various components. Worn or damaged parts should be repaired or
replaced. Such maintenance can result in detection of deterioration before an
overt malfunction occurs. Documentation of the maintenance program should be
kept indicating the nature and date of the work performed, as well as the
name of the trained individual servicing the equipment.
·
Implement
a monitoring program to measure airborne levels of waste gases in the breathing
zone or immediate work area of those most heavily exposed (e.g.,
anesthesiologist, nurse anesthetist, oral surgeon) in each anesthetizing
location and PACU. Periodic monitoring (preferably at least semiannually) of
waste gas concentrations is needed to ensure that the anesthesia delivery
equipment and engineering/environmental controls work properly and that the
maintenance program is effective. Monitoring may be performed effectively
using conventional time-weighted average air sampling or real-time air
sampling techniques.
·
Encourage
or promote the use of scavenging systems in all anesthetizing locations where
inhaled agents are used, recognizing that a waste gas scavenging system is
the most effective means of controlling waste anesthetic gases.
·
Implement
an information and training program for employees exposed to anesthetic
agents that complies with OSHA’s Hazard Communication Standard (29 CFR 1910.1200)
so that employees can meaningfully participate in, and support, the
protective measures instituted in their workplace.
·
Define
and implement appropriate work practices to help reduce employee exposure.
Training and educational programs covering appropriate work practices to
minimize levels of anesthetic gases in the operating room should be conducted
at least annually. Employers should emphasize the importance of implementing
these practices and should ensure that employees are properly using the
appropriate techniques on a regular basis.
·
Implement
a medical surveillance program for all workers exposed to waste gases.
·
Ensure
the proper use of personal protective equipment during clean-up and
containment of major spills of liquid anesthetic agents.
·
Manage
disposal of liquid agents, spill containment, and air monitoring for waste
gases following a spill.
·
Comply
with existing federal, state, and local regulations and guidelines developed
to minimize personnel exposure to waste anesthetic gases, including the proper
disposal of hazardous chemicals.
4.
Personal
Protective Equipment
Personal protective equipment should not be
used as a substitute for engineering, work practice, and/or administrative
controls in anesthetizing locations and PACUs. In fact, exposure to waste
gases is not effectively reduced by gloves, goggles, and surgical masks. A
negative-pressure, high-efficiency particulate air (HEPA) filter used for
infection control is also not appropriate to protect workers from waste
gases. Air-supplied respirators with self-contained air source are ideal for
eliminating exposure but are not a practical alternative.
During clean-up and containment of spills of
liquid anesthetic agents, personal protective equipment should be used in
conjunction with engineering, work practice, and/or administrative controls
to provide for employee safety and health. Gloves, goggles, face shields, and
chemical protective clothing (CPC) are recommended to ensure worker
protection. Respirators, where needed, should be selected based on the
anticipated contamination level.
When selecting gloves and CPC, some of the
factors to be considered include material chemical resistance, physical
strength and durability, and overall product integrity. Permeation,
penetration, and degradation data should be consulted if available. Among the
most effective types of gloves and body protection are those made from Viton®, neoprene, and nitrile. Polyvinyl
alcohol (PVA) is also effective but it should not be exposed to water or
aqueous solutions.
When the gloves and the CPC being used have
not been tested under the expected conditions, they may fail to provide
adequate protection. In this situation, the wearer should observe the gloves
and the chemical protective clothing during use and treat any noticeable
change (e.g., color, stiffness, chemical odor inside) as a failure until
proved otherwise by testing. If the work must continue, new CPC should be
worn for a shorter exposure time, or CPC of a different generic material
should be worn. The same thickness of a generic material such as neoprene or
nitrile supplied by different manufacturers may provide significantly
different levels of protection because of variations in the manufacturing
processes or in the raw materials and additives used in processing.
Professional judgement must be used in
determining the type of respiratory protection to be worn. For example, where
spills of halogenated anesthetic agents are small, exposure time brief, and
sufficient ventilation present, NIOSH-approved chemical cartridge respirators
for organic vapors should provide adequate protection during cleanup
activities.
Where large spills occur and there is
insufficient ventilation to adequately reduce airborne levels of the
halogenated agent, respirators designed for increased respiratory protection
should be used. The following respirators, to be selected for large spills,
are ranked in order from minimum to maximum respiratory protection:
·
Any
type 'C' supplied-air respirator with a full facepiece, helmet, or hood
operated in continuous-flow mode.
·
Any
type 'C' supplied-air respirator with a full facepiece operated in
pressure-demand or other positive-pressure mode.
·
Any
self-contained breathing apparatus with a full facepiece operated in
pressure-demand or other positive-pressure mode.
This section describes engineering
and work practice controls specific to hospital ORs, PACUs, dental
operatories, and veterinary clinics and hospitals. Operational procedures
relating to engineering controls are also discussed where appropriate.
1.
Hospital Operating Rooms
For years anesthesia providers tolerated
exposure to waste anesthetic gases and regarded it as an inevitable
consequence of their work. Since the 1970s anesthesiologists have steadily
worked to improve equipment and technique to reduce workplace exposures to
waste anesthetic gases, and significant progress has been made. In early
delivery equipment, waste gases were exhausted through the APL or
"pop-off" valve into the face of the anesthesia provider and were
distributed into the room air. Present practice which utilizes an efficient
scavenging system avoids this type of contamination by collecting the excess
gases immediately at the APL valve.
a.
Engineering Controls
Waste gas evacuation is required for every
type of breathing circuit configuration (Huffman 1991; Azar and Eisenkraft
1993), with the possible exception of a closed circuit, because most
anesthesia techniques typically use more fresh gas flow than is required.
Appropriate waste gas evacuation involves collection and removal of waste
gases, detection and correction of leaks, consideration of work practices,
and effective room ventilation (Dorsch and Dorsch 1994). To minimize waste
anesthetic gas concentrations in the operating room the recommended air
exchange rate (room dilution ventilation) is a minimum total of 15 air
changes per hour with a minimum of 3 air changes of outdoor air (fresh air)
per hour (American Institute of Architects 1996-1997). Operating room air containing
waste anesthetic gases should not be recirculated to the operating room or
other hospital locations.
b.
Work Practices
In most patients, a circle absorption system
is used and can be easily connected to a waste gas scavenging system. In
pediatric anesthesia, systems other than those with a circle absorber may be
used. Choice of the breathing circuit that best meets the needs of pediatric
patients may alter a clinician’s ability to scavenge waste gas effectively.
Breathing circuits frequently chosen for neonates, infants, and small
children are usually valveless, have low resistance, and limit rebreathing.
The Mapleson D system and the Jackson-Rees modification of the Ayre’s T-piece
are examples of limited rebreathing systems that require appropriate scavenging
equipment.
The following work practices may be employed
with any of the above breathing circuits:
·
Empty
the contents of the reservoir bag directly into the anesthetic gas scavenging
system and turn off the flow of N2O and any halogenated anesthetic agent
prior to disconnecting the patient circuit.
·
Turn
off the flow of N2O and the vaporizer, if appropriate, when the patient circuit is
disconnected from the patient, for example, for oral or tracheal suctioning.
·
Test
daily for low-pressure leaks throughout the entire anesthesia system. All
leaks should be minimized before the system is used. Starting anesthetic gas
flow before the actual induction of anesthesia begins is not acceptable. For
techniques to rapidly induce anesthesia using inhaled agents (single-breath
mask induction), the patient connector should be occluded when filling the
breathing circuit with nitrous oxide or halogenated agent prior to applying
the mask to the patient's face.
If the circle absorber system (Figure 6) is
used, the following additional work practices can be employed:
·
Adjust
the vacuum needle valve as needed to regulate the flow of waste anesthetic
gases into the vacuum source in an active scavenging system. Adjustments
prevent the bag from overdistending by maintaining the volume in the
scavenging system reservoir bag between empty and half-full (Bowie and
Huffman 1985; Huffman 1991). In machines that use an open reservoir to
receive waste gas, a flowmeter is used to adjust the rate of gas flow to the
vacuum system.
