chapter 13. controlling trace gas levels

8/11/2019 Chapter 13. Controlling Trace Gas Levels

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Chapter 13

Controlling Trace Gas LevelsIntroduction

Before scavenging was inst ituted, excess anesthetic gases and vapors were

discharged into room air. As a consequence, operating room personnel were

exposed to low concentrat ions of these drugs with l i t t le c oncern about any

detrimental effects that could possibly result from such exposure. Questions have

been raised about possible hazards from exposure to trace amounts of anesthetic

gases and vapors (1). (For the remainder of this chapter, anesthetic gases and

vapors wil l be referred to as gases , because most vapors behave as gases.)

A trace lev el of an an es the tic gas is a conc entrat ion far below that needed for

clinical anesthesia or that can be detected by smell (2 ). Trace gas levels are

usually expressed in parts per mil l ion (ppm), which is volume/volume (100% of a

gas is 1,000,000 ppm; 1% is 10,000 ppm).

Reported trace gas concentrat ions in operating rooms vary greatly, depending on

the fresh gas f l ow, the venti lat ion system, the length of t ime that anesthesia has

been administered, the measurement site, anesthetic technique, and other

variables. Trace gas levels tend to be higher with pediatric anesthesia (3,4), in

dental operatories (1,5), and in poorly venti lated postanesthetic care units

(recovery rooms) (6,7 ,8,9,10).

Methods of Study

Despite many studies and much discussion, opinions dif fer o n whether or not a

problem exists and what levels should be allowed in the working environment ( 11).

To interpret the data, it is f irst necessary to understand how it was collected. Four

basic methods of study have been used. All have major l imitat ions and

disadvantages.

An im a l In v e s t i g a t i o n s

During animal studies, laboratory animals are exposed to v arying levels of gases

for varying periods of t ime and are studied to determine the effects.

These studies should be interpreted warily. Large numbers of animals need to be

studied to achieve s tat ist ical signif icance. In animals, diet affects tumor

susceptibi l i ty, and stress affects reproduction (12). Toxicity usually depends on

both exposure t ime and concentrat ion, and it is dif f icult to correlate exposure t ime

in animals with that in humans because their l i fe spans are so d if ferent. Finally,


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variat ions in drug effects among species create uncertainty about the relevance of

these f indings to h umans.

Hum an Vo l u n t e er S t u d i es

Human volunteers have been used to study the effects of trace gases on skil led

performance, immune responses, and drug metabolism.

Epidemiologic Studies of Exposed Humans

A number of ep id emio log ic stud ies of ex pose d pe rsonn el have be en pe rfo rmed.

Most were retrospective and used questionnaires. They suffer from low response

rates, inappropriate control groups, poor recollect ions and biases on the part of the

respondents, poor wording, fai lure to include signif icant points in the

questionnaires,

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and misinterpretat ions due t o dif ferences in education and experience on the part

of the respondents (12 ,13 ,14 ,15 ,16). Data interpretation is hampered by a lack of

agreement as to what level of s ignif icance to accept (12 ,17 ). Finally, many of the

studies have not b een designed to test the cause-and-effect relat ionship between

trace gases and problems in exposed personnel. Some studies show increased risk

for specif ic groups but not for other equally exposed groups (18 ). Others have

shown problems in groups with and without exposure to trace gases, suggesting

that the risk may be related to some other factor. Finally, many of the studies were

performed before scavenging and other methods to control trace gas levels were

implemented.

Mo r t a l i t y St u d i es

Studies on the causes of death and the age at which death occurred among

anesthesiologists have p rovided interest ing data. One s tudy found that

anesthesiologists do not have an increased risk of death from cancer or heart

disease but do have higher rates of death due to suicide, substance abuse, other

external causes and cerebrovascular disease than internists (19 ). There was also

an increased rate of death from HIV and v iral hepatit is in male anesthesiologists.

Another stu dy found th at wh ile anes th es iolog is ts hav e a s ign if ic an tly yo un ge r me an

age of death, there was no stat ist ical dif ference in age-specif ic mortality (20).

Problems Attributed to Trace Gases


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Sp o n t an e o u s A b o r t i o n

Epidemiologic StudiesEpidemiologic s tudies f rom several countries have shown higher rates of

spontaneous abortion in operating room and dental operatory personnel than in

women in dif ferent environments (21 ,22,23 ,24,25 ,26,27,28,29,30,31 ,32,33). Other

studies have failed to f ind signif icant increases in spontaneous abort ions in

exposed personnel (34,35,36,37 ,38 ,39,40). One study found that the f requency of

miscarriages among nurses working in intensive care units was approximately equal

to that of nurses in the operating room, suggesting that other factors such as stress

may play a role (27 ,41 ). A study of midwives who were often exposed to nitrous

oxide found that the risk of spontaneous abort ion was not increased with nitrous

oxide exposure but was increased with night work and high workload ( 42 ).

Animal Studies

Ha l o g e n at e d A g e n t s

Investigat ions have found no evidence of increased spontaneous abort ion in mice

exposed to high levels of isof lurane, enflurane, or halothane

(43,44,45,46 ,47 ,48,49 ).

N i t r o u s O x i d eOne study found that prolonged exposure to 1 ,000 ppm nitrous oxide caused fetal

death, but no effect was seen when 500 ppm was used (50). A later study found

that the threshold for fetal death was higher (between 1,000 and 5,000 ppm) with

intermittent exposure (51).

M i x t u r e s

Investigat ions us ing mixtures of halothane and nitrous oxide found no effect with

concentrat ions as high as 1,600 ppm halothane plus 100,000 ppm ni trous oxide

(49). Nitrous oxide 500,000 ppm plus isoflurane 3,500 ppm also had no effect on

spontaneous abortions (52 ).

Sp o n t an e o u s A b o r t i o n i n S p o u s es

Al though sev era l s tu dies hav e shown an in cre as ed spontaneous ab ort io n ra te in

wives of exposed males (23,53,54), the majority suggest that there is no increase

(16,21,25,40 ). One study found no changes in s perm concentrat ion or morphology

in male anesthesiologists working in health care facil i t ies with s cavenging

equipment (55 ). Studies have failed to show any adverse effect on reproductive


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processes of male animals exposed to up to 5,000 ppm enflurane (44,56 ) or 10 ppm

halothane plus 500 ppm nitrous oxide (57 ).

I n f e r t i l i t y

Epidemiologic Studies

Several studies have found higher-than-expected rates of involuntary infert i l i ty

among exposed personnel (24 ,53 ,58 ). A more recent study found no evidence that

female anesthetists have increased risk of infert i l i ty (59 ). One study found

decreased fert i l i ty in dental operatory s taffs who were exposed to considerably

higher levels of ni trous oxide than those who were operating room personnel (5).

One study found no effect from paternal exposure (25), and no changes in sperm

count or morphology were found in male anesthesiologists working in scavenged

operating rooms (55).

Animal Studies

Ha l o g e n at e d A g e n t s

Numerous s tudies of animals exposed to high concentrat ions of isof lurane,

enflurane, or halothane showed l i t t le or no effect on fert i l i ty (43 ,45,56,60,61,62 ,63 ).

N i t r o u s O x i d e

No changes in male fert i l i ty and no sperm abnormalities were found in mice after

exposure to up to 800,000 ppm (60,64 ). However, prolonged exposure of male rats

to 200,000 ppm resulted in abnormalit ies in spermatogenic cells (65). Exposure to

up to 800,000 ppm caused no changes in fert i l i ty i n male or female f l ies (61 ).

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M i x t u r e s

Decreased fert i l i ty was seen in female rats exposed to halothane 10 ppm plus

nitrous oxide 500 ppm before mating (57). Male rats exposed to these

concentrations showed greater frequency of chromosomal aberrations in

spermatogenic cells, but the aberrations were probably too infrequent to cause

decreased fert i l i ty (61 ).

B i r t h Def e c t s



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Several studies in humans h ave found an increase in congenital abnormalit ies in

children of exposed personnel (21,23 ,24 ,28,34,37 ,53 ,54,66 ,67 ). Interpretat ions of

the data have been questioned (12,17,18 ,68 ). Several investigators have found no

increase in birth defects among the offspring of exposed parents

(16,25,26,27 ,29 ,36,40 ). No increase in chromosomal abnormalit ies in exposed

nurses or changes in sperm morphology have been found in male anesthesiologists

working in operating rooms (55,69).

Animal Studies

Studies of laboratory animals exposed to concentrations of inhalat ional agents well

above those found in even unscavenged operating rooms have failed to f ind any

signif icant teratogenic effects (44 ,45,47 ,48 ,49,50 ,52 ,57,60,64,70,71,72 ).

Im pa i r e d Per f o rm ance

Operating room personnel are subjected to many s t imuli that require precise, rapid,

and complicated responses. Because the patient 's survival depends on the

alertness and performance of the professional team, anything that interferes with

its abil i ty to perceive changes and react quickly and appropriately may result in

harm to a patient.

