Waste Anesthetic Gases and Scavenging Systems

Trace Concentrations of Anesthetic Gases

Concern over trace concentrations of anesthetic gases dates back to 1967, when Vaĭsman reported findings of a survey of 354 anesthesiologists in Russia. All worked in poorly ventilated operating rooms (ORs) and used nitrous oxide (N ₂ O), halothane, and ether. Of the total, 303 responded to the survey; and of these, 110 were female. Female responders reported 31 pregnancies, 18 of which ended in spontaneous abortion. One pregnancy resulted in a congenitally abnormal child. Vaĭsman concluded that these problems in pregnancy—as well as other reported effects, such as nausea, irritability, and fatigue—were due to a combination of long-term inhalation of anesthetic vapors, emotional strain, and excessive workload. Although uncontrolled and largely anecdotal, this study drew attention to the possibility that trace concentrations of anesthetics may be harmful. The matter was taken up by investigators in Europe ^, and in the United States. Their results, and those from animal studies, gave cause for further inquiry.

In 1970, the U.S. Congress passed the Occupational Safety and Health Act, the purpose of which was to ensure “safe and healthful working conditions for all men and women in the nation.” The act established the National Institute for Occupational Safety and Health (NIOSH), which was given the responsibility to conduct and fund research in exposure hazards and to recommend safety standards. The act also established the Occupational Safety and Health Administration (OSHA), which, after due procedure, would enact into law and then enforce NIOSH recommended standards. NIOSH funded a number of studies, one of which was the National Survey of Occupational Disease Among Operating Room Workers, conducted in conjunction with the American Society of Anesthesiologists (ASA) Ad Hoc Committee on Waste Anesthetic Gases. This study surveyed 49,000 people who were potentially exposed, namely members of the ASA, American Association of Nurse Anesthetists (AANA), Association of Operating Room Nurses (AORN), Association of Operating Room Technicians (AORT), and 26,000 unexposed personnel, namely members of the American Academy of Pediatrics and American Nurses Association. The results, published in 1974, showed an increased reported incidence among women of spontaneous abortion, liver and kidney disease, and cancer, and a higher incidence of congenital abnormalities in the offspring of exposed women. Among the exposed men, the incidence of cancer did not increase, but that of hepatic disease did. NIOSH sponsored further related studies that included investigation of methods for reducing exposure to waste gases. The organization planned to introduce scavenging of waste anesthetic gases (WAGs) and repeat this survey to determine whether scavenging did indeed reduce these adverse health effects and show an association between trace anesthetic gases and disease.

In March 1977, before a repeat survey was commenced, NIOSH published Criteria for a Recommended Standard: Exposure to Waste Anesthetic Gases and Vapors. This report estimated that 214,000 workers were potentially exposed to trace concentrations of anesthetic gases on a day-to-day basis. The document reviewed all the available data and found that, although not definitive, the evidence suggested a relationship between health hazards and trace concentrations of anesthetic gases. No cause-and-effect relationship was established, and no safe exposure levels could be identified. However, the document recommended that risks be minimized as much as possible by maintaining “exposures as low as is technically feasible.” The document also recommended measures to reduce exposure and to monitor exposure levels, and it advocated extensive recordkeeping regarding the health of OR personnel.

NIOSH recommended environmental limits for the upper boundary of exposure. The occupational exposure level is based on the weight of the agent collected from a 45-L air sample by charcoal adsorption over a sampling period of 1 hour. The levels set were lower than the levels seen to cause harm in animal experiments; however, the exposure levels cannot conclusively be defined as safe due to inadequate understanding of the potential adverse effects of WAGs. NIOSH’s recommendation is as follows: “Occupational exposure to halogenated anesthetic agents shall be controlled so that no worker is exposed at concentrations greater than 2 ppm [parts per million] of any halogenated anesthetic agent. When such agents are used in combination with N ₂ O, levels of the halogenated agent well below 2 ppm are achievable. In most situations, control of N ₂ O to a time-weighted average (TWA) concentration of 25 ppm during the anesthetic administration period will result in levels of approximately 0.5 ppm of the halogenated agent. Occupational exposure to N ₂ O, when used as the sole anesthetic agent, shall be controlled so that no worker is exposed at TWA concentrations greater than 25 ppm during anesthetic administration. Available data indicate that with current control technology, exposure levels of 50 ppm, and less for N ₂ O, are attainable in dental offices.”