·
Cap
any unused port in a passive waste gas scavenging configuration.
Figure 6. Circle breathing system
connected to a closed reservoir scavenging interface. (Reproduced by
permission of North American Dräger, Telford, Pennsylvania).
2.
Postanesthesia Care in Hospitals and Stand-Alone Facilities
Because the patient is the main source of
waste anesthetic gases in the PACU, it becomes more difficult to control
health-care workers’ exposures to waste anesthetic gases. The unique PACU
environment coupled with the patient’s immediate condition upon arrival from
surgery require different work practices than those routinely used in ORs.
Patients undergoing general anesthesia usually have their airways secured
using a tracheal tube with an inflatable cuff that seals the tube within the
trachea. The seal between the tracheal tube cuff and the trachea (or between
the face mask and the face) is essential for maintaining a gas-tight system
that permits effective scavenging in the OR. The tracheal tube connects the
patient with the breathing circuit which is connected to the scavenging
system in the OR. Once the patient reaches the PACU, scavenging systems such
as those used in the OR are no longer effective, since the patient is no
longer connected to the breathing circuit. Other less-effective methods of
waste gas removal are thus relied upon.
a.
Engineering Controls
As a result of using appropriate anesthetic
gas scavenging in ORs, the levels of contamination have been decreased. In
the PACU, however, the principle of scavenging as practiced in the OR is not
widely accepted due to medical considerations and consequently is
infrequently employed as a source-control method for preventing the release
of waste anesthetic gases into the PACU environment. Most PACUs provide care
to multiple patients in beds without walls between them, and convective
currents move the gases from their source to other areas. Therefore, in the
PACU, a properly designed and operating dilution ventilation system should be
relied upon to minimize waste anesthetic gas concentrations. This system
should provide a recommended minimum total of 6 air changes per hour with a
minimum of 2 air changes of outdoor air per hour to adequately dilute waste
anesthetic gases (American Institute of Architects 1996-1997). Room exhaust
containing waste anesthetic gases should not be recirculated to other areas
of the hospital.
b.
Work Practices
PACU managers should consider:
·
Periodic
exposure monitoring with particular emphasis on peak gas levels in the
breathing zone of nursing personnel working in the immediate vicinity of the
patient’s head. Methods using random room sampling to assess ambient
concentrations of waste anesthetic gases in the PACU are not an accurate
indicator of the level of exposure experienced by nurses providing bedside
care. Because of the closeness of the PACU nurse to the patient, such methods
would consistently underestimate the level of waste anesthetic gases in the
breathing zone of the bedside nurse.
·
Application
of a routine ventilation system maintenance program to keep waste gas
exposure levels to a minimum.
3.
Dental Operatory
Mixtures of N2O and oxygen have been used in
dentistry as general anesthetic agents, analgesics, and sedatives for more
than 100 years (McGlothlin et al. 1992). The usual analgesia equipment used
by dentists includes a N2O and O2
delivery system, a gas mixing bag, and a nasal mask with a positive pressure
relief valve (Dorsch and Dorsch 1994). The analgesia machine is usually
adjusted to deliver more of the analgesic gas mixture than the patient can
use.
Analgesia machines for dentistry are designed
to deliver up to 70 percent (700,000 ppm) N2O to a patient during dental
surgery. The machine restricts higher concentrations of N2O from being administered to
protect the patient from hypoxia. In most cases, patients receive between 30
and 50 percent N2O during surgery. The amount of time N2O is administered to a patient
depends on the dentist’s judgment of patient needs and the complexity of the
surgery. The most common route of N2O delivery and exhaust is through
a nasal scavenging mask applied to the patient.
Some dentists administer N2O at higher concentrations at the
beginning of the operation, then decrease the amount as the operation
progresses. Others administer the same amount of N2O throughout the operation. When
the operation is completed, the N2O is turned off. Some dentists
turn the N2O on
only at the beginning of the operation, using N2O as a sedative during the
administration of local anesthesia, and turn it off before operating
procedures. Based on variations in dental practices and other factors in room
air, N2O
concentrations can vary considerably for each operation and also vary over
the course of the operation.
Unless the procedure is performed under
general anesthesia in an OR, halogenated anesthetics are not administered,
nor does the patient undergo laryngoscopy and tracheal intubation. In the
typical dental office procedure, the nasal mask is placed on the patient,
fitted, and adjusted prior to administration of the anesthetic agent. The
mask is designed for the nose of the patient since access to the patient’s
mouth is essential for dental procedures.
A local anesthetic, if needed, is typically
administered after the N2O takes effect. The patient’s mouth is opened and the local anesthetic
is injected. The dental procedure begins after the local anesthetic takes
effect. The patient opens his/her mouth but is instructed to breathe through
the nose. Nonetheless, a certain amount of mouth breathing frequently occurs.
The dentist may periodically stop the dental procedure for a moment to allow
the patient to close the mouth and breath deeply to re-establish an
appropriate concentration of N2O in the patient’s body before resuming the
procedure. Depending on the nature of the procedure, high velocity suction is
regularly used to remove intraoral debris and, when used, creates a negative
air flow and captures some of the gas exhaled by the patient.
At the end of the procedure, the nosepiece is
left on the patient while the N2O is turned off and the oxygen flow is
increased. The anesthetic mixture diffuses from the circulating blood into
the lungs and is exhaled. Scavenging is continued while the patient is
eliminating the N2O.
a.
Engineering Controls
The dental office or operatory should have a
properly installed N2O delivery system. This includes appropriate scavenging equipment with
a readily visible and accurate flow meter (or equivalent measuring device), a
vacuum pump with the capacity for up to 45 L/min of air per workstation, and
a variety of sizes of masks to ensure proper fit for individual patients.
A common nasal mask, shown in Figure 7,
consists of an inner and a slightly larger outer mask component. The inner
mask has two hoses connected that supply anesthetic gas to the patient. A
relief valve is attached to the inner mask to release excess N2O into the outer mask. The outer
mask has two smaller hoses connected to a vacuum system to capture waste
gases from the patient and excess gas supplied to the patient by the
analgesia machine. The nasal mask should fit over the patient’s nose as
snugly as possible without impairing the vision or dexterity of the dentist.
Gases exhaled orally are not captured by the nasal mask. A flow rate of
approximately 45 L/min has been recommended as the optimum rate to prevent
significant N2O
leakage into the room air (NIOSH 1994).
Figure 7. A nasal mask designed to allow
waste gases to be scavenged through the nose piece.
A newer type of mask is a frequent choice in
dental practice: a single patient use nasal hood. This mask does not require
sterilization after surgery because it is used by only one patient and is
disposable.
In a dental operatory, a scavenging system is
part of a high-volume evacuation system used with a dental unit. The vacuum
system may dispose of a combination of waste gases, oral fluid, and debris,
and is not limited to waste gas removal. The exhaust air of the evacuation
system should be vented outside the building and away from fresh-air inlets
and open windows to prevent re-entry of gas into the operatory.
The general ventilation should provide good
room air mixing. In addition, auxiliary (local) exhaust ventilation used in
conjunction with a scavenging system has been shown to be effective in
reducing excess N2O in the breathing zone of the dentist and dental assistant, from
nasal mask leakage and patient mouth breathing (NIOSH 1994). This type of
ventilation captures the waste anesthetic gases at their source. However,
there are practical limitations in using it in the dental operatory. These
include proximity to the patient, interference with dental practices, noise,
and installation and maintenance costs. It is most important that the dentist
not work between the patient and a free-standing local exhaust hood. Doing so
will cause the contaminated air to be drawn through the dentist’s breathing
zone. These auxiliary ventilation systems are not now commercially available.
b.
Work Practices
·
Prior
to first use each day of the N2O machine and every time a gas cylinder is
changed, the low-pressure connections should be tested for leaks.