Al though a few s tu die s hav e fou nd tha t volun tee r exposure to tr ace gas

concentrat ions caused signif icant decreases in performance (73 ,74 ,75,76 ), efforts

to duplicate these results have failed (41 ,77,78,79,80,81,82 ,83,84,85 ,86 ). These

studies found that the concentrations needed to decrease performance were

hundreds of times greater than the average levels found in unscavenged operating

rooms. In another study, neuropsychological symptoms and tiredness were reported

more by individuals in operating rooms where scavenging was used less often (87 ).

One study determined that personnel exposed to 51 to 54 ppm of nitrous oxide had

slowed reaction times compared with workers not exposed to trace gases ( 88 ).

Cance rEpidemiologic Studies

A la rge s tud y foun d no inc re ase in cancer in ex pos ed ma le s but indicate d that

females in the operating room were at higher risk for cancer than nonexposed

females (21). The signif icance of these data has been questioned (17,18 ). Similar

results have been reported for female dental operatory assistants (23). Two studies

of dentists have shown that the incidence of c ancer is not signif icantly dif ferent

among those exposed and those not exposed to trace concentrations of anesthetics


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(23,54). One study found a higher rate of melanoma among anesthesiologists (89).

A re v ie w of comb ine d data f ro m six stud ies found an inc reased can cer risk among

women but not men (41,90 ).

Mortality Studies

There is no increased death rate f rom cancer in male anesthesiologists

(19,91,92,93 ,94 ,95,96 ). The death rate from cancer among female

anesthesiologists is higher when compared with male anesthesiologists and control

groups (93), but the numbers are too small to permit any strong conclusions.

Various therapeutic modalit ies have resulted in higher cancer cure rates, so the

incidence of cancer cannot be inferred by using only mortality data.

Animal StudiesStudies have found no evidence of increased carcinogenicity in animals exposed to

up to 5,000 ppm halothane or 10,000 ppm enflurane (97 ,98,99 ). One s tudy found

hepatic neoplasms in mice exposed during gestat ion and early l i fe to 1,000 to 5,000

ppm isoflurane (100 ), but the validity of this study has been questioned, and it

appears that the increased incidence of tumors may have been the result of other

factors. In later studies, no evidence of increased carcinogenicity was found in

animals exposed to up to 6,000 ppm isoflurane (98 ,101). No evidence of increased

carcinogenicity in mice has been found with exposure to up to 800,000 ppm nitrousoxide (98,102). No increase in neoplasms was found in rats exposed to 10 ppm

halothane plus 500 ppm nitrous oxide (103).

Mutagenicity Testing

Huma n S t u d i es

Cytogenetic methods are increasingly used for evaluating the effects of exposure to

potential mutagens in the environment

(105 ,106,107,108 ,109,11 0,111 ,112 ,113,114 ,115 ). Some studies found no

associat ion between occupational exposure to waste anesthetic gases and

cytogenetic damage (104,10 8,110 ,11 3,116,117 ,118 ,119 ,12 0). Others suggest that

there may be an association (105,106,107,11 1,11 2,11 4,115,121,12 2,123 ). One

study found that the waste gas levels recommended by the National Inst itute for

Occupational Safety and Health (NIOSH) appear to be safe, whereas exposure to

higher levels were associated with an increase in c hromosome damage (10 9).

A n i m a l S t u d i es


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Several studies have found that halothane and its metabolites are not mutagenic

(124 ,125,126,127 ,128,12 9). Others have found that halothane and/or its

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metabolites a re weakly mutagenic (130,131 ,132 ,133). Investigators have been

unable to demonstrate mutagenic effects f rom enflurane (125 ,134,13 5) or isof lurane

(125 ,127,135). Investigations have found that nitrous oxide is not mutagenic

(125 ,136). One investigation found that halothane plus nitrous oxide did not

increase mutagenesis (128). Another found that nitrous oxide had no effect on the

mutagenicity of halothane (130). The same study found that there was no

mutagenicity with mixtures of nitrous oxide and enflurane or isof lurane.

L i v e r Di sease


Studies have found that operating room and dental operatory personnel have

higher-than-expected rates of hepatic disease (11,19 ,21,23 ,54 ,89,90 ,13 7).

Interpretation of these data has been questioned (17 ).

Recurrent hepatit is following exposure to halothane has been demonstrated in a

few individuals (138,13 9,140,141,14 2), and exposure to trace anesthetic agents

enhances hepatic metabolism of some drugs (143,144). Elevated serum

autoantibodies that react with specif ic hepatic proteins have been found in

anesthesia personnel, especially females and pediatric anesthesiologists (145).

The relevance of these facts to the effects of trace concentrat ions is not clear.

Animal Studies

Halothane exposure in concentrat ions as low as 20 ppm may be associated with

mild toxic effects to the l iver in rats (14 6,147 ,148). No evidence of such effects has

been found from exposure to enflurane or isoflurane (146,14 8).

Renal DiseaseOne study found that female operating room nurses, technicians, and

anesthesiologists had a higher r isk of kidney disease than did comparable groups

outside the operating room (21). These results have been questioned (17). Another

study failed to f ind any increase in kidney disease in male anesthesiologists (11 ).

A stu dy showed an inc rea se in renal di sease in expos ed de ntists and fema le chai r-

side assistants (23 ). No increase in deaths caused by renal disease has been found

among anesthesiologists (93).


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Hem a to l o g i c S t u d i e s

Epidemiologic StudiesIn one s tudy, a higher-than-expected rate of leukemia was found in female

anesthesiologists, but the small database made any valid conclusions dif f icult (21).

Other studies found no s ignif icant alterat ions in hematologic function in exposed

individuals (149,150,15 1,152 ). However, 3 of 20 dentists exposed to c oncentrat ions

of nitrous oxide higher than those normally found in operating rooms showed

abnormalit ies in their bone marrow, and two had abnormalit ies in their peripheral

blood (153).

Animal Studies

In mice, no hematologic effects were found from exposure to 500 ppm halothane

(97). Exposure to 3,000 ppm enflurane had no effect on hematopoiesis in mice

(154 ). Exposure to 10 ,000 ppm nitrous oxide caused no c hanges in hematopoiesis

in rats (15 5). Cytogenetic damage to bone marrow was found in rats exposed to 10

ppm halothane plus 500 ppm nitrous oxide (56).

Neu r o l o g i c S ymp t om s

A nonspec if ic polyne uro path y fol lo wing chron ic exposure to ni trous ox id e has been

described (15 6,157). Two studies found an increase in neurologic symptoms(numbness, t ingling, and/or muscle weakness) in dentists and female chair-side

assistants who are exposed to anesthetic gases (23,158). Another study showed no

dif ference in neurologic symptoms or signs, sensory p erception, or nerve

conduction between dentists who use nitrous oxide extensively and those who use

it sparingly or not at all (159 ). High levels of nitrous oxide have not been shown to

cause neuromuscular or neurologic abnormalit ies in animals (15 9).

A l t e r at i o n s i n Imm une Res p o n s e

Several studies have found that work in operating rooms does not change the

immunologic prof i le of individuals (160,161 ,162,163 ,164 ). A study of people

working in unscavenged rooms with trace gas concentrat ions several t imes the

recommended levels had changes that reversed when they were removed from that

environment (16 5).

Ca rd i a c D i sease

Studies have shown a greater-than-expected f requency of hypertension and

dysrhythmias (11,16 6), and there is one case report of atr ial f ibri l lat ion secondary


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to halothane exposure (167). However, mortality studies give no evidence that

anesthesiologists have a higher-than-expected risk of dying from heart disease

(91,92,93,94 ,95 ,96).

M i s ce l l a n eou s

Various studies have reported higher-than-expected incidences of bone and joint

disease (11), ulcers (11 ,166 ), ulcerat ive colit is (16 6), gallbladder disease (11 ),

migraine (166 ), and headache and fat igue (168) in exposed personnel. Case

reports of exposed personnel who developed asthmatic symptoms ( 169), laryngit is

(170 ), ophthalmic

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hypersensit ivity (171 ), conjunctivit is (172), exacerbation of myasthenia gravis

(173 ), and skin eruptions (174 ,175 ) have been published. Mortality s tat ist ics show a

high incidence of suicide and substance abuserelated deaths among

anesthesiologists (19,91,93).

Summa r y

The evidence that trace anesthetic gases are harmful is at present suggestive

rather than conclusive (176). The hazard, if it exists, is not great and is more

properly regarded as disquieting than alarming. Researchers who have

systematically examined the published data have concluded that reproductive

problems in women were the only health effects for which there is reasonably

convincing evidence (14 ). While it is reassuring to note that studies have shown

that anesthesiologists have a mortality rate less than that expected for physicians

or the general populat ion (92,93 ,94 ), reproductive problems are not reflected in

mortality data, and high cure rates may be responsible for the low mortality. One

study showed an increased rate of early ret irement as a result of permanent i l l

health and a high rate of deaths while working among anesthesia personnel (177).A cause-and-ef fec t re la tions hip between occup ati onal exposure and th e problems

described has not been f irmly established. I f there is an increased risk, i t may be

related to other factors such as mental and physical stress; strenuous physical

demands; dis turbed night rest; need for constant a lertness; long and inconvenient

working hours that often interfere with domestic l i fe; irregular routine; exposure to

transmissible infect ions, solvents, propellants, c leaning substances, lasers,

methylmethacrylate, radiat ion, or ultraviolet l ight; pre-exist ing health and

reproductive problems; hormonal or dietary disturbances; the physical or emotional


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makeup of those who choose to work in operating rooms; socio-economic f actors;

or some other as ye t undefined factor ( 17 8).