These recommended limits were based on two studies. First, Whitcher and colleagues showed that these levels were readily attainable in the OR when certain precautionary measures were taken. Second, Bruce and Bach found no decrement in the psychomotor capacities of volunteers exposed for 4 hours at the recommended levels. The newer volatile agents (isoflurane, sevoflurane, desflurane) had not been introduced at the time these recommendations were made, so exposure limits for these agents were not assessed.

There is no universal standard for WAG exposure. Many countries have set their own limit values and there is a great deal of variability among these limits. Values for a small sampling of countries are summarized in Table 5.1 . Some organizations such as NIOSH use a 1-hour TWA, while others use an 8-hour TWA.

Table 5.1

International Occupational Exposure Limits of Trace Anesthetic Gases

	Nitrous Oxide (ppm)	Halothane (ppm)	Isoflurane (ppm)	Sevoflurane (ppm)
USA	25	2
UK	100	10	50
Canada (Ontario)	25	2	2
Germany	100	5
Sweden	100	5	10	10
Finland	100	1	10	10

ppm, Parts per million.

To provide a perspective on how small 1 ppm is, consider that the presence of 25 ppm of nitrous oxide (N ₂ O) in the atmosphere represents a concentration of one four-hundredth of 1% (100% N ₂ O is 1 million ppm [by volume]):

100% N ₂ O = 10 ⁶ ppm
1% N ₂ O = 10,000 ppm
(1/100) × 1% N ₂ O = 0.01% N ₂ O = 100 ppm
(1/400) × 1% N ₂ O = 0.0025% N ₂ O = 25 ppm

Similarly, 2 ppm and 0.5 ppm of halothane represent a concentration of one five-thousandth and one twenty-thousandth of 1% halothane:

100% halothane = 10 ⁶ ppm
1% halothane = 10,000 ppm
(1/5000) × 1% halothane = 0.0002% = 2 ppm
(1/20,000) × 1% halothane = 0.00005% = 0.5 ppm

To express this in terms of anesthetic potency, divide the value expressed in parts per million by the minimum alveolar concentration (MAC) of halothane (0.76%) at 1 atmosphere pressure:

2 ppm = 0.0002 % = \frac{2 \times 10^{- 4}}{0 {. 76 × 10}^{- 2}} M A C = 0.00026 M A C h a l o t h a n e

What levels of trace anesthetics may be found in the OR? When no attempt has been made to reduce leakage or to scavenge waste gases, trace gas levels of 400 to 600 ppm of N ₂ O and from 5 to 10 ppm of halogenated agents may be detected. Effective scavenging alone can reduce these levels more than 10-fold.

The volume of N ₂ O that must be released into an OR to reach the NIOSH maximum recommended limit of 25 ppm can be calculated by the following equation. Assume the size of an OR is 5 m ² × 4 m. The volume of the OR is therefore given by:

500 cm × 500 cm × 400 cm = 100 × 10 ⁶ mL = 100,000 L

Therefore, the NIOSH limit is 25/10 ⁶ . If the OR volume is

100 × 10 ⁶ mL, this limit is reached by release of 2500 mL (25 × 100) or 2.5 L of N ₂ O.

This calculation assumes uniform mixing of all gases and no ventilation or air conditioning of the OR. If it is assumed that the OR ventilation system produces 12 air changes per hour, in the OR described above, a leakage rate of 2.5 L/5 min or 0.5 L/min N ₂ O would be necessary to maintain the ambient air N ₂ O level at 25 ppm.

In 2000, OSHA revised its recommendations on WAGs in the light of current knowledge. The revised recommendations are published on the Internet for informational purposes only and are regularly updated as information becomes available. The document is not published in the standard OSHA manual on occupational hazards, however. These recommendations are all advisory and have not been promulgated as a standard; rather, they are to be seen as guidelines. OSHA recommends scavenging of WAGs in all anesthetizing locations and advocates work practices to reduce trace levels of anesthetic gases in the ambient air. A documented maintenance program should be in place for all anesthesia workstations, and an ongoing education program for all personnel to inform them of these recommendations must exist. OSHA recommends a program for monitoring trace anesthetic gases and also recommends a preemployment medical examination for all employees. Each institution also should have a mechanism in place for employees to report any work-related health problems.

Contamination with trace concentrations of anesthetic gases also may occur in the corridors of an OR suite, in the anesthesia workroom, and in the N ₂ O storage area. Poorly ventilated postanesthesia care units (PACUs) also may be contaminated with exhaled anesthetic agents.