High-pressure line connections should be tested for leaks quarterly. A soap
solution may be used to test for leaks at connections. Alternatively, a
portable infrared spectrophotometer can be used to detect an insidious leak.
·
Prior
to first use each day, inspect all N2O equipment (e.g., reservoir bag,
tubing, mask, connectors) for worn parts, cracks, holes, or tears. Replace as
necessary.
·
Connect
mask to the tubing and turn on vacuum pump. Verify appropriate flow rate
(i.e., up to 45 L/min or manufacturer’s recommendations).
·
A
properly sized mask should be selected and placed on the patient. A good,
comfortable fit should be ensured. The reservoir (breathing) bag should not
be over- or underinflated while the patient is breathing oxygen (before
administering N2O).
·
Encourage
the patient to minimize talking, mouth breathing, and facial movement while
the mask is in place.
·
During
N2O administration, the reservoir
bag should be periodically inspected for changes in tidal volume, and the
vacuum flow rate should be verified.
·
On
completing anesthetic administration and before removing the mask,
non-anesthetic gases/agents should be delivered to the patient for a
sufficient time based on clinical assessment that may vary from patient to
patient. In this way, both the patient and the system will be purged of
residual N2O. Do
not use an oxygen flush.
4.
Veterinary
Clinics and Hospitals
Inhalation anesthesia in veterinary hospitals
is practiced in a manner similar to that in human hospitals. Generally,
animals are initially given an injectable anesthetic, followed by general
anesthesia maintained by an inhalation technique. In animal anesthesia, there
are five basic methods by which inhalation anesthetics are administered:
open-insufflation, semiopen without nonrebreathing valves, semiopen with
nonrebreathing valves, semiclosed, and closed. Figure 8 illustrates a circle
breathing system. Oxygen and anesthetic are transported to the animal’s lungs
from the anesthesia machine through a face mask or tracheal tube. An
inflatable cuff on the distal end of the tracheal tube facilitates a seal
with the inner wall of the trachea.
A. Oxygen
source |
F.
Y-Piece connecting inspiratory |
B.
Pressure reducing valve |
And expiratory hoses |
C.
Flow meter |
G.
Expiratory valve |
D.
Vaporizer |
H.
Reservoir bag |
E.
Inspiratory valve |
I. Carbon
dioxide absorber |
|
J.
Pop-off valve |
Figure 8. Circle breathing system used
for veterinary anesthesia. (Reproduced by permission of American Industrial
Hygiene Association, Fairfax, Virginia).
Unidirectional valves allow flow from the
vaporizer to the animal upon inspiration and route the exhaled gases through
a carbon dioxide absorber during expiration. High fresh-gas flows are
typically used with all techniques except closed-system breathing circuits.
During expiration, excess or waste gas exits the breathing circuit at the
adjustable pressure-limiting (APL) or pop-off valve and escapes into the room
unless it is appropriately scavenged.
Controlled rebreathing systems used for very
small animals allow exhaled gases to be immediately expelled from the system
into the room air. Because these systems do not include a carbon dioxide
absorber, greater fresh-gas flows are required to ensure removal of carbon
dioxide from the system. A higher fresh-gas flow may lead to an increase in
ambient waste gas levels.
a.
Engineering
Controls
The basic principles of scavenging used to
capture excess anesthetic gases in hospital surgical suites are appropriate
for application in veterinary anesthesia. The APL or pop-off valve is
connected to the scavenging interface valve. A waste gas reservoir bag is
attached to the interface valve and collects excess anesthetic gases.
In general, the disposal pathway for waste
anesthetic gases generated in a veterinary facility can be any one of those
mentioned (e.g., ventilation system, central vacuum system, dedicated blower
[exhaust] system, passive duct system, or adsorber) and described in detail
on pages [15-17] of this document. A
vacuum source, if present, is connected to the interface valve and waste gas
reservoir bag, where gas is stored until the vacuum can move it to the
outside air. If only halogenated compounds are used, an activated charcoal
adsorption system can be used.
b.
Work
Practices
The following are recommended work practices
for reducing gas leakage:
·
Avoid
turning on N2O or a
vaporizer until the circuit is connected to the animal. Switch off the N2O and vaporizer when not in use.
Maintain oxygen flow until the scavenging system is flushed.
·
Select
the optimal size tracheal tube for the animal and make sure the cuff, if
present, is adequately inflated. Adequacy of cuff inflation may be evaluated
by delivering a positive-pressure breath while the APL or pop-off valve is
closed and listening for a leak originating from around the tracheal tube
cuff.
·
Occlude
the Y-piece if the breathing circuit must be disconnected during surgery.
·
Once
anesthesia is discontinued, empty the breathing bag into the scavenging
system rather than into the room. Releasing anesthetic gases into the OR
could significantly increase the overall waste gas concentration within the
room.
·
At
the end of the surgical procedure, continue to administernon-anesthetic
gases/agents as long as clinically necessary, using high oxygen flow rates
through the breathing circuit to wash the anesthetic gases out of the system
and the animal. This allows exhaled anesthetic gases to be collected by the
scavenging system.
·
It
is possible to close an anesthetic circle and reduce fresh-gas flow rates. In
a circle system where oxygen is the only carrier gas, the amount of fresh gas
flowing to the animal should be adjusted to closely match the animal’s
metabolic oxygen requirement.
·
Select
masks to suit various sizes and breeds encountered in veterinary practice.
When a mask is used for induction or maintenance of anesthesia, use a mask that
properly fits the contour of the animal’s face to minimize gas leakage.
Minimize the time of mask anesthesia to reduce waste.
·
Use
a box for induction of anesthesia in small, uncooperative animals. As with
the mask technique, the induction box method requires high gas-flow rates,
with substantial anesthetic spillage. Methods to minimize this spillage
include tight seals on the box and placement of the box near the ventilation
port of a well-ventilated room. The box can also be connected to an anesthetic
gas-scavenging system to evacuate the gases in the box prior to removing the
animal.
·
Make
certain that the reservoir bag, used to store excess anesthetic waste gas
until the vacuum system can remove it, is adequate to contain all scavenged
gas. This reservoir bag is especially designed to connect to anesthetic
gas-specific fittings.
Small volumes of liquid anesthetic
agents such as halothane, enflurane, isoflurane, desflurane, and sevoflurane
evaporate readily at normal room temperatures, and may dissipate before any
attempts to clean up or collect the liquid are initiated. However, when large
spills occur, such as when one or more bottles of a liquid agent break,
specific cleaning and containment procedures are necessary and appropriate
disposal is required (AANA 1992). The recommendations of the chemical
manufacturer’s material safety data sheet (MSDS) that identify exposure
reduction techniques for spills and emergencies should be followed.
In addition, OSHA Standard for
Hazardous Waste Operations and Emergency Response (29 CFR 1910.120) would
apply if emergency response efforts are performed by employees. The employer
must determine the potential for an emergency in a reasonably predictable
worst-case scenario, and plan response procedures accordingly. Only adequately
trained and equipped workers may respond to spills. When the situation is
unclear or data are lacking on the exposure level, the response needs to be
the same as for high levels of exposure. Responses to incidental releases of
liquid anesthetic agents where the substance can be absorbed, neutralized, or
otherwise controlled at the time of release by employees in the immediate
release area, or by maintenance personnel do not fall within the scope of
this standard.
Because of the volatility of
liquid anesthetics, rapid removal by suctioning in the OR is the preferred
method for cleaning up spills. Spills of large volumes in poorly ventilated
areas or in storage areas should be absorbed using an absorbent material,
sometimes called a sorbent, that is designed for clean-up of organic
chemicals."Spill pillows" commonly used in hospital laboratories,
vermiculite, and carbon-based sorbents are some of the materials commercially
available and regularly used for this purpose. Caution should be exercised if
broken glass bottles pose a hazard.