The Committee on Occupational Health of Operating Room Personnel suggests that

health care inst itut ions bring to the attention of operat ing and recovery room

personnel pert inent information on the claimed risks of excess anesthetic gases

and ways by which these risks can be minimized (179). A sample letter is available

(180 ).

Control Measures

Complete elimination of all anesthetic molecules from the ambient atmosphere is

impossible. The goal should be to reduce concentrat ions to the lowest level

consistent with a reasonable expenditure of effort and money. To achieve this,

attention should be focused on four areas: scavenging, equipment leaks, work

techniques, and the room ventilation system.

Scav e n g i n g S y s t ems

Scavenging is the collect ion of excess gases f rom equipment used to administer

anesthesia or ex haled by the patient and the removal of these gases to an

appropriate place of discharge outside the work environment. Scavenging sys tems

are also referred to as evacuation systems, waste anesthetic gas disposal s ystems,

anesthesia waste exhaust, and excess anesthetic gas-scavenging systems.

Installat ion of an eff icient scavenging system is the most important step in reducing

trace gas levels, lowering ambient concentrat ions by up to 90%

(181 ,182,183,184 ,185,18 6,187 ,188 ,189,190 ,191 ).

A scaveng in g sys te m cons is ts of f ive bas ic part s (Fig. 13.1): a gas-collect ing

assembly, which captures gases at the site of emission; a transfer tubing, which

conveys collected gases to the interface; the interface, which provides posit ive

(and sometimes negative) pressure relief and may provide reservoir capacity; the

gas-disposal tubing, which conducts the gases from the interface to the gas-disposal system; and the gas disposal system, which conveys the gases to a point

where they are discharged. Frequently, some or all of these components are

combined.

A U.S . sta ndard (192) and an international standard (193) for scavenging systems

have been published. The international standard dif fers from the U.S. standard in

that some f it t ings are male rather than female and vice versa.

Gas-collecting Assembly


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The gas-collect ing assembly (scavenger adapter; gas-capturing assembly, device,

or valve; scavenging trap or v alve; collect ing or collect ion valve; scavenging exhale

valve; evacuator; antipollut ion valve; ducted expiratory valve; collect ing system

exhaust valve; scavenging trap, collect ing system) collects excess gases and

delivers them to the transfer means. It may attach to, or be an integral part of, a

source. Frequently, the outlets of two or more sources are joined together. The

Ame ric an Soc iety of Te sting an d Ma te rials (ASTM) s tanda rd and in te rnati ona l

standards (19 2,193 ) specify that the outlet connection must be a 30-mm male

f it t ing. In the past, 19-mm fit t ings were permitted, but they a re being phased out.

The size is important, because it should not be possible to connect components of

the breathing system to the outlet. Some early assemblies had 22-mm fit t ings, and

cases of misconnection with breathing system tubes occurred (194,195 ).

B r e at h i n g S y s t em s

Systems Containing an Adjustable Pressure-Limiting Valve

Systems with an adjustable pressure-l imit ing

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(APL) valve (Chapter 7) include the circle system and the Mapleson A, B, C, and D

systems. The APL valve is essential ly f i t ted with a shroud (Fig. 13.2). With thecircle, Bain, and Lack va riant of the Mapleson D systems, the weight of the

assembly can be s upported by the anesthesia machine, and the transfer means can

be quite short. Smaller and l ighter APL valves with gas -collect ing assemblies are

available for the other Mapleson systems.


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View Figure

Figure 13.1Complete scavenging system. The gas-collecting assembly may be an integral part of the breathingsystem, ventilator, gas monitor, or extracorporeal pumpoxygenator. The interface may be an integral part of thegas-collecting assembly or some other portion of thescavenging system.

An APL valv e ma y have a bui l t-in mechan ism th at prevents positi ve or nega tiv e

pressure from the scavenging system being transmitted to the breathing system

(196 ).

View Figure

Figure 13.2Gas-collecting assembly attached to an APLvalve.

T-piece Systems without an Adjustable Pressure-Limiting Valve

Numerous devices f or removing gases from the bag have been described

(197 ,198,199,200 ,201,20 2,203 ,204 ,205,206 ,207 ,208 ,209,210 ,211 ,212,213,21 4).

Other methods use a container that is attached to s uction (215 ,21 6). Other methods


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attach the gas-collect ing assembly between the bag and its attachment to the

tubing (217 ,218,219 ,22 0).

Re s u s c i t a t i o n E q u i pm e n t

Nonrebreathing valves with a scavenging adapter are commercially available. I t is

fair ly simple to a ttach a collect ion assembly to the exhalat ion port of some exist ing

nonrebreathing v alves without affect ing v alve function.

Ma s k s o r N a s a l C a n n u l a e

I t is common pract ice in some inst itut ions to administer anesthetic gases to

patients through a nasal cannula or face mask for sedation. Placing a tent or hood

around the patient 's head and attaching a suction source can reduce the ambient

concentrations of gases (221,222,22 3). A double mask consist ing of a smaller inner

mask separated from a larger outer mask by a space connected to a scavenging

device wil l reduce ambient concentrat ions (224,225 ).

Ve n t i l a t o r s

Anesthes ia ven ti la to rs are now equippe d wi th gas-c ol lec t in g assemb lies . The was te

gas is di rected to the same interface that is used by the breathing system. In most

cases, the drive gas which is composed of oxygen, air, or a combination of the two,

is expelled into the room.

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On some older venti lators, the exhaust includes not only excess breathing system

gases but also the driv ing gas for the venti lator. In this situat ion, a disposal system

that is capable of handling high gas f lows is required. A sc avenging system that

functions eff icient ly with s pontaneously breathing or manually v enti lated patients

may fail to do so when used with venti lators that discharge the driving gas into the

scavenging system (226).

E x t r a c o r p o r e al P um p O x y g e n a t o r s

The outlet port of an extracorporeal pump oxygenator is a potential source of

anesthetic pollut ion (227,228). Gas-collect ing assemblies f or these are available

(229 ). I t is important to provide an effect ive interface with these devices because

signif icant posit ive or negative pressure alterat ions at the outf low port can

markedly alter oxygenator functioning (230).

Re s p i r at o r y G a s Mo n i t o r s


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A diverting gas mon itor (Chapter 22) withdraws gas from the breathing system and

transports it into the monitor. The gas then needs to be either returned to the

breathing system or diverted into the scavenging system. This source of

contamination is often ignored (231,232 ,233,234 ). Monitors manufactured in recent

years are equipped with an outlet to facil i tate scavenging (23 5) (Fig. 13.3).

View Figure

Figure 13.3A:Gas monitor with sample gas outlet (atupper right). B:Connection of transfer tubing near theinterface.

C r y o s u r g i c a l U n i t s

Some cryosurgical units use ni trous oxide. These can contribute to operating room

contamination (236 ). These units should be f i t ted with scavengers, or carbon

dioxide should be used instead of ni trous oxide (23 7).

L e a k S i t e s

When there is a definite leak site (such as when a face mask is used or a vaporizer

is f i l led), or in the postanesthesia care unit , close (local) scavenging of

contaminated air t hrough a separate scavenging device or a low negative p ressure

hood can be used to lower ambient concentrations (210,238 ,239,240,241). A face

mask that has the ability to reduce the concentration of anesthetic gases in the

recovery room has been described (8).

Transfer Tubing


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The transfer tubing (exhaust tubing or hose, transfer means, transfer system)

conveys gas from the collect ing assembly to the interface when the interface is not

an integral part of the gas-collect ing assembly.

The transfer tubing is most commonly a length of tubing with a connector at either

end. The inlet and outlet f i t t ings should be either 19 or 30 mm. It should be as short

as possible (this is facil i tated by mounting the interface on the anesthesia machine)

and wide enough to carry a high flow of gas without a significant increase in

pressure. I t should be resistant to kinking. I t should not touch the f loor, but if i t

does, it should be designed to prevent occlusion. I t should be easily seen and easy

to disconnect from the gas-collecting assembly in the event of malfunction or

scavenging system occlusion. To discourage misconnections, it is recommended

that the transfer tubing be dif ferent (by color and/or configurat ion) from the

breathing system tubing (Fig. 13.6).