Sources of Anesthetic Gas Contamination

Potential sources of anesthetic gas contamination are the adjustable pressure-limiting (APL) or “pop-off” valve, the high-and low-pressure systems of the anesthesia machine, the anesthesia ventilator, cryosurgery units, and other miscellaneous sources.

Adjustable Pressure-Limiting Valve

The APL valve of the anesthesia breathing circuit is the outlet for WAGs during spontaneous or assisted ventilation. Depending on the inflow rate of fresh gas, more than 5 L of gas can exit the circuit through this valve every minute. The effect of such spillage on the level of anesthetic contamination in the OR is given by the following equation:

C = (L × 60 × 10 ⁶ ) / (NV)

where C is the OR pollutant level in parts per million, L is the pollutant spillage in liters per minute, V is the OR total air volume in liters, and N is the number of air exchanges per hour. For example, if L = 3 L/min, V = 100,000 L, and N = 10 exchanges per hour, then

C = (3 × 60 × 10 ⁶ ) / (12 × 100,000) = 150 ppm

Very large volumes of anesthetic gas are discharged through the relief valve of nonrebreathing systems, such as the Bain circuit (Mapleson D) or Jackson-Rees (Mapleson F). Without scavenging and proper room ventilation, N ₂ O levels as high as 2000 ppm have been reported in the breathing zone of anesthesiologists.

High- And Intermediate-Pressure Systems Of The Anesthesia Machine

The high- and intermediate-pressure systems of the anesthesia machine include the N ₂ O central supply pipeline and reserve tanks and the internal piping of the machine that leads to the N ₂ O flowmeter ( Fig. 5.1 ). The pressure in this system can range from 26 to 750 pounds per square inch gauge (psig); leaks therefore are likely to contribute significantly to the contamination of the OR. Common sources of leaks are defective connectors in the N ₂ O central supply line “quick connects” (see Fig. 5.1 ) and defective yokes for the N ₂ O reserve tanks. In one OR suite, major high-pressure leaks were detected in 50% of the anesthesia machines. When the OR is not being used, background N ₂ O contamination is primarily caused by high-pressure leaks.

Fig. 5.1, Sources of leaks in the high-pressure system. A : Diameter index safety system (DISS) N 2 O hose connection to machine N 2 O inlet; B : Hanger yoke for N 2 O reserve tank.

Low-Pressure System Of the Anesthesia Machine

The low-pressure system of the anesthesia machine includes the N ₂ O flowmeter, the vaporizers, the fresh gas delivery tubing from the anesthesia machine to the breathing circuit ( Fig. 5.2F ), the carbon dioxide absorber ( Fig. 5.2E ), the breathing hoses ( Fig. 5.2D ), the unidirectional valves, the ventilator, and the various components of the waste gas scavenging system. Leaks occur most commonly in the carbon dioxide absorber because of loose screws, worn gaskets, granules of absorbent on the gaskets, and an open petcock. Other leaks may occur as a result of breaks in the rubber and plastic components of the breathing circuit, loose domes on the unidirectional valves ( Fig. 5.2A,B ), or disposable breathing circuits, especially the swivel type, which are particularly prone to leakage due to imperfections in the manufacturing process. Although vaporizers usually are gas tight, they too occasionally leak as a result of loose mounts, defective seals and gaskets, or incompletely closed fill ports.

Fig. 5.2, Sources of leaks in the low-pressure system. A and B : Domes of unidirectional valves; C : oxygen sensor; D : breathing circuit hoses; E : CO 2 absorber canister; F : fresh gas delivery tube; G : breathing system reservoir bag.

A direct linear relationship exists between the peak pressure in the breathing circuit and the amount of gas that escapes through a leak in the low-pressure system: the higher the pressure, the greater the leak. Despite effective scavenging, as much as 2 L/min may leak from the breathing circuit, increasing the N ₂ O concentration in the atmosphere by 100 to 200 ppm.

The scavenging system itself may be a source of leakage. The rubber parts and safety valves may leak anesthetic gases, as may improperly sized hoses, such as 22-mm hoses that have been “adapted” to fit the 19-mm or 30-mm scavenger connections. The system also may be overloaded because of inadequate vacuum or ventilation systems; consequently, waste gases are spilled into the OR atmosphere.

Anesthesia Ventilator

The anesthesia ventilator may be a major source of leakage. Some ventilators leak internally and cause anesthetic gases to mix with the nonscavenged driving gas of the ventilator, although many GE machines do scavenge drive gas. In one OR, the N ₂ O level increased from 5 to 80 ppm each time the ventilator was used.