Both enflurane and desflurane are
considered hazardous wastes under the EPA regulations because these chemicals
contain trace amounts of chloroform (a hazardous substance), a by-product of
the manufacturing process.Consequently, sorbents that have been saturated
with enflurane or desflurane should be managed as an EPA hazardous waste
material due to the trace concentrations of chloroform present. Isoflurane
and halothane do not contain trace amounts of chloroform or any other regulated
substance and are therefore not considered hazardous wastes by EPA.
To minimize exposure to all liquid
anesthetic agents during clean-up and to limit exposure during disposal
procedures, the following general guidelines are recommended. The waste material
should be placed in a container, tightly sealed, properly labeled, and
disposed of with other chemical wastes sent to a facility’s incinerator or
removed by a chemical waste contractor. After a large spill has occurred and
the appropriate response action taken, airborne monitoring should be
conducted to determine if the spill was effectively contained and cleaned up.
Determination of appropriate
disposal procedures for each facility is the sole responsibility of that
facility. Empty anesthetic bottles are not considered regulated waste and may
be discarded with ordinary trash or recycled. Furthermore, the facility as
well as the waste handling contractor must comply with all applicable
federal, state, and local regulations.
To minimize exposure to waste
liquid anesthetic agents during clean-up and disposal, the following general
guidelines are recommended by the manufacturers of liquid anesthetic agents:
·
Wear
appropriate personal protective equipment. (Refer to section E. 4. on personal
protective equipment).
·
Where
possible, ventilate area of spill or leak. Appropriate respirators should be
worn.
·
Restrict
persons not wearing protective equipment from areas of spills or leaks until
clean-up is complete.
·
Collect
the liquid spilled and the absorbent materials used to contain a spill in a
glass or plastic container. Tightly cap and seal the container and remove it
from the anesthetizing location. Label the container clearly to indicate its
contents.
·
Transfer
the sealed containers to the waste disposal company that handles and hauls
waste materials.
·
Health-care
facilities that own or operate medical waste incinerators may dispose of
waste anesthetics by using an appropriate incineration method after verifying
that individual incineration operating permits allow burning of anesthetic
agents at each site.
Air monitoring is one of the
fundamental tools used to evaluate workplace exposures. Accordingly, this
section presents some of the appropriate methods that can be used to detect
and measure the concentration of anesthetic gases that may be present in the
health-care environment. The data provided by monitoring are necessary to
establish proper engineering, work practice, and administrative controls to
ensure the lowest reasonably achievable gas levels in the operatory and PACU
room air.
OSHA recommends that air sampling
for anesthetic gases be conducted every 6 months to measure worker exposures
and to check the effectiveness of control measures. Furthermore, OSHA
recommends that only the agent(s) most frequently used needs to be monitored,
since proper engineering controls, work practices and control procedures
should reduce all agents proportionately. However, the decision to monitor
only selected agents could depend not only on the frequency of their use, but
on the availability of an appropriate analytical method and the cost of
instrumentation. [ASA emphasizes regular maintenance of equipment and
scavenging systems, daily check-out procedures for anesthesia equipment, and
education to ensure use of appropriate work practices. It does not believe
that a routine monitoring program is necessary when these actions are being
carried out. ASA prefers to use monitoring when indicated such as in the
event of known or suspected equipment malfunction. The Academy of General
Dentistry also emphasizes properly installed and maintained analgesia
delivery systems.]
Three fundamental types of air
samples can be taken in order to evaluate the workplace: personal, area, and
source samples. Personal samples give the best estimate of a worker’s
exposure level since they represent the actual airborne contaminant
concentration in the worker’s breathing zone during the sampling period. This
is the preferred method for determining a worker’s time-weighted average
(TWA) exposure and should be used to assess personal exposures during anesthetic
administration and in the PACU. Where several health-care workers perform the
same job, on the same shift, and in the same work area, and the length,
duration, and level of waste gas exposures are similar, an employer may
sample a representative fraction of the employees instead of all employees.
Area sampling is useful for
evaluating overall air contaminant levels in a work area and for
investigating cross-contamination with other areas in the health-care
facility. Source sampling can be used to detect leaks in the anesthesia
delivery and scavenging systems as well as ineffective capture by the
scavenging system. Thus, how samples are taken is a critical point in any
safety program.
The OSHA Chemical Information
Manual contains current sampling technology for several of the anesthetic
gases that may be present in anesthetizing locations and PACUs. Some of the
sampling methods available are summarized below.
1.
Time-Integrated
Sampling
a.
Nitrous
Oxide
Personal N2O exposures can be determined by
using the VAPOR-TRAK nitrous oxide passive monitor (sometimes called
a"passive dosimeter" or"diffusive sampler") as referenced
in the 2000 OSHA Chemical Information Manual under IMIS:1953. The
minimum sampling duration for the dosimeter is 15 minutes; however, it can be
used for up to 16 hours of passive sampling. This sampler has not been
validated by OSHA. Other dosimeters are commercially available and can be
used. Although not validated by OSHA at this time, they may be validated in
the future. Five liter, 5-layer aluminized gas sampling bags can also be used
to collect a sample.
b.
Halogenated
Agents
Three chlorofluorocarbon-based anesthetic
agents (halothane, enflurane, and isoflurane) and one fluorocarbon-based
agent (desflurane) are listed in the Chemical Information Manual. The
OSHA sampling procedure for halothane is listed under IMIS:0395; for
enflurane, under IMIS:1038; for isoflurane, under IMIS:F118; and for
desflurane, under IMIS:R218.
The current recommended media sampling for
halothane, enflurane, and isoflurane requires an Anasorb 747 tube (140/70 mg
sections) or an Anasorb CMS tube (150/75 mg. sections). The sample can be
taken at a flow rate of 0.5 L/min. Total sample volumes not exceeding 12
liters are recommended. The current recommended sampling media for desflurane
requires an Anasorb 747 tube (140/70 mg sections). The sample can be taken at
a flow rate of 0.05 L/min. Total sample volumes not exceeding 3 liters are
recommended. All four sampling methodologies are fully validated analytical
procedures.
2.
Real-Time
Sampling
Sampling that provides direct, immediate, and
continuous (real-time) readout of anesthetic gas concentrations in ambient
air utilizes a portable infrared spectrophotometer. Since this method
provides continuous sampling and instantaneous feedback, sources of
anesthetic gas leakage and effectiveness of control measures can be
immediately determined.
3.
Additional
Sampling Guidelines
If it should ever be necessary to enter an
operating room to conduct air sampling, the following guidelines provide the
information needed. Individuals performing air sampling should be familiar
with and follow all OR procedures for access into and out of the surgical
suite with particular attention to sterile and nonsterile areas. The patient
is the center of the sterile field, which includes the areas of the patient,
operating table, and furniture covered with sterile drapes and the personnel
wearing sterile attire. Sampling in the breathing zone of surgeons and other
nursing or technical personnel who work in the sterile field must conform to
the principles of sterile field access. Strict adherence to sound principles
of sterile technique and recommended practices is mandatory for the safety of
the patient.
Generally speaking, each hospital has its own
guidelines for proper OR attire and other safety procedures. These rules
should be strictly followed by anyone entering the OR. There are standard
uniform guidelines that apply to all hospitals. Only clean and/or freshly
laundered OR attire is worn in the OR. Proper attire consists of body covers
such as a two-piece pantsuit (scrub suit), head cover (cap or hood), mask,
and shoe covers. A sterile gown is worn over the scrub suit to permit the
wearer to come within the sterile field. Other attire such as gloves and
eyewear may be required. Some hospitals, but not all, may allow persons
coming into the OR to wear a clean gown (in addition to the cap, the mask,
and the shoe covers) over their street clothes if they are not going to
remain in the OR for longer than 10-15 minutes.
In regard to decontaminating outside
equipment, each hospital has its own policy. However, the common practice is
to "wipe off" all surfaces with a chemical disinfectant. Most
hospitals use Wescodyne or other phenolic solutions. Good physical cleaning before
disinfection helps reduce the number of microorganisms present and enhances
biocidal action.