Interface

The interface (balancing valve or device, pressure balancing valve or device,

interface system or block, intermediate site, safety block, air break receiver,

receiver unit , air b reak, receiving system, interface valve, scavenging valve,

reservoir) serves to prevent pressure increases or decreases in the scavenging

system from being t ransmitted to the breathing system, venti lator, or extracorporeal

oxygenator. The U.S. and international standards (19 2,19 3) require that the

pressure immediately downstream of the gas-collect ing assembly be l imited to

between -0.5 and +3.5 cm H2O during normal operating condit ions and up to +15

cm H2O with obstruct ion of the scavenging system.

The interface inlet must have a 1 9- or 30-mm (preferred) male connector. The size

of the outlet is optional but should be dif ferent from breathing system connectors

and from the inlet connector if the device is sensit ive to the direct ion of f low.

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The interface should be situated as close t o the gas-collect ing assembly as

possible, where it can be readily observed and reached by anesthesia personnel.

There are three basic elements to an interface: posit ive pressure relief, negative

pressure relief, and reservoir capacity. Irrespective of what type of disposal system

is used, posit ive pressure relief must be provided to protect the equipment and

patient if occlusion of the scavenging system occurs. I f an act ive disposal system is


16/98

used, negative pressure relief is needed to l imit s ubatmospheric pressure. A

reservoir is necessary to match the intermittent f low from the gas-collect ing

assembly to the continuous f low of the disposal s ystem. A device that gives an

audible signal may be f it ted to the interface to indicate operation of the posit ive or

negative pressure relief device. A f low indicator may be provided to monitor f low

from the interface to the gas-disposal system (Figs. 13.4, 13.5).

The reservoir may be a rigid container, wide tubing, a bag, or a c ombination of

these. A distensible bag allows gas removal by the scavenging system to be

monitored. I t should only be used with ac t ive disposal systems and should be of a

dif ferent color from, and situated away from, the reservoir bag in the breathing

system. The connection between the bag and the interface should be a dif ferent

size from the mount for the reservoir bag in the b reathing system.

Interfaces can be divided into two types: open and c losed, depending on the means

to provide posit ive and negative pressure relief.

Open In t e r f a c e

An open interface (a i r brea k receiv er uni t) (242,243 ,244 ) (Fig. 13.4A) has one or

more openings to atmosphere (allowing posit ive and negative pressure relief) and

contains no valves. I t s hould be used only with an act ive disposal system. The

inlet, the disposal system connection, and the opening(s) to atmosphere should be

arranged so that waste gases are removed before room air is entrained.

Because the discharge of waste gases is usually intermittent and f low through an

active disposal assembly is continuous, a reservoir is needed to hold the surges of

gas that enter the interface until the disposal system removes them. The reservoir

allows the f low rate in the disposal system to be kept just above the average f low

rate of gases from the gas-collect ing assembly.

I t is important that the reservoir have adequate capacity, especially if a venti lator in

which the driving gas mixes with waste gases is used or if high t idal volumes or

high nitrous oxide f lows are used (245).

The safety afforded by an open system depends on the patency of the vents to

atmosphere, so it is important to have redundancy in case some are accidentally

blocked (246 ,247 ). The vents should be checked

P.382

and cleaned regularly. Plast ic bags and surgical drapes should be kept away from

the vents.


17/98

View Figure

Figure 13.4A:An open interface. Note the escape-inletports at the top of the reservoir. These provide positive andnegative pressure relief. B,C:are closed interfaces. (A)and(B)are active systems. (C)is a passive system.

View Figure

Figure 13.5Open interfaces. The open ports at the top ofthe canister provide positive and negative pressure relief.The flow control valve is used to regulate the scavengingflow. The flowmeter indicates whether or not the flow iswithin the range recommended by the manufacturer. Thefloat should be between the two markings on the flowmeter.

Inside the canister, one tube conducts waste gases to thebottom. The other tube conducts gases from the bottom tothe disposal system.

Open interface are shown in Figs. 13.4A, and 13.5. Anesthetic gases from the

transfer means enter at the top and are c onducted to the base where they are

dispersed. A parallel tube is connected at the top to the reservoir. The space

around both tubes acts as a reservoir. The holes at the top are open to

atmosphere. A f lowmeter measures the a mount of vacuum that is applied to the

interface by the act ive disposal s ystem. I t also provides a visual indicat ion that the

vacuum is turned ON. Usually, there are two marks between which the indicator

should be located.


18/98

The open interface is simple but may pollute the atmosphere if the reservoir does

not have suff icient volume to contain the boluses of waste gases. The act ive

disposal system must supply adequate f low to remove the scavenged gases from

the interface. Turbulence wil l inc rease the volume of air contaminated with

anesthetic gases (242). Turbulence is greatest when gases from the breathing

system f low against the disposal system f low and l east when f low is in the same

direct ion.

C l o s e d In t e r f a c e s

A closed inte rf ac e (Fig. 13.4B,C) makes its connection(s) to atmosphere through

valve(s). A posit ive pressure relief valve is always required to allow gases to be

released into the room if there is obstruction of the scavenging system downstreamof the interface. I f an act ive disposal system is to be used, a negative pressure

relief (pop-in, inlet relief) valve is necessary to allow air to be entrained when the

pressure in the reservoir fal ls below atmospheric.

A re serv oi r is not re qu ire d wi th a c los ed in te rf ace and should not be used un less an

active disposal system is used. I f an act ive disposable system is used, a

distensible bag is useful for monitoring scavenging system function.

Positive Pressure Relief Only

A closed inte rf ac e wi th only posi ti ve pressur e rel ie f sho uld be used on ly wi th apassive disposal system. An example is shown in Figure 13.4C. The posit ive

pressure relief valve remains closed unless there is a problem downstream of the

interface. The relief device may be spring loaded or work by gravity.

Positive and Negative Pressure Relief

I f an act ive disposal system is used, a negative pressure relief valve must also be

present. Subatmospheric pressures greater than -0.5 cm H2O can raise or lower the

opening pressure of some APL valves (24 8).

Examples of the closed interfaces are shown in Figures 13.4B and Cand Figure

13.6. When a passive disposal s ystem is used, the negative pressure relief wil l

P.383

remain closed at all t imes. I f an act ive disposal system is used, it should close

during high peak f low rates from the gas-collect ing assembly and open when the

gas-disposal assembly f low is greater than the f low of gases entering the gas-

collect ing assembly from the breathing sys tem.


19/98

View Figure

Figure 13.6Closed interfaces. Note the wide-bore flexibletransfer tubing that is different in appearance from the

breathing system tubes.

The rate of f low i nto the gas-disposal assembly should be adjusted to the optimal

level by observing the bag (if present) and the posit ive and negative relief valves.

In an o ptimally adjusted system, the bag expands and deflates but never becomes

overextended or completely deflated (249,25 0,25 1). I f the bag is continually

collapsed or the negative pressure relief valve opens frequently, the flow should be

lowered. I f the bag becomes distended or the posit ive pressure relief valve opens

frequently, f low should be increased.

A closed inte rf ac e can be used wi th an y ty pe of disposal sys tem, but valv es ad d to

the complexity. They must be designed so that they do not st ick or leak. Interfaces

with two negative pressure relief valves are available and add a margin of safety.

Gas-disposal Tubing

The gas-disposal tubing (receiving hose, disposal tubing) connects the interface to

the disposal system (Fig. 13.1). To avoid misconnections, it should be dif ferent in

size and appearance from the breathing system hoses. I t should be resistant to

collapse and free of leaks. Wi th a passive gas-disposal system, it is important that

the hose be as s hort and wide as pract ical to minimize resistance.

Ideally, the gas-disposal tubing should be run overhead to minimize the risk of

occlusion and to avoid the dangers of personnel tr ipping over it or other apparatus

becoming entangled in it. It may be hidden in a false ceiling. If the tubing must be

run across the f loor, i t should be routed where it is l east l ikely to be stepped on or

have equipment rolled over it . I f i t must pass a doorway, it should follow the door

frame.


20/98

Gas-disposal System

The gas-disposal system (elimination s ystem or route, disposal-exhaust route,

disposal assembly) removes waste gases from the anesthetizing location. The

gases must be vented at a point that is isolated from personnel and any air intakes.

Gas-disposal systems are of two types: act ive, in which a f low-inducing device

moves the gases, and passive, in which the pressure is raised above atmospheric

by the patient exhaling, by manually squeezing the reservoir bag, or by a venti lator.

With an act ive s ystem, there

P.384

wil l be negative pressure in the gas-disposal tubing. With a passive system, thepressure wil l be posit ive.

Active sys te ms are usually mo re ef fec ti ve at keep ing po llut ion lev els low, becau se

most leaks wil l be inward (182 ,252,25 3). They allow small-bore tubing to be used,

and excessive resistance is not a problem. They also aid room air exchange. They

are, however, expensive in terms of energy costs. They are not automatic and must

be turned ON and OFF. I f they are not turned ON, air pollut ion wil l occur; i f they

are not turned OFF, there wil l be needless waste of energy. Active systems are

more complex than passive ones. Their use requires that the interface have

negative pressure relief.

Passive systems are simpler but may not be as effect ive in lowering trace gas

levels, because the posit ive p ressure encourages outward leaks. They are less

expensive to operate than act ive s ystems.