Errors In Anesthesia Technique

With a leak-proofed anesthesia workstation and breathing system and an effective scavenging system, 94% to 99% of all OR anesthetic contamination is caused by errors in anesthesia technique. The following are significant errors in technique:

1.
Incorrect insufflation techniques.
2.
Turning the N ₂ O flowmeter and/or a vaporizer on while the breathing circuit is not connected to the patient causes direct loss of anesthetic gases into the room. This often occurs at the beginning and end of anesthesia administration and during intubation. When the breathing circuit is not connected to the patient, the anesthesiologist should turn off the fresh gas flows or utilize the “pause fresh gas flow” function; otherwise, anesthetic-laden gas in the circuit will flow into the room.
3.
A poorly fitting face mask, laryngeal mask, or other airway device permits leakage of anesthetic gases around the rim.
4.
An uncuffed tracheal tube that is too small relative to the tracheal diameter or a poorly seated cuffed tracheal tube may allow anesthetic gases to leak and spill into the room air.
5.
At the conclusion of surgery, if a deeply anesthetized patient is disconnected from the breathing circuit, relatively high concentrations of anesthetic gases can be exhaled into the room air.
6.
Accidental spillage of liquid volatile anesthetic agent while a vaporizer is refilled adds vapor to room air; each milliliter of spilled liquid adds about 200 mL of vapor (see Chapter 3 ).
7.
Emptying the breathing circuit of anesthetic gases while the circuit is disconnected from the patient spills anesthetic gases directly into the room air.

Cryosurgery

Cryosurgery is used by gynecologists, ophthalmologists, otolaryngologists, and dermatologists and is a major source of OR contamination. A jet of 20 to 90 L/min of liquid N ₂ O is used as a surgical tool. The liquid rapidly evaporates and raises the level of N ₂ O in the air. Air contamination with N ₂ O is particularly high when cryosurgery is used in a small, poorly ventilated office. These units should be scavenged.

Miscellaneous Sources Of Anesthetic Contamination

When a potent volatile anesthetic agent is used during cardiopulmonary bypass, the waste anesthetic vapor is discharged into the room air because scavenging of pump oxygenators affects their performance. Diffusion of anesthetic vapors from rubber and plastic goods in the anesthesia workroom is another cause of anesthetic contamination of the atmosphere. As much as 300 mL of anesthetic vapor may be released from a reusable rubber breathing circuit. Cardiopulmonary bypass machines are not sold with a scavenging system. Also, any vaporizer must be added in-house. A scavenging system should be set up from the exit port of the membrane oxygenator if inhaled anesthetic agents are used. ^,

A minor cause of anesthetic contamination is diffusion of anesthetic gases from the surgical wound and the patient’s skin. The concentration of halothane in the air immediately adjacent to the operating field was found to be three to six times higher than in room air. Higher levels were also found under the surgical drapes, possibly as a result of diffusion of halothane from the patient’s skin.

An infrequent cause of OR contamination is accidental crossing of the fresh air intake and exhaust ducts of the OR ventilation system. Also, installing the system exhaust port in a position upwind from the fresh air intake port may result in contamination of all ORs supplied by that ventilation system.

Another cause of OR contamination is failure to scavenge the exhaust from a sidestream-sampling capnograph or multigas analyzer. These monitors generally draw from 100 to 300 mL/min of gas via an adapter at the Y-piece in the breathing system. After the sampled gas has passed through the analyzer, it should be directed either to the waste gas scavenging system of the workstation or returned to the breathing system.

Operating Room Ventilation Systems

The OR ventilation system is an important factor in reducing anesthetic contamination of the OR atmosphere. Unventilated ORs may have trace levels of anesthetic gases that are four times as high as those with proper ventilation. The nine components of a typical OR ventilation system are as follows:

1.
Fresh air intake from outside
2.
Central pump
3.
Series of filters
4.
Air conditioning units
5.
Manifold that distributes fresh air to the OR
6.
Manifold that collects air from the OR
7.
Fresh air inflow port in each OR
8.
Exhaust port to the hospital air conditioning system in each OR
9.
Exhaust port to the outside

The OR fresh air inflow port is located in the ceiling, and the exhaust port is located on an adjacent wall 6 inches above the floor. In general, OR ventilation systems are one of two types: nonrecirculating or recirculating. The following sections describe the various features of each system.