Any person not familiar with the OR is
usually instructed by a scrub nurse on all the safety procedures pertaining
to the hospital. The scrub nurse will also provide instructions on hand
scrubbing and other procedures that may be necessary. Persons entering the OR
must follow these guidelines and instructions.
In addition, it should be recognized that the
patient’s welfare, safety, and rights of privacy are paramount.
In all locations where anesthesia
is administered, engineering controls such as a scavenging system to remove
waste anesthetic gases and adequate room ventilation should be utilized.
Medical surveillance of personnel working in scavenged operating rooms is
intended primarily to establish a baseline. Routine annual follow-up is
primarily educational and at minimum, might consist of a health
questionnaire. Examinations and laboratory testing should be available for
conditions suspected of being related to occupational exposure. A sample
program might include:
·
A
preplacement medical questionnaire that includes a detailed work history
(including past exposures to waste anesthetic gases); a medical history with
emphasis on: hepatic (liver), renal (kidney), neurological (nervous system),
cardiovascular (heart and circulation), and reproductive functions. Pertinent
positive response(s) to the questionnaire should be followed by an
appropriate medical evaluation (i.e., in-depth history and physical
examination where appropriate) and, where relevant, suitable laboratory
tests, such as liver function tests.
·
An
annual questionnaire emphasizing the issues mentioned above. Again, the need
for physical examination or laboratory work may be based on questionnaire
responses.
·
A
system should be created for employees to report health problems which they
believe may be associated with anesthetic exposure. Employees should be
informed of this reporting system and of the method by which reports can be
submitted.
·
An
acute exposure ( i.e., a sudden, high-level exposure) should be documented.
Any subsequent health effects should trigger a medical history, and a
physical examination (where appropriate).
·
A
reproductive hazards policy should also be in place at the facility and
should address worker exposure and reproductive health effects in male and
female employees. The facility should provide training in the known and
potential adverse health effects, including reproductive effects, of waste
anesthetic gases, as is required for chemicals covered by the Hazard
Communication Standard.
·
A
final medical review upon job transfer or termination. This should be in the
form of a questionnaire that includes any acute or significant exposures as
well as a review of symptoms and signs detected during employment, along with
a medical evaluation when appropriate.
·
Medical
and exposure records developed for employees who may be exposed to hazardous
chemicals such as N2O and halogenated anesthetic agents must be retained, made available,
and transferred in accordance with OSHA Standard for Access to Employee
Exposure and Medical Records (29 CFR 1910.1020). The occurrence of injury or
illness related to occupational exposure must be recorded in accordance with
OSHA recordkeeping regulations (29 CFR 1904).
In accordance with the Hazard
Communication Standard (29 CFR 1910.1200), employers in health-care
facilities must develop, implement, and maintain at the workplace a written,
comprehensive hazard communication program that includes provisions for
container labeling, collection and availability of material safety data
sheets (MSDSs), and an employee training and information program. The
standard also requires a list of hazardous chemicals in the workplace as part
of the written hazard communication program.
Any chemicals subject to the
labeling requirements of the FDA are exempt from the labeling requirements
under the Hazard Communication Standard. This includes such chemicals as
volatile liquid anesthetics and compressed medical gases. However, containers
of other chemicals not under the jurisdiction of the FDA must be labeled,
tagged, or marked with the identity of the material and must show appropriate
hazard warnings as well as the name and address of the chemical manufacturer,
importer, or other responsible party. The hazard warning can be any type of
message --words, pictures, or symbols-- that conveys the hazards of the
chemical(s) in the container. Labels must be legible, in English (plus other
languages if desired), and prominently displayed.
Each MSDS must be in English,
although the employer may maintain copies in other languages as well, and
must include information regarding the specific chemical identity of the
anesthetic gases or hazardous chemical and its common names. In addition,
information must be provided on the physical and chemical characteristics of
the hazardous chemical, known acute and chronic health effects and related
health information, primary route(s) of entry, exposure limits, precautionary
measures, emergency and first-aid procedures, and the identification of the
organization responsible for preparing the sheet. As a source of detailed
information on hazards, copies of the MSDS for each hazardous chemical must
be readily accessible during each work shift to employees when they are in
their work area(s).
Employers must prepare a list of
all hazardous chemicals in the workplace, and the list should be checked to
verify that MSDSs have been received for each chemical. If there are
hazardous chemicals used for which no MSDS has been received, the employer
must contact the supplier, manufacturer, or importer to obtain the missing
MSDS.
Health-care employers must
establish a training and information program for all personnel who are
involved in the handling of, or who have potential exposure to, anesthetic
gases and other hazardous chemicals to apprise them of the hazards associated
with these chemicals in the workplace. Training relative to anesthetic gases
should place an emphasis on reproductive risks. Training and information must
take place at the time of initial assignment and whenever a new hazard is
introduced into the work area. At a minimum, employees must be informed of
the following:
·
The
Hazard Communication Standard (29 CFR 1910.1200) and its requirements.
·
Any
operations and equipment in the work area where anesthetic agents and hazardous
chemicals are present.
·
Location
and availability of the written hazard communication program including the
required lists of hazardous chemicals and the required MSDS forms.
The employee training program must
consist of the following elements:
·
How
the hazard communication program is implemented in the workplace, how to read
and interpret information on the MSDS and label of each hazardous chemical,
and how employees can obtain and use the available hazard information.
·
The
physical and health hazards of the chemicals in the work area.
·
Measures
employees can take to protect themselves from these hazards, including
specific procedures put into effect by the employer to provide protection
such as engineering controls, appropriate work practices, emergency procedures
for spill containment, and the use of personal protective equipment.
·
Methods
and observations that may be used to detect the presence or release of
anesthetic gases and other hazardous chemicals in the work area (such as
monitoring conducted by the employer, continuous monitoring devices, and the
appearance or odor of chemicals when released).
Personnel training records are not
required to be maintained, but such records would assist employers in
monitoring their programs to ensure that all employees are appropriately
trained. Employers can provide employees information and training through
whatever means are found appropriate and protective. Although there would
always have to be some training on-site (such as informing employees of the
location and availability of the written program and MSDSs), employee
training may be satisfied in part by general training about the requirements
of the hazard communication standard and about chemical hazards on the job
which is provided by, for example, professional associations, colleges,
universities, and training centers. In addition, previous training,
education, and experience of a worker may relieve the employer of some of the
burdens of informing and training that worker. The employer, however,
maintains the responsibility to ensure that their employees are adequately
trained and are equipped with the knowledge and information to do their jobs
safely.
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American Conference of
Governmental Industrial Hygienists (ACGIH) is an organization devoted to the
development of administrative and technical aspects of worker health
protection. The ACGIH is a professional organization, not a government
agency.
ACGIH threshold limit
value-time-weighted average (TLV-TWA) refers to the time-weighted average airborne
concentration of a substance, for a normal 8-hour workday and a 40-hour
workweek, to which nearly all workers may be repeatedly exposed, day after
day, without adverse effect.
Adapters are fittings used to establish
functional continuity between otherwise disparate and incompatible
components.
Adjustable Pressure-Limiting (APL)
Valve, also known
as a "pop-off" valve, is a user-adjustable valve that releases
gases to the atmosphere or a scavenging system and is intended to provide
control of the pressure in the breathing system. The volume of gas above that
needed to achieve the required patient pressure is vented.
Air is the elastic, invisible mixture of gases
(chiefly nitrogen and oxygen) that may be used with medical equipment; also
called medical air.
Anesthesia machine is equipment intended for
dispensing and delivering anesthetic gases and vapors into a breathing
system.
Anesthesia system is any of a variety of assemblies
designed to administer an anesthetic.
Anesthetic agent is a drug that is used to reduce
or abolish the sensation of pain, e.g., halothane, enflurane, isoflurane,
desflurane, sevoflurane, and methoxyflurane.