Pas s i v e S y s t em s

Room Ven tilation System

Venti lat ion systems in operating rooms are of two types: nonrecirculat ing (one

pass, single pass, 100% fresh air) and recirculat ing (187). A nonrecirculatingsystem takes in exterior ai r and processes it by f i l tering and adjust ing the humidity

and temperature. The processed air is circulated through the room and then all of i t

is exhausted to atmosphere. This type of venti lat ion system can be used for waste

gas disposal by securing the disposal tubing to a convenient exhaust gri l le. Air

f lowing into the gri l le wil l remove the gases from the room.

Economic concerns has l ed to increased use of v enti lat ion systems that recirculate

air. With this type of sys tem, a small amount of air is taken in from the atmosphere,

while the remaining air is recirculated. Most of the gases exhausted from the room


21/98

are shunted back into the intake and recirculated, while a volume equal to the fresh

air admitted is exhausted. With this type of system, waste gases must be vented

beyond the point of recirculat ion.

The health care facil i ty engineer should know which type of v enti lat ion system is

present. I f not, absence of recirculat ion can be determined by sampling the room

air inlet to see if i t is f ree of trace gases after they have been released in another

room.

An important con sid era ti on when using the room ven ti la ti on sys te m for was te ga s

disposal is the negative pressure in the exhaust duct. I f waste gases are introduced

at the gri l le, the negative pressure is usually minimal and its effect negligible (246 ).

I f they are introduced at a distance downstream in the duct (as they must be with a

recirculat ing system), negative pressure relief must be provided in the interface.

In many operating rooms, the exhaust gri l les are not located close to the

anesthesia machine, posing problems with tubing on the f loor where it may be

occluded. In some cases, the disposal tubing can be ex tended to a wall- or ceil ing-

mounted connection that leads to a pipe in the wall (187). The pipe then connects

to the exhaust duct.

Piping Direct to Atmosphere

Piping direct to the atmosphere is also known as a direct duct or v ent, specialized

duct system, direct disposal l ine, or through-the-wall system (252 ,25 3,254,25 5).

Excess gases are vented through the wall, window, ceil ing, or f loor to the outside,

using only the sl ight pressure of the gases and leaving the gas-collect ing assembly

to provide the f low. This type of system is not suitable for an operating room that is

far from an outside wall (24 6).

To prevent cross f low between rooms, each room should have its own duct. The

inlet to the duct should be close to the anesthesia machine. There should be a

means to cap the opening to the duct when it is not connected to the gas-disposal

tubing. The duct should be short with a large diameter to avoid excessive back

pressure. A unidirectional valve may be placed in the duct to prevent outside air

from entering the operating room and to minimize the effects of wind pressure on

the disposal system (254 ).

The discharge point on the outside should be s elected so that it is away from wind

pressures, ignit ion hazards, windows, and the i nlets for the v enti lat ion system. I t

may be advantageous to attach a short T-piece as a terminal (256 ). The open


22/98

end(s) should point downward to prevent water and dirt from entering and be fitted

with netting to prevent insects, rodents, and foreign matter from entering the pipe.

The direct piping disposal system is e asy to use but requires a special installat ion.

In redesigning an exist ing operating room or designing a new room, construct ion of

such a system should be considered. I f the operating rooms are not near the

outside of the building, this type of disposal assembly may not be pract ical.

Problems that can occur with the direct disposal sys tem include both posit ive and

negative pressure caused by wind currents, obstruct ion from ice buildup, and

accumulat ion of foreign matter at the outlet (255 ,257). There needs to be a means

to determine system patency. It is important to do trace gas monitoring with the

system in use in order to make certain a f low-inducing device is not needed. A

study of this type of system found that it worked eff icient ly and had low

maintenance costs (258).

Adsorption Device

An adsorpti on de vic e remo ves some or all ex ces s an esthetic agen ts by adsorb ing

them or converting them to harmless substances

(185 ,255,259,260 ,261,26 2,263 ,264 ). Canisters of varying shape and capacity that

are f i l led with ac t ivated charcoal have been used for waste gas disposal. Some can

be regenerated by autoclaving (265). Dif ferent volat i le agents are adsorbed with

varying eff iciency. The eff iciency of adsorption also

P.385

depends on the f low rate through the canister (266). Moisture may reduce the

eff iciency (26 7).

Ads orp ti on dev ic es are s imple and port ab le and do no t re qu i re ex pens iv e

installation or maintenance. An additional advantage is that halogenated anesthetic

vapors are not released to the ozone layer (266 ). An act ivated charcoal f i l ter has

been used successfully to scavenge nitr ic oxide and nitrogen dioxide (268 ).

Ads orp ti on dev ic es hav e a numb er of disadvan tages . At pres ent, th ere is no

adsorption device for nitrous oxide. They are fair ly expensive and are effect ive for

only short periods of t ime. They must be replaced regularly and pose storage and

disposal problems. In order to determine whether or not the adsorber is saturated

requires monitoring or weighing. Finally, a large canister may impose signif icant

resistance (259). I t is recommended that adsorpt ion devices be l imited to situat ions


23/98

where nitrous oxide is not being employed and no other means of eliminating waste

gases is available.

Concerns about the release of anesthetic waste gases into the atmosphere and

their contribut ion to global warming and ozone deplet ion have been voiced

(255 ,269,270,271 ). Zeolites may be used to adsorb halogenated agents from the

outlet of the scavenging system, thereby reducing atmospheric pollut ion

(269 ,272,273).

Cat a l y t i c D e c om p o s i t i o n

Catalyt ic decomposit ion can be used to convert nitrous oxide to nitrogen and

oxygen, reducing its contribut ion to the greenhouse effect (274 ,275).

A c t i v e S y s t em s

Piped Vacuum

The central vacuum system is a popular method of gas disposal (27 6,27 7,278). The

system should be capable of providing high volume (30 L/minute) f low, but only

slight negative pressure is needed. There should be a means to allow the user to

control the f low. This wil l conserve energy, reduce the load on the central pumps,

and reduce the noise level. For s ome units, this is done by observing the bag and

the posit ive and negative pressure relief valves. Others have a means to allow the

user to adjust the flow to that recommended by the manufacturer (Figs. 13.5, 13.6).

A re s tr ic ti ve ori f ice ma y be plac ed in the vacuum n ip ple to li mi t the f low (279).

There are a number of problems associated with using a central piped vacuum

system for sc avenging.

In a d e q u a t e Num b e r o f V a c u um O u t le t s

Many operating rooms have only two vacuum outlets. This is barely enough for

some surgical procedures, let alone anesthesia use. Ideally, anesthesia personnel

should have two vacuum outlets available, one for suct ioning the airway and one

for scavenging waste gases.

If there are not e nough outlets, a Y may be i nserted into the vacuum line to create

two lines. Unfortunately, this may reduce the flow so that it becomes inadequate for

either purpose.

Some anesthesia providers use a single vacuum line for sc avenging and patient

suctioning. The vacuum line remains attached to the interface most of the t ime and

is detached when needed for patient suct ioning. I f the f low of anesthesia gases is

not turned OFF, there wil l be escape of anesthetic gases into the room air.


24/98

In c o n v e n i en t O u t l e ts

I f a suction outlet is not near the anesthesia machine, long tubings must reach

across the f loor, with the ha zards of occlusion, tr ipping, and entanglement with

other apparatus.

S y s t em O v e r lo a d

Because scavenging requires high f lows, the c entral vacuum system may become

overloaded if too many devices are in us e at the same t ime. Overcoming this

problem may require a major renovation of the system. System overload can be

reduced if anesthesia personnel adjust the f low to that necessary to prevent gases

from being spilled into room air and turn off the flow after use.

D am a g e to t h e V a c u um S y s t em

Wear and tear on the vacuum pump can be expected to increase if the central

vacuum system is used for disposal of waste gases. Vacuum pump failure and

pump f ires have been reported (280).

P e rs o n n e l E x p o s u r e

If the exhaust from the central vacuum pump goes to an area frequented by

personnel or is situated near an air intake, open window, or door, this wil l result in

addit ional exposure of personnel to waste gases. I t may be necessary to relocate

the pump exhaust.

I n c o n v e n i e n c e

To conserve energy, the vacuum system should be turned ON just before

anesthesia is begun and turned OFF at the termination of a procedure. For further

energy conservation, the anesthesia provider should adjust the vacuum f low

according to the volume of waste gases. If these duties are neglected, there will be

either wasted energy or operating room pollution.

Active Duct System

The other type of ac t ive disposal assembly is a dedicated evacuation system that

leads to the outside and employs f low-inducing devices (fans, pumps, blowers, etc.)

that can move large volumes of gas at low pressures

(246 ,252,278,281 ,282,28 3,284 ) (Fig. 13.7). It has been recommended that two flow-

inducing devices be provided and arranged so that if one fai ls, the second one wil l

run. Several ducts may be connected together to a common duct that leads outside.