Nonrecirculating Ventilation System

Most ORs are equipped with a nonrecirculating ventilation system ( Fig. 5.3 ). This type of system pumps in fresh air from the outside and removes and discards all stale air. The number of air exchanges per hour varies significantly, even among ORs in the same suite. A survey of one OR suite revealed that the number of air exchanges varied from less than 5 to more than 30 per hour.

The number of air exchanges per hour is an important determinant of the level of anesthetic contamination in the OR atmosphere. A rate of 15 or more exchanges per hour is recommended. Lower rates might permit creation of hot spots, air pockets highly contaminated by anesthetics; higher rates may create air turbulence that may cause discomfort to OR personnel.

The airflow pattern in the OR and the location of the anesthesia workstation in relation to the airflow also influence the level of anesthetic contamination ( Fig. 5.4 ). When airflow generates floor-to-ceiling eddies, causing extensive air mixing, hot spots are reduced in size and number. On the other hand, hot spots are more likely to form with laminar airflow, which reduces air mixing.

Fig. 5.4, Operating room air flow pattern with ceiling-to-floor eddy.

Recirculating Ventilation System

A recirculating ventilation system partially recirculates stale air. Each air exchange consists of part fresh outdoor air and part filtered and conditioned stale air. This is more economical than the nonrecirculating systems because it requires less air conditioning. The recirculating system is particularly popular in locations with extremely hot or cold climates. Because filtering does not cleanse air of anesthetic gases, a recirculating system may contaminate clean ORs by recirculating air from contaminated ORs to clean ORs.

The American Institute of Architects published recommendations for ventilation in ORs and PACUs. New ORs are required to have 15 to 21 air exchanges per hour, of which three must be fresh outside air. For PACUs in use, the minimum number of total air changes is six per hour with a minimum of two air changes per hour of outdoor air.

Positive- Versus Negative-Pressure Ventilation

Operating room ventilation systems must also take infection control into consideration. The ventilation pattern can affect the distribution of airborne particles from the patient’s skin and from OR personnel. Increasing the total air flow of a room is advantageous, because it has a dilutional effect on airborne particles. However, increased flow may also create turbulence, which increases the distribution of microbes. Low-velocity unidirectional flows are ideal for minimizing the distribution of microbes. Positive-pressure ventilation is typically used in ORs to create a protective environment with unidirectional flow from the OR outwards to the corridor. Negative-pressure ventilation with unidirectional flow inwards may be used in rare circumstances if there is a highly infective patient in the OR. Negative-pressure ventilation is commonly used in bronchoscopy suites.

Waste Gas Scavenging Systems

Modern anesthesia workstations are factory equipped with scavenging systems. Scavenging systems can be active (requiring active suction) or passive (relying on the passive flow of gas downstream out of the system). The Joint Commission (TJC; formerly known as The Joint Commission on Accreditation of Healthcare Organizations) requires that all WAGs be scavenged using active scavenging methods. Although it is highly unlikely, anesthesia machines that are not equipped with such systems may still be in use. Some anesthesia personnel arrange to discharge the waste anesthesia gases toward the floor, believing that the denser-than-air anesthetic gases form a layer on the floor and flow out via the positive-pressure OR ventilation system. In reality, WAGs are effectively mixed with room air by air eddies created by OR traffic. When the anesthesia machine is equipped with a properly functioning scavenging system, the trace concentrations of WAGs and vapors are reduced by 90%.

General Properties Of Scavenging Systems

An anesthesia waste gas scavenging system collects the WAGs from the breathing circuit and discards them. A properly designed and assembled system will not affect the dynamics of the breathing circuit, nor will it affect ventilation and oxygenation of the patient. The American Society for Testing and Materials (ASTM) document F1343-02, last published in 2002, provided standard specifications to serve as guidelines for the manufacturers of equipment that removes excess anesthetic gases from the working environment. The ASTM standard was superseded by the International Standard Organization (ISO) 8835-3 (2007) standard.

A typical scavenging system consists of four parts: (1) a relief valve through which gas leaves the breathing circuit, (2) conducting tubing to move the gas from the breathing circuit to a scavenging interface, (3) the scavenging interface, and (4) a disposal line. As mentioned previously, scavenging systems are classified as either active or passive. In an active scavenging system, a substantial negative pressure (hospital vacuum) is applied to the disposal line connected to the interface, and waste gas is sucked away from the interface. In a passive scavenging system, waste gases flow under their own pressure via a wide-bore tubing to the OR ventilation exhaust grille.