Anesthetic agent vapor is the gaseous phase of an
anesthetic agent that is normally a liquid at room temperature and
atmospheric pressure.
Anesthetic gas is any gaseous substance, e.g.,
nitrous oxide, used in producing a state of anesthesia.
Anesthetic vaporizer is a device designed to
facilitate the change of an anesthetic from a liquid to a vapor.
Anesthetizing location is any area in a facility where
an anesthetic agent or drug is administered in the course of examination or
treatment. This includes operating rooms, delivery rooms, emergency rooms,
induction rooms, and other areas.
Area sample is a sample collected at a fixed
point in the workplace. The data from the area sample may or may not
correlate with an individual’s personal sample results due to the often high
degree of variability in exposures.
Breathing system is a gas pathway in direct
connection with the patient's lungs, through which gas flow occurs at
respiratory pressures, and into which a gas mixture of controlled composition
may be dispensed. The function of the breathing system is to convey oxygen
and anesthetic gases to the patient's lungs and remove waste and anesthetic
gases from the patient's lungs. Scavenging equipment is not considered part
of the breathing system. The system is also referred to as breathing or
patient circuit, respiratory circuit or system.
Breathing system, semiclosed is a system that allows some of
the expired gases to leave the circuit; the remainder mixes with the fresh
gases and is reinhaled. A CO2 absorber is used in this system.
Breathing tubes are large-bore, nonrigid tubes
composed of rubber or plastic and used in most breathing systems to convey
gases to and from the patient's airway. They are usually corrugated to
prevent obstruction due to kinking and increase flexibility.
Breathing zone is defined as the area
immediately adjacent to the employee’s nose and mouth; a hemisphere forward
of the worker’s shoulders with a radius of approximately 6 to 9 inches.
Calibrated vaporizer is an instrument designed to
facilitate the change of a liquid anesthetic into its vapor and to add a
controlled amount of this vapor to the fresh gas flow.
Carbon dioxide (CO2) is a colorless, odorless gas, and is a
normal end product of human metabolism. It is formed in the tissues and
eliminated by the lungs.
Carbon dioxide absorber is a device used to remove CO2 chemically from exhaled patient
gas. Primarily used in the closed or semiclosed circle breathing system,
which requires carbon dioxide absorption to make reinhalation of previously
exhaled gas possible.
Carcinogenicity is the ability of a substance to
cause cancer.
Check valves are also known as unidirectional
valves, one-way valves, and inspiratory and expiratory valves (refer to
definition of unidirectional valve).
Common (fresh) gas outlet is the port through which the
mixture of gases and vapors dispensed from the anesthesia machine is
delivered to the breathing system. Also referred to as the machine outlet.
Compressed gas is defined as any material or
mixture having in the container an absolute pressure exceeding 40 psig at
70°F or having an absolute pressure exceeding 104 psig at 130°F.
Congenital anomaly is a structural or functional
abnormality of the human body that develops before birth but is not
inherited. One type of birth defect.
Connectors are fittings intended to join
together two or more components.
Cylinder supply source is a cylindrical-shaped tank that
is color-coded and pin-indexed or Compressed Gas Association (CGA)
valve-specific and used to contain a specified medical gas. It supplies
compressed gas to the anesthesia machine if a pipeline supply source is not
available or if the pipeline fails. Cylinders range in size from B (smallest)
to H (largest).
Cylinder pressure gauge monitors the pressure of gas
within a cylinder.
Diameter Index Safety System
(DISS) provides
threaded noninterchangeable (gas-specific) connections for medical gas lines
at pressures of 200 psig or less to minimize the risk of misconnection.
Embryolethal refers to a substance that is
lethal to the developing embryo, the product of conception up to the end of
the eighth week of human pregnancy.
Epidemiology is the study of health and
illness in human populations. It is the study of trends and events in similar
populations, for example, one exposed to a chemical and one not exposed.
Excess gases are those gases and anesthetic
vapors that are delivered to the breathing circuit in excess of the patient’s
requirements and the breathing circuit’s capacity. These gases are released
from the breathing circuit via the APL or pop-off valve or the ventilator
pressure relief valve and are ultimately removed from the breathing circuit
by the waste gas scavenging system.
Exhalation check valve, also known as expiratory unidirectional
valve, refers to that valve placed in the vicinity of the CO2 absorber that ensures that
exhaled gases flow away from the patient and into the absorber.
Flow control valve, also known as the needle valve,
controls the rate of flow of a gas through its associated flow meter by
manual adjustment of a variable orifice.
Flowmeter is a device that measures and
indicates the flow rate of a gas passing through it.
Gas is defined as a formless fluid that expands
readily to fill any containing vessel, and which can be changed to the liquid
or solid state only by the combined effect of increased pressure and
decreased temperature.
Gas-tight seal is a connection that does not
allow bubbling when immersed in water and subjected to a differential
pressure.
General anesthesia is a state of unconsciousness in
which there is an absence of pain sensation.
Hanger yoke is a device used to attach a
reserve gas cylinder to the anesthesia machine. The functions of the hanger
yoke are to orient and support the cylinder, provide a gas-tight seal, and
ensure a unidirectional flow of gas into the machine. It is pin-indexed
according to a gas-specific safety system in order to prevent the connection
of a cylinder of one gas to a yoke intended for another.
HVAC system, also known as the heating,
ventilating, and air conditioning system, supplies outdoor replacement
(make-up) air and environmental control to a space or building. It conditions
the air by supplying the required degree of air cleanliness, temperature
and/or humidity.
Inhalation check valve, also called inspiratory
unidirectional valve, refers to the valve placed in the vicinity of the CO2 absorber that ensures that the
gases flow toward the patient.
In vitro describes studies that are done
in the laboratory, literally"in glass," using, for example, cells,
as distinct from studies performed using whole living animals.
Medical gas is any gaseous substance that
meets medical purity standards and has application in a medical environment.
Examples are oxygen, nitrous oxide, helium, air, nitrogen, and carbon
dioxide.
Medical gas mixture is a mixture of two or more
medical gases to be used for a specific medical application.
Mutagenicity is the ability of a substance to
cause changes in the genetic material.
NIOSH RELs (recommended exposure
limits) are
occupational exposure limits recommended by NIOSH as being protective of
worker health and safety over a working lifetime. These limits are generally
expressed as 8- or 10-hour TWAs for a 40-hour workweek. The REL may also be
expressed as a short-term (TWA) exposure limit or a ceiling limit.
Nitrous oxide (N2O) is used as an anesthetic agent in medical,
dental, and veterinary operatories. It is a weak anesthetic with rapid onset
and rapid emergence. In hospitals, it may be used with oxygen as a carrier
gas for other, more potent anesthetics. In dental offices, it is administered
with oxygen, primarily as an analgesic (an agent that diminishes or
eliminates pain in the conscious patient) and as a sedative to reduce
anxiety.
Nonrecirculating ventilation
system takes in
fresh outside air and processes it by filtering and adjusting the humidity
and temperature. The processed air is circulated through the various rooms in
a facility, and then all of it is exhausted to the atmosphere. Whatever
volume of fresh air is introduced into a room is ultimately exhausted
outdoors.
Occupational exposure to waste anesthetic gases
includes exposure to any inhalation anesthetic agents that escape into
locations associated with, and adjacent to, anesthetic procedures. Such
locations include, but are not limited to, operating rooms, delivery rooms,
recovery rooms, and dental operatories.
Oxygen (O2) is an element which, at atmospheric
temperatures and pressures, exists as a colorless, odorless, tasteless gas.
Its utstanding properties are its ability to sustain life and to support
combustion. Although oxygen is nonflammable, materials which burn in air will
burn much more vigorously and create higher temperatures in oxygen or
oxygen-enriched atmospheres.
Oxygen flush valve is a separate valve designed to
rapidly supply a large volume of oxygen to the breathing system.
PACU (postanesthesia care unit) is also known as the recovery
room.
Patient end is the end of the component part
nearest the patient.
PEEP valve is a device installed in the
exhalation limb of the patient circuit that allows positive end-expiratory
pressure to be delivered to the patient's airway and adjusted as needed.
Personal sample is a sample collected from an
individual’s breathing zone.
Pin Index Safety System is a safeguard to eliminate
cylinder interchanging and the possibility of accidentally placing the
incorrect gas on a yoke designed to accommodate another gas. Two pins on the
yoke are so arranged that they project into the cylinder valve. Each gas or
combination of gases has a specific pin arrangement.
Pipeline supply source is a permanently installed piped
distribution system that delivers medical gases such as oxygen, nitrous
oxide, and air to the operating room.
Pneumatic means pertaining to or operated
by air or other gas under pressure.
Power outlet is an accessory outlet located on
an anesthesia machine that supplies a driving gas for auxiliary equipment
such as a ventilator. Driving gas is normally oxygen, but medical air may be
used.
Pressure relief valve is a mechanical device that
eliminates system overpressure by allowing the controlled or emergency escape
of liquid or gas from a pressurized system. The relief valve may or may not
be adjustable.
Prospective study or cohort study follows a
population from a set time into the future. It is an epidemiological method
for identifying the future relationship, if any, between exposure to an agent
and the increased incidence of some adverse health effect in a population.
PSIG stands for pounds per square inch gauge,
which is the difference between the measured pressure and surrounding
atmospheric pressure. Most gauges are constructed to read 0 at atmospheric
pressure.
Recirculating ventilation system returns part of the exhaust air to
the air supply duct. The system takes in an amount of fresh outside air that
varies as a function of the outside temperature. Air exhausted from a room is
filtered for particulate matter and bacteria, not anesthetic gases, and then
recirculated through several rooms by means of a common mixing (plenum)
chamber. In this process, some fresh air is added and a equal amount of
recirculating air is exhausted.
Recovery room is the patient care location
where recovering patients are awakened and stabilized and/or awakened after
surgical anesthesia. Anesthetic gases are exhaled by recovering patients (who
received inhalation anesthetics) as they breathe.
Reservoir bag is also known as the respiratory
bag or breathing bag. It allows accumulation of gas during exhalation so that
a reservoir is available for the next inspiration. It provides a means
whereby anesthesia personnel may assist or control ventilation. It can serve,
through visual and tactile observation, as a monitor of a patient’s
spontaneous respirations and acts to protect the patient from excessive
pressure in the breathing system.
Respiration is the process by which a rapid
exchange of oxygen and carbon dioxide takes place between the atmosphere and
the blood coming to the pulmonary capillaries. Oxygen is taken up, utilized
in metabolic processes, and a proportional amount of carbon dioxide is
released.
Retrospective study or case control study examines
two populations. The first population consists of individuals who demonstrate
the effect of interest, and the second is made up of those who do not. The
two populations are matched as well as possible with respect to all other
variables, e.g., age, socioeconomic status, and so on. Then the past
histories of exposure of the two populations are investigated to determine if
some differences can be identified that might be related to the toxic effects
observed.
Scavenging is defined as the collection of
excess gases from the breathing circuit and removal of these gases to an
appropriate place of discharge outside the working environment.
Scavenging system is defined as a device (assembly
of specific components) that collects and removes the excess anesthetic gases
that are released from the breathing circuit. Scavenging systems are also
called evacuation systems, waste anesthetic gas disposal systems, and excess
anesthetic gas-scavenging systems.
Source-control technology is an engineering control
designed to collect and remove excess anesthetic gases at the point of origin
(i.e., from the breathing circuit or in close proximity to the patient’s
mouth and nose). It can be either a scavenging system or local (auxiliary)
exhaust ventilation system.
Source sample is a sample collected at the
origin of contamination (source of emission).
Teratogenicity is the ability of a substance to
cause birth defects in offspring, as a result of maternal (before or after
conception) or paternal exposure to the toxic substance.
Tracheal tube also called the endotracheal
tube, intratracheal tube, and catheter is inserted into the trachea and is
used to conduct gases and vapors to and from the lungs.
TWA is a time-weighted average concentration. It
is a way of expressing exposure such that the amount of time spent exposed to
each different concentration level is weighted by the amount of time the
worker was exposed to that level.
Unidirectional valve is a valve that allows gas flow
in one direction only. Two unidirectional valves are used in each circle
system to ensure that the gases flow toward the patient in one limb of the
circle breathing system and away in the other. They are usually part of the
absorber assembly.
Vapor is the gaseous phase of a substance which at
ordinary temperature and pressure exists as a liquid.
Ventilation is (1) the physical process of
moving gases into and out of the lungs. (2) It is also defined for the
purposes of industrial hygiene engineering as a method for providing control
of an environment by strategic use of airflow. The flow of air may be used to
provide either heating or cooling of a work space, to remove a contaminant
near its source of release into the environment, to dilute the concentration
of a contaminant to acceptable levels, or to replace air exhausted from a
space.
Waste anesthetic gases are those gases that are
inadvertently released into the workplace and/or can no longer be used. They
include all fugitive anesthetic gases and vapors that are released into
anesthetizing and recovery locations, from equipment used in administering
anesthetics under normal operating conditions, as well as those gases that
leak from the anesthetic gas scavenging system, or are exhaled by the patient
into the workplace environment. Waste gases are also those excess gases in
the breathing circuit that are ultimately scavenged. Spills of liquid
anesthetic agents also contribute to ambient levels of waste gases. Waste
anesthetic gases may include N2O and vapors of potent inhaled volatile
anesthetic agents such as halothane, enflurane, isoflurane, desflurane and
sevoflurane.
This checkout, or a reasonable
equivalent, should be conducted before administration of anesthesia. These
recommendations are only valid for an anesthesia system that conforms to
current and relevant standards and includes an ascending bellows ventilator
and at least the following monitors: capnograph, pulse oximeter, oxygen
analyzer, respiratory volume monitor (spirometer) and breathing system
pressure monitor with high and low pressure alarms. This is a guideline which
users are encouraged to modify to accommodate differences in equipment design
and variations in local clinical practice. Such local modifications should
have appropriate peer review. Users should refer to the operator’s manual for
the manufacturer’s specific procedures and precautions, especially the
manufacturer’s low pressure leak test (step #5).
Note: *If an anesthesia provider uses the same
machine in successive cases, these steps need not be repeated or may be
abbreviated after the initial checkout.
Emergency Ventilation Equipment
*1. Verify Backup Ventilation
Equipment is Available & Functioning
High-Pressure System
*2. Check Oxygen Cylinder Supply
a. Open O2
cylinder and verify at least half full (about 1000 psi).
b. Close cylinder.
*3. Check Central Pipeline
Supplies
a. Check that hoses are connected and pipeline gauges read about 50 psi.
Low-Pressure System
*4. Check Initial Status of
Low-Pressure System
a. Close flow control valves and turn vaporizers off.
b. Check fill level and tighten vaporizer’s filler caps.
*5. Perform Leak Check of Machine
Low-Pressure System
a. Verify that the machine master switch and flow control valves are OFF.
b. Attach"Suction Bulb" to common (fresh) gas outlet.
c. Squeeze bulb repeatedly until fully collapsed.
d. Verify bulb stays fully collapsed for at least 10 seconds.
e. Open one vaporizer at a time and repeat"c" and"d"
as above.
f.
Remove suction bulb, and reconnect fresh gas
hose.
* 6. Turn On Machine Master Switch and all other necessary
equipment.
* 7. Test Flowmeters
a. Adjust flow of all gases through their full range, checking for smooth
operation of floats and undamaged flowtubes.
b. Attempt to create a hypoxic O2/N2O
mixture and verify correct changes in flow and/or alarm.
Scavenging System
* 8. Adjust and Check Scavenging
System
a. Ensure proper connections between the scavenging system and both APL
(pop-off) valve and ventilator relief valve.
b. Adjust waste gas vacuum (if possible).
c. Fully open APL valve and occlude Y-piece.
d. With minimum O2 flow,
allow scavenger reservoir bag to collapse completely and verify that absorber
pressure gauge reads about zero.
e. With the O2 flush
activated, allow the scavenger reservoir bag to distend fully, and then
verify that absorber pressure gauge reads <10 cm H2O.
Breathing System
* 9. Calibrate O2 Monitor
a. Ensure monitor reads 21% in room air.
b. Verify low O2 alarm
is enabled and functioning.
c. Reinstall sensor in circuit and flush breathing system with O2.
d. Verify that monitor now reads greater than 90%.
10. Check Initial Status of
Breathing System
a. Set selector switch to"Bag" mode.
b. Check that breathing circuit is complete, undamaged and unobstructed.
c. Verify that CO2
absorbent is adequate.
d. Install breathing circuit accessory equipment (e.g., humidifier, PEEP
valve) to be used during the case.
11. Perform Leak Check of the
Breathing System.
a. Set all gas flows to zero (or minimum).
b. Close APL (pop-off) valve and occlude Y-piece.
c. Pressurize breathing system to about 30 cm H2O with O2 flush.
d. Ensure that pressure remains fixed for at least 10 seconds.
e. Open APL (pop-off) valve and ensure that pressure decreases.
Manual and Automatic Ventilation
Systems
12. Test Ventilation Systems and
Unidirectional Valves
a. Place a second breathing bag on Y-piece.
b. Set appropriate ventilator parameters for next patient.
c. Switch to automatic ventilation (Ventilator) mode.
d. Fill bellows and breathing bag with O2 flush and then turn ventilator ON.
e. Set O2 flow to
minimum, other gas flows to zero.
f.
Verify that during inspiration bellows
delivers appropriate tidal volume and that during expiration bellows fills
completely.
g. Set fresh gas flow to about 5 L/min.
h. Verify that the ventilator bellows and simulated lungs fill and
empty appropriately without sustained pressure at end expiration.
i.
Check for proper action of
unidirectional valves.
j.
Exercise breathing circuit accessories to
ensure proper function.
k. Turn ventilator OFF and switch to manual ventilation (Bag/APL) mode.
l.
Ventilate manually and assure inflation and
deflation of artificial lungs and appropriate feel of system resistance and
compliance.
m. Remove second breathing bag from Y-piece.
Monitors
13. Check, Calibrate and/or Set
Alarm Limits of all Monitors
Capnometer
Oxygen Analyzer
Pressure Monitor with High and Low Airway Alarms
Pulse Oximeter
Respiratory Volume Monitor (Spirometer)
Final Position
14. Check Final Status of Machine
a. Vaporizers off
b. APL valve open
c. Selector switch to"Bag"
d. All flowmeters to zero
e. Patient suction level adequate
f.
Breathing system ready to use
The interface serves to prevent
potentially dangerous increases or decreases of pressure in the anesthetic
waste gas disposal system from reaching the patient’s breathing circuit. In
order to do this, the interface has three components: positive pressure
relief, negative pressure relief, and a reservoir.
Irrespective of the type of
disposal system used (i.e., active or passive), positive pressure relief must
be provided to protect the equipment and patient if occlusion of the scavenging
system outlet occurs. If the scavenging system outlet becomes occluded, the
positive-pressure relief vent opens to prevent transmission of high pressure
to the breathing circuit. If an active disposal system is used, negative
pressure relief is needed to prevent negative (suction) pressure from the
disposal system from reaching the patient’s breathing circuit. A reservoir is
necessary to allow the scavenging system to accommodate an increased volume
of excess anesthetic gas which may transiently exceed the per-minute removal
capacity of the system. It may also serve as a monitor of the scavenging
system if the reservoir is a distensible bag. Overdistension of the bag could
indicate inadequate function of the system and the need to adjust the needle
valve to allow more gas to flow through.
Interfaces can be divided into two
types: open and closed, depending on the means to provide positive and
negative pressure relief. An open reservoir interface is one that is always
open to atmosphere and contains no valves. It relies on open ports for
positive and negative pressure relief. A closed interface uses
"spring-loaded or weighted" valves for positive and negative
pressure relief.
The open reservoir interface
(Figure 9) should be used only with an active disposal system. Because the
discharge of waste gases from the breathing system is usually intermittent
and flow through an active disposal assembly is continuous, a reservoir is
needed to accommodate the surges of gas that enter the interface at a flow
rate greater than that at which the disposal system removes them. The
reservoir allows the flow rate in the disposal system to be kept just above
the average, rather than at the peak flow rate of gases from the
gas-collecting assembly.
A closed interface is one in which
the connection(s) with the atmosphere is(are) through valve(s). A positive
pressure relief is always required to allow release of gases into the room if
there is an obstruction of the scavenging system downstream of the interface.
If an active disposal system is to be used, a negative pressure relief valve
is necessary to allow entrainment of room air when the pressure falls below
atmospheric.
Figure 9. Open reservoir scavenging
interface. Reproduced by permission of North American Dräger, Telford,
Pennsylvania).
The interface typically consists
of a manifold with four ports and two relief valves (Azar and Eisenkraft 1993;
Dorsch and Dorsch 1994). Figure 10 shows the flow of waste gases from the
breathing circuit as it enters the intake ports of the interface. This figure
shows the pathway of gas flow in an active scavenging system that uses
a facility’s vacuum source (wall suction) for gas disposal (Huffman 1991).
As gas is drawn through the
suction nipple, located on the right of the drawing in Figure 10, it flows
through the manifold and past the two relief valves. The upper relief valve
limits positive pressure, and the lower valve limits negative pressure. A
3-liter bag is shown attached in the diagram and serves as the waste gas
reservoir. When more flow is passing into the manifold than the vacuum can
remove, waste gas is temporarily stored in the reservoir bag.
The rate of gas flow through the
interface is controlled by adjusting the needle valve in such a way that the
reservoir bag is not allowed to become filled. In the ideal situation, this
rate of flow should maintain the volume in the reservoir bag between empty
and half-filled. Adjusting the needle valve alters the flow of waste gases
into the vacuum source. This adjustment does not regulate vacuum or suction.
If the flow is insufficient and the reservoir bag is allowed to distend, the
positive pressure relief valve will open and vent some of the exhaled gases
into the room. This situation is corrected simply by adjusting the needle
valve to increase the flow of waste gases to the vacuum. If the flow is too
great and the bag collapses, the negative pressure relief valve will open and
let in as much room air as needed.
The purpose of these valves is to
protect the breathing circuit from extremes of pressure. The positive
pressure relief valve will not be activated if the flow is properly adjusted
and the contour of the bag is observed to monitor its volume. In an active
scavenging system, any unused nipple must be capped or the vacuum will draw
in room air and also provide the opportunity for waste gases to diffuse into
the room.
Figure 10. The flow of waste gases through
the scavenging interface that is connected to a vacuum source. (Reproduced by
permission of Datex·Ohmeda, Madison, Wisconsin).
A passive scavenging system
for waste gas evacuation, shown in Figure 11, uses the facility’s ventilation
system instead of the vacuum system to dispose of waste gases. In this
configuration, flow of waste gases through the interface is basically the
same as in the active system. Gas pressure is limited by positive and
negative relief valves. Transfer of the waste gases from the interface to the
disposal system relies solely on the pressure of the waste gases since a
vacuum is not used.
In a passive system the adjustment
knob must remain in the down position to close the needle valve. As shown
below, a 19 mm corrugated hose is used to connect the interface with the
room’s ventilation exhaust grille (Azar and Eisenkraft 1993). A passive
system (unlike an active system) is not connected to a vacuum or source of
negative pressure and does not need to be adjusted regularly.
Figure 11. The flow of waste gases through
the interface in a passive scavenging system.
(Reproduced by permission of
Datex·Ohmeda, Madison, Wisconsin).