The f low-inducing device is located in the common duct and provides movement of

gases at a low negative pressure. Balancing dampers should be provided to prevent


25/98

pressure imbalances from developing between the operating rooms that are

connected to the system (24 8,278). The

P.386

negative pressure helps to ensure that cross contamination between operating

rooms will not occur and prevents atmospheric conditions from affecting the outflow

from the system. The outlet to atmosphere should be away from windows and

venti lat ion intakes. A means to adjust the f low may be incorporated into the

common duct.

View Figure

Figure 13.7Part of a piped anesthetic gas evacuation

system with a shut-off valve. The gauge is at the right.

Each operating room is supplied with an evacuation inlet ( Fig. 13.8). It should not

be interchangeable with other systems, including the piped vacuum system. I t is

recommended that there be a means to indicate to the user that the scavenging

system is operational.

The advantages of the active duct system are that resistance is not a problem, and

wind currents do not affect the system. Disadvantages include those of any ac t ive

system: added complexity and the need f or negative pressure relief and reservoir

capacity in the interface. I t requires a s pecial installat ion, which should be

considered during renovation or when a new anesthetizing location is being

designed. The f low-inducing device means a dded energy consumption and requires

regular maintenance.

A l t e r at i o n s i n Wo r k P r a c t i c es


26/98

A number of work pract ic es al lo w anes the tic gases to ente r ro om air

(285 ,286,287,288 ). Adhering to the following pract ices wil l signif icantly reduce

contamination. Most can be followed without compromising safety, and some of

them are beneficial to the patient. Trace gas monitoring can be used to

demonstrate to personnel the techniques needed to avoid polluting room air.

Adheri ng to good work pra ct ic es shou ld no t dis trac t from patie nt comf ort and

safety. For example, in pediatric anesthesia, use of uncuffed tracheal tubes may

necessary, and holding the mask t ight ly against the face may be fr ightening to a

child.

Checking Equipment Before Use

Before start ing an anesthetic, al l c omponents of the scavenging system should besecurely connected and patent.

P.387

If an act ive gas disposal assembly is to be used, the f low should be turned ON.

View Figure

Figure 13.8Inlet for anesthetic gas evacuation (at right). Aprobe attached to the gas transfer disposal tubing is insertedinto this.

Leaks in the anesthesia machine and breathing system can contribute to operating

room contamination. The preuse checkout (Chapter 33) should reveal these leaks

so that they can be corrected (289).

Nitrous oxide should be turned ON only momentari ly during the preuse equipment

checkout. Most tests should be conducted by using oxygen or air.

Using Scavenging Equipment


27/98

Failure to use av ailable scavenging equipment correct ly is common (290,291). In

some cases, the reasons relate to equipment design and specif ic c ircumstances.

More frequently, however, lack of concern is the problem.

Proper Mask Fit

Obtaining a good mask f it requires skil l but is c rit ical to maintain the lowest

possible levels of anesthetic gases in t he room. Mask f it is especially important

during assisted or c ontrolled venti lat ion, when higher pressures wil l magnify the

leak between the patient and the mask. Anesthesia by face mask causes the

highest levels of pollut ion (23 4,240 ,289 ,292,293 ,294,295). Pollut ion is also a

problem with supraglott ic airway devices, although lower levels of anesthetic gases

are found with these devices than with face masks (23 4,29 2,29 3,29 6,29 7). Anactive scavenging device near the mask (210 ,24 1,29 8) or the use of a d ouble mask

can reduce room pollut ion from a poor mask f it (299,300 ).

Preventing Anesthetic Gas Flow Directly into the Room

Nitrous oxide and other agents should not be turned ON unti l the mask is f i t ted to

the patient 's face. Turning the gas f low (but not the vaporizer) OFF during

intubation is also good pract ice (30 1,302,303 ,304,305,30 6). This maintains

postintubation concentrat ions close to preintubation levels and decreases operating

room pollut ion.The patient connection port on the breathing system can be blocked during

intubation (30 7,308,30 9,31 0), but care should be taken that part of the blocking

device does not become dislodged and enter the breathing system (311). The fresh

gas f low should be turned OFF or the APL valve opened to prevent the bag from

overf i l l ing.

Disconnections can be prevented by making certain that all connections are t ight

before use. Disconnections for act ivit ies such as taping the tracheal tube or

posit ioning the patient should be kept to a minimum. If i t is necessary to make a

disconnection, release of anesthetic gases into the room can be minimized if the

reservoir bag is f irs t gradually emptied into the scavenging system and the fresh

gas f low is turned OFF. Alternately, the patient port can be occluded and the APL

valve opened so that the gases wil l enter the scavenging system. I f a venti lator

(which has its own spil l valv e) is being used, the APL valve does not need to be

opened.

Washout of Anesthetic Gases at the End of a Case


28/98

At the end of a cas e, 100% oxygen shou ld be adm iniste red be fore ex tuba ti on or the

face mask or supraglott ic device is removed to f lush most of the anesthetic gases

into the sc avenging system.

Preventing Liquid Agent Spills

I t is easy to spil l l iquid agent when f i l l ing a vaporizer, so care should be exercised.

The use of an agent-specif ic f i l l ing device (Chapter 6) wil l reduce spil lage. Devices

that reduce spil lage when using funnel-f i l l vaporizers are available.

Using local scavenging wil l reduce contamination associated with f i l l ing and

draining vaporizers (17 6,240). A portable vaporizer may be f i l led in a hood with gas

extract ion.

The connections for f i l l ing and draining a v aporizer should be kept t ight. I f one ofthese connections is loose, agent may escape (312 ).

Avoiding Certain Techniques

Insuff lat ion techniques in which an anesthetic mixture is introduced into the

patient 's respiratory s ystem during inhalat ion are sometimes used for laryngoscopy

and bronchoscopy. These techniques result in f looding the air around the face with

anesthetic agents. High f low rates are required to avoid dilut ion with room air and

result in a cloud of anesthetic gases escaping into the room air. Local scavenging

should be used to remove the anesthetic gases if an insuff lat ion technique is used.

Proper Use of Airway Devices

The use of cuffed tracheal tubes wil l reduce environmental c ontamination from

waste anesthetic gases (313). Only small leaks should be permitted around

uncuffed tubes in pediatric patients. When using an uncuffed tube, contamination

can be reduced by placing a suction catheter in the mouth (31 4) and using a throat

pack (17 6).

Supraglott ic ai rway devices usually have a greater leak than c uffed tracheal tubes

but contribute less to trace gas c ontamination than anesthesia conducted with amask (234 ,292,293,29 6,29 7).

Where it is not possible to use a leak-t ight device, a hood can be placed around the

head and suction used to remove the trace gases (313 ,315 ,316).

Disconnecting Nitrous Oxide Sources

Nitrous oxide and oxygen pipeline hoses leading to the machine should be

disconnected at the end of the operating schedule. The disconnection should be

made as


29/98

P.388

close to the terminal unit as possible and not at the back of the anesthesia machine

so that if there is a leak in the hose, no gases wil l escape to room air while the

hose is disconnected. This wil l result in lower levels of ni trous oxide in the

operating room and conserve gases.

When cylinders are used, the cylinder valv e should be closed at the end of the

operating schedule. Gas remaining in the machine should be bled out and

evacuated through the scavenging system.

Using Low Fresh Gas Flows

Using low fresh gas f lows wil l reduce the pollut ion result ing from disconnections inthe breathing system and from ineff icient scavenging (317,31 8,31 9). I t also allows

low removal f lows to be used with act ive disposal assemblies, result ing in energy

conservation and reduced wear and tear on the disposal device. The use of a trace

gas monitor may lead to use of lower fresh gas f lows (320). Using low gas f lows

does not make scavenging unnecessary, because high f lows must st i l l be used at

t imes.

Using Intravenous Agents and Regional Anesthesia

Using intravenous induction techniques signif icantly reduces trace gas exposure(321 ).

Keeping Scavenging Hoses off the Floor

A scaveng in g hose on the fl oo r can be obs tructed or da maged by equipment rol l ing

over i t , reducing scavenging.

L e a k Con t r o l

Some leaks are unavoidable, but they should be minimized

(14,25,198,28 7,288,32 2,32 3). Leak control may require replacement of equipment

that cannot be made gas t ight.

Most anesthesia machines are serviced at regular intervals. Unfortunately, this

servicing does not always identify or correct all leak points. In addit ion, leaks in

some equipment develop fair ly frequently, so quarterly servicing is not suff icient.

In-house monitoring and maintenance are necessary to minimize leakage.

Pressure Terminology

Some literature on scavenging has referred to all equipment upstream of the f l ow

control valves as the high-pressure system and all equipment between the f low


30/98

control valves and the patient plus the scavenging equipment as the low-pressure

system (28 7). In this book, the high-pressure system refers to those components

that contain gas whose pressure is normally above 50 psig (340 kPa). This includes

the components between the cylinder and the regulator. The intermediate-pressure

system includes components normally subjected to a pressure between 50 and 55

psig. This includes the pipeline hoses and the components of the machine between

the pressure regulators or pipeline inlets and the f low c ontrol valves. The low-

pressure system consists of components downstream of the f low control valves to

the patient, plus the scavenging system.