Relief Valves

During spontaneous or manually assisted ventilation with a circle breathing system, the WAGs leave the circuit via an APL, or “pop-off,” valve ( Fig. 5.5 ). Contemporary APL valves are spring- loaded and calibrated so that opening pressure can be adjusted by the user. When set to zero the valve requires only minimal positive pressure to open and allow escape of the waste gases from the circuit (see also Chapter 4 ). The valve has a single exhaust port ( Fig. 5.6 ). To prevent accidental connection to the breathing circuit, this exhaust port is usually a 19-mm male fitting with a 1:40 conical shape. Some older designs of APL valve were not spring- loaded or calibrated, rather they were needle valves in which the adjustment knob controlled the opening between the needle valve and its seat. The size of the needle valve opening controlled the rate of gas flow into the scavenging system and the inspiratory pressure during manually assisted (bag) ventilation.

Fig. 5.5, Points of exit for waste gas from a typical circle system. APL, Adjustable pressure-limiting valve; PEEP, positive end-expiratory pressure.

Fig. 5.6, Datex-Ohmeda adjustable pressure-limiting valve.

When an anesthesia ventilator is in use, the APL valve is out of circuit and the WAGs leave the circuit via the ventilator pressure relief (PR) valve (see Fig. 5.5 ). During inspiration, this valve is held closed by positive pressure transmitted from the ventilator driving gas circuit in bellows ventilators, or some other mechanism, as in the case of piston ventilators. The exhaust port of the PR valve is also 19 mm in diameter, as with the APL valve.

Conducting Tubing

The conducting tubing moves the WAGs from the APL and PR valve to the scavenging interface. Tubing is specified to be of 19 mm or 30 mm diameter to avoid accidental connection to the breathing circuit, and it is made sufficiently rigid to prevent kinking.

Scavenging Interface

The scavenging interface system is a safety mechanism equipped with relief valves interposed between the breathing circuit and the hospital’s vacuum or ventilation system. The interface is important for protecting the patient and the ventilator from excessive positive or negative pressure. A scavenging interface system may contain either a closed or an open reservoir.

Closed-Reservoir Scavenging Interface Systems

A closed-reservoir interface system includes a reservoir bag (different from the patient reservoir bag), which contains exhaled WAGs, and spring-loaded valves that prevent the hospital evacuation system from exerting excessively high or low pressures on the breathing circuit ( Fig. 5.7 ). The waste gases can be evacuated from the reservoir bag using either the hospital vacuum or ventilation system.

Fig. 5.7, Schematic of the Dräger scavenging safety interface. This is a closed-reservoir active scavenging system. A : Normal state of distension of the scavenger reservoir bag. B : Overdistended bag, due to excess delivery or inadequate removal of waste gases from the interface. C : Collapsed position of the bag due to application of excessive vacuum to the interface. DISS, Diameter index safety system.

When the hospital vacuum system is used to evacuate WAGs, a failure of the system or insufficient suction results in excessive pressure buildup in the reservoir. If this occurs a pop-off valve opens, and the WAGs are vented into the room. Such pop-off valves historically were set to open when the positive pressure in the reservoir reached 5 cm H ₂ O, but newer systems, such as the GE Aisys™ workstation, open at 10 cm H ₂ O. In contrast, if the hospital vacuum system generates excessive negative pressure, the negative-pressure relief, or “pop-in,” valve will open and allow room air to be sucked into the reservoir. This avoids application of negative pressure to the breathing circuit. The negative-pressure relief valves open inward when the pressure in the reservoir falls below −0.5 to −1.8 cm H ₂ O.

Fig. 5.8 depicts a closed-reservoir interface system evacuated by the hospital ventilation system. No vacuum is connected to the interface, and the needle valve is closed. This is an example of a passive scavenging system.

Fig. 5.8, GE-Datex-Ohmeda closed-reservoir scavenging interface used as a passive system. The vacuum is not connected (compare with Fig. 5.7 ), and gas flows passively to the operating room exhaust ventilation duct. Note: If a 30 mm scavenging hose is not available, a 30 mm to 19 mm reducing coupler may be used.

In addition to containing the WAGs during exhalation, the reservoir bag of a closed-reservoir scavenging interface provides a visual indication of whether the scavenging system is functioning properly (see Fig. 5.7 ). An overdistended bag indicates a weak vacuum or occluded disposal route. A collapsed bag indicates excessive vacuum. When the scavenging system is operating properly, the bag fills during exhalation and empties during inhalation.

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