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Scavenging is the collection and the subsequent removal of vented gases from the operating room.[151] The amount of gas used to anesthetize a patient commonly far exceeds the patient's needs. Scavenging minimizes operating room pollution. In 1977, the National Institute for Occupational Safety and Health (NIOSH) prepared Criteria for a Recommended Standard: Occupational Exposure to Waste Anesthetic Gases and Vapors.[152] Although it was maintained that a safe level of exposure could not be defined,
| Anesthetic Gas | Maximum TWA * Concentration (ppm) |
|---|---|
| Either agent alone |
|
| • Halogenated agent alone | 2 |
| • Nitrous oxide | 25 |
| Combined halogenated agent and nitrous oxide |
|
| • Halogenated agent | 0.5 |
| • Nitrous oxide | 25 |
| Dental facilities (nitrous oxide alone) |
|
| • Nitrous oxide | 50 |
| NIOSH, National Institute for Occupational Safety and Health; ppm, parts per million; TWA, time-weighted average. | |
| Adapted