Identifying Leak Sites

There are several techniques for locating leak sites (234 ). A c ontinuous infrarednitrous oxide analyzer can be used. The equipment under test is pressurized with

nitrous oxide and the sampling probe directed at suspected leak sites. The me ter

reading indicates the presence or absence of leaks. This wil l identify most leaks.

An ex cep tion would be le ak ag e into a vapo ri zer.

Some leak sites can be identif ied by applicat ion of a solut ion of 50% liquid soap

and 50% water or a commercial leak test solut ion. Another method is to put alcohol

on one's hands and move the hands over the equipment. A leak wil l cause cooling.

Leakage can be assessed by test ing the capacity of the equipment to sustain

pressurizat ion. The total leak rate is de termined, after which a co mponent is

excluded and the leak rate determined again. The dif ference is the leak rate for that

component.

H i g h - p r e s s u r e S y s t em

To test for leaks in the high-pressure system, the pipeline hoses should be

disconnected and the f low c ontrol valves closed. The valve on a nitrous oxide

cylinder should be opened fully, the pressure recorded, and the cylinder valve

closed. The pressure should be recorded again 1 hour later. I f l i t t le or no p ressure

drop has occurred, there is no signif icant leakage. I f i t fal ls, the high-pressure

system is not t ight. The test should be repeated with the other nitrous oxide

cylinder if there is a double yoke.

If a signif icant leak is found, the most common site is the yoke, and applicat ion of a

leak test solut ion wil l demonstrate a poor seal. Tightening the c ylinder in its yoke

wil l often seal the leak. Other easily correctable causes include double, absent, or

deformed washers. I f damaged parts are found, they should be replaced. I f f ixing


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these problems does not cause the pressure to hold, the leak is inside the machine

and must be co rrected by the manufacturer's service representat ive.

Because leakage in this area does not occur often, checking every 2 to 4 months as

well as after a cylinder has been changed should be suff icient (202,285,287 ).

In t e r m e d i a t e- p r es s u r e S y s t em

Leaks in the intermediate-pressure system components can be determined by

measuring the nitrous oxide concentrat ions in the operating room when no

anesthesia is being administered (287). The survey should begin at least 1 hour

after administrat ion of anesthesia has been discontinued. I f a

P.389

recirculat ing air condit ioning system is in use, a longer period may be required. The

early morning is an excellent t ime to perform this test.

Flow control valves should be closed, pipeline hoses connected, and cylinder

valves closed. Room air should be s ampled at the anesthesia breathing zone (4 to

5 feet above the f loor within 3 feet of the f ront of the anesthesia machine) and the

room air intake and outlet. Nitrous oxide concentrat ions should be less than 5 ppm

(153 ,230). I f a higher level is found, the pipeline hoses should be disconnected and

the measurements repeated after a period of t ime. I f a high level is st i l l present,

this indicates a leak in the nitrous oxide pipe leading into the room or the stat ion

outlet and should be reported to the health care faci l i ty engineer. I f the level fal ls,

this indicates a l eak in the p ipeline hose or the anesthesia machine.

Common problems with pipeline hoses include worn or leaking connections

(especially quick connects), deformed compression f it t ings, and holes. These

should be corrected or the hoses replaced. Leaks inside the anesthesia machine

require correct ion by a service representat ive.

Once leaks are corrected, it is suggested that test ing of the intermediate system be

performed every 2 to 4 months (198,230 ,285 ,28 7).

L o w - p r e s s u r e S y s t em

The low-pressure port ion of the system develops leaks more frequently than other

parts. The preuse test for leaks in the breathing system (Chapter 3 3) is suff icient

for the safe c onduct of anesthesia, yet can miss leaks that emit large amounts of

anesthetic gases i nto room air.

One way to quantify leakage in most of the low-pressure system is shown in Figure

13.9. The breathing system is assembled for use. All components that are normally


32/98

used should be present in their usual posit ions. The patient port is occluded. The

bag is removed and the bag mount occluded. This is necessary because the bag's

compliance makes it hard to quantitate low leak rates. The bag should be tested

separately for leaks. A vaporizer on the anesthesia machine should be turned ON.

The APL valve should be fully open and the scavenging system occluded upstream

of the interface. The oxygen f low control v alve is now opened suff icient ly to

establish and maintain a steady pressure of 30 cm H 2O on the pressure gauge in

the breathing system. The f low on the oxygen f lowmeter is the leak rate and should

be less than 1,000 mL/minute. Leakage of 1,000 mL/minute of nitrous oxide would

result in a mean concentrat ion of only 30 ppm in a 4,000 cubic foot room with 15 air

changes per hour (248 ). The leakage test should be repeated with the other

vaporizers turned ON.

View Figure

Figure 13.9Test for quantifying low-pressure leakage. (1)The reservoir bag is removed, and the bag mount is

occluded. (2)The patient port is occluded. (3)The APLvalve is opened fully. (4)The transfer means is occluded

just upstream of the interface. (5)Oxygen flow is turned onand adjusted to maintain a pressure of 30 cm H2O on the

pressure gauge in the breathing system.

I f the leak rate exceeds 1,000 mL/minute, the APL valve should be closed and the

leak rate again determined. The dif ference is the leak rate in the scavenging

system. The remaining leakage can be divided into that associated with the

machine and that associated with the breathing system by attaching a

sphygmomanometer bulb to the anesthesia machine common gas outlet a nd

determining the oxygen f low necessary to achieve and maintain a pressure of 22

mm Hg. This is the port ion of the low-pressure leakage associated with the

machine. The machine leak site can be further ref ined by turning the vaporizer OFF

and again determining the leak rate.


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Problems in the scavenging system may be as simple as a c rack in a tubing

(especially where it becomes kinked) or a poor connection.

The breathing system is the most common location for signif icant low-pressure

leaks, and the most common site is the absorber. Common problems include

defect ive gaskets or seals, improper closure, inadequate t ightening, and open or

leaking drain cocks. Absorbent granules on the gaskets can prevent a t ight s eal.

Disposable canisters may be cracked during transit and leak after being installed.

Most of these problems are easily corrected. Complicated repairs s hould be done

only by the service representat ive.

The above test does not check for leaks in the venti lator. The venti lator and the

low-pressure system can be tested by using an infrared nitrous oxide analyzer. The

anesthesia machine and breathing system are set up for cl inical use. The patient

port outlet is occluded and the bag/ventilator selector switch put in the bag mode.

The APL valve is closed. Using the f lowmeters, the breathing system is pressurized

to 30 cm H2O with a 50% mixture of nitrous oxide and oxygen. The machine and

breathing system are scanned for nitrous oxide leaks. The selector valve is then put

in the v enti lator mode and the f lowmeters set to deliv er 2 L/minute oxygen and 2

L/minute ni trous oxide. The venti lator is turned ON and set to a

P.390

tidal volume such that a peak pressure of 30 cm H2O is reached. The scavenger

system is act ivated. The machine, venti lator, breathing system, and scavenging

system are scanned. Readings should not be greater than 25 ppm nitrous oxide.

A ven ti la tor wi th a s ta nd ing be l lo ws can be checked fo r lea ks by f i l l ing the be llows

with gas, then switching the bag/venti lator selector switch to the bag posit ion. The

bellows should remain fully inf lated. A hanging bellows can be t ested for leaks by

stopping it during inspirat ion and placing the bag/venti lator selector switch in the

bag posit ion. The bellows should remain compressed.

It is controversial as to how often the low-pressure system should be tested for

leakage. Suggested intervals vary from daily (230 ,285) to every other week (287) to

monthly (198 ). It should be repeated with new equipment and when the absorbent is

changed.

Room Ven t i l at i o n S y s t em

The room venti lat ion system serves as an important adjunct to trace gas c ontrol by

dilut ing and removing anesthetic gases result ing from leaks, e rrors in technique,


34/98

and scavenging system malfunctions (324 ,32 5,32 6,327,328). Recirculat ing systems

are less effect ive at removing trace gases than nonrecirculat ing systems. A

downward displacement venti lat ion system is more effect ive than a turbulent f low

system (18 3). A turnover rate of 20 exchanges per hour is considered necessary to

prevent bacteria from sett l ing (329).

The anesthesia machine should be placed as close to the exhaust gri l le as

possible. This will ensure maximum gas removal by the ventilation system and

make it easy to use the venti lat ion system as the gas-disposal system. This should

be taken i nto considerat ion when constructing a new operating room or renovating

an older one.

Online ambient air control has been proposed (10,33 0). This would permit the room

venti lat ion to be matched to the actual contamination level.

Hazards of Scavenging Equipment

M i s a s s emb l y

Misconnections involving the scavenging system are not uncommon (331 ,332 ). Most

scavenging components have 19- or 30-mm connections rather than the 15- and 22-

mm sizes found in b reathing systems. This wil l not completely prevent

misconnections, because there may be other apparatus in the room that will accept

19- or 30-mm connections (32 5,325A), and sometimes a 19- or 30-mm connector

can be f it ted onto a 22-mm one (33 3,33 4). The safety provided by 19- and 30-mm

connectors can be bypassed by using cheater adapters or tape for making

connections.

A ci rc le system ho se ma y be con nec ted to th e ou tl et of the APL val ve col le ct ing

assembly (194 ,195 ,335 ,336 ). Measures to prevent this include turning the exhaust

port of the gas-collect ing assembly so that it points in the opposite direct ion from

the breathing system ports, use of transfer and gas-disposal tubings of dif ferent

colors and/or configurat ions f rom breathing system tubes, and using 30-mmconnections in the scavenging system.

P r es s u r e A l t er a t io n s i n t h e B r e a t h i n g S y s t em

The scavenging system extends the breathing system all the way to the gas-

disposal point. When a scavenging system malfunctions or is misused, posit ive or

negative pressure can be transmitted to the breathing system. This is more l ikely to

occur with c losed interfaces.


35/98

Measures to prevent these untoward incidents include employing collapse-resistant

material in all disposal lines, making the transfer means easy to disconnect, using

scavenging tubing that has a dist inct ive appearance, incorporat ing posit ive and

negative pressure relief valves in the interface, regularly checking the valves for

proper functioning, using an open interface, and using airway pressure monitors

(Chapter 23).

Positive Pressure

Posit ive pressure in the scavenging system can result from an occluded transfer or

gas-disposal tubing. This can be caused by the wheel of an anesthesia machine or

other equipment roll ing onto the tubing (19 5,337 ,338 ,339,340), ice (257 ), insects,

water, or other f oreign matter. Another cause is defect ive components (341).Misassembly of the connection to the exhaust grille (342) and failure to include an

opening between the inner and outer tubes of a tube-within-a-tube interface ( 343 )

have been reported.

These malfunctions may not result in a pressure buildup when a posit ive pressure

relief mechanism is incorporated into the interface. The posit ive pressure relief

mechanism may be incorrect ly assembled, may not open at a low enough pressure,

or may be blocked (344). Obstruction or misconnection of the transfer tubing may

occur (334 ,34 5,346 ,347 ,348,34 9). Because these problems are on the patient side

of the interface, disconnecting the transfer means from the gas-collect ing assembly

may be necessary to prevent a dangerous increase in pressure. In one reported

case, the transfer tubing was kinked, causing back pressure to dev elop in the gas

jacket of an extrac orp ore al ox ygenato r. This resu lted in gas be ing fo rced in to th e

blood (350). All tubings that conduct scavenged gas should be off the f loor or

protected so that they cannot become obstructed (339).

With some older APL valves, subatmospheric pressure can result in obstruct ion and

a buildup of posit ive

P.391

pressure in the breathing system (34 3,351). In one reported case, subatmospheric

pressure in the scavenging system drew a venti lator relief v alve diaphragm onto its

seat and closed the valve, result ing in a pressure increase in the system (35 2).

In another reported case, low scavenging flow resulted in an increase in pressure in

the bag at the interface. This caused the v enti lator to fai l, and there was sustained


36/98

posit ive pressure in the patient circuit (353). On newer models, only ful l f low can be

used for scavenging.

Negative PressureI f an act ive disposal system is in use and the APL valve is ful ly open, there is

danger that subambient pressure wil l be applied to the breathing system.

Monitoring expired volume (but not ai rway pressure) may fail to detect a

disconnection in the breathing system because the scavenging system may draw a

considerable flow of room air through the expiratory pathway ( 354,35 5,36 7).

Gas may be ev acuated from the breathing system if the APL v alve allows gas to be

drawn through it and into the scavenging system at a pressure less than that

needed to open the negative pressure valve on the interface (356,35 7). Thisproblem can be corrected by part ial ly closing the APL v alve (358 ), increasing the

fresh gas f low, or lowering the f low in the gas disposal system.

The negative pressure relief mechanism may malfunction

(344 ,359,360,361 ,362,36 3). Another problem is us ing an interface designed for a

passive system (which has no means to prevent a subatmospheric pressure) in an

active scavenging system (346 ). In some scavenging systems that use the central

vacuum system, a restrict ive orif ice is incorporated into the vacuum hose f it t ing to

limit gas evacuation, regardless of the pressure applied by the central v acuum

source (27 9). If this orif ice is omitted or becomes damaged, excessive vacuum will

be applied to the interface and the capacity of the negative pressure relief

mechanism may be exceeded.

Ways to prevent negative pressure f rom being transmitted to the breathing system

include provision of one or more negative relief mechanisms in the interface with an

active disposal system (36 4), adjust ing the f low through the gas disposal system to

the minimum necessary, and protecting the openings to atmosphere from accidental

occlusion.

L o s s o f Mo n i t o r i n g In p u t

A scaveng in g sys te m ma y mask the s tr ong odo r of a volati le anestheti c agent,

delaying recognit ion of an overdose (34 5,365). Use of anesthetic agent monitoring

(Chapter 22) should largely eliminate this problem.

A l a rm Fai l u r e

A cas e has bee n rep ort ed in wh ich nega t iv e pre ssure from the scav eng ing sys te m

interface prevented the venti lator bellows from collapsing when a disconnection in


37/98

the breathing system occurred (366). The low airway pressure alarm in the

venti lator was not act ivated. In another case, room air was d rawn into the breathing

system through a disconnection, preventing the low minute v olume alarm from

sounding (36 7).

Monitoring Trace Gases

Ra t i ona l e

Ai r monito ri ng is the be st indi cator of th e suc cess of a was te ga s contro l pro gra m.

It ref lects how well leaks and errors in technique are being controlled as well as the

eff iciency of the scavenging and room venti lat ion systems and documents that low

trace levels a re being maintained. Some anesthetic departments do not monitor

trace gas levels in the belief that scavenging devices have solved the problem

(368 ).

Monitoring is necessary because a sc avenging system that appears adequate in

design may perform ineff icient ly in use. Sites where gas can leak are diverse,

frequently obscure, and sometimes inaccessible. Even relat ively large leaks may be

inaudible. Nitrous oxide is odorless, and the threshold for smell ing halogenated

agents may be as high as 300 ppm (369 ). Without monitoring, operat ing room

personnel may be unaware that atmospheric contamination is at unacceptable

levels. A properly conducted monitoring program can provide a construct ive method

of reminding anesthesia personnel to avoid careless work habits. Another

advantage of monitoring is that it can detect problems with gas delivery to

equipment (370).

Al though such a pro gra m wi ll increas e a hea lth care fac il ity's ope ra ting ex pense, it

wil l help to reduce the inst itut ion's l iabil i ty to claims by employees alleging that

exposure to waste gases contributed to a spontaneous abortion or other medical

problems. Correct ing certain leaks such as those associated with the pipeline or

pipeline hose can result in a savings to the facil i ty.

In - h o u s e v er s u s Comm e r c i al L a b o r a t o r y

The monitoring program should be directed by an interested and qualif ied person,

preferably from the anesthesia department. Samples may be analyzed by either

facil i ty-based personnel or outside commercial laboratories. The use of an outside

laboratory avoids the cost of purchasing, operat ing, maintaining, and calibrat ing a

gas analyzer. The responsibil i ty for record keeping is shared.

P.392


38/98

The chief disadvantage is the delay in report ing results. The precise circumstances

at the time the samples were taken are likely to have been forgotten, and the effect

of correct ive measures cannot be immediately assessed. In addit ion, analysis of a

large number of samples is expensive.

Adv an ta ges of in -house an al ys is inc lu de a virtually unlimited number of ana lyses at

modest cost and immediate on-site reporting. Leaks can be f ound quickly and the

effect iveness of the correct ion assessed immediately. An on-site continuous

monitor is useful for demonstrat ing the effects of technique errors on trace gas

levels and the results of correct ions.

A smal l fac i li ty mi gh t pe rio dical ly lease an ins tr um en t or share one wi th o ther

health care facil i t ies in the area rather than purchase its own.

Equ i pm en t f o r De t erm i n i n g T r ac e Gas Con c e n t r a t i o n s

Infrared Analyzers

Infrared gas analyzers were discussed in Chapter 22. These monitors are the most

pract ical for the average health care inst itut ion because they are reliable, relat ively

inexpensive, and easy to use (37 1). They are useful for locating leaks, especially

those in unusual locations. They give continuous measurements so that exposed

personnel and those responsible for air monitoring are given an i mmediate reading.

When operated on battery power, a number of locations can be sampled quickly. A

recording attachment may be helpful.

These instruments are most often used for monitoring nitrous oxide. Unfortunately,

carbon dioxide and water vapor in high concentrat ions wil l interfere with the

analysis. This can be avoided by sampling at least 6 to 10 inches away from

personnel. Analyzers capable of measuring halogenated anesthetics are available

but have many technical dif f icult ies; a lcohols and other substances in the operating

room cause interference (66

chapter 13. controlling trace gas levels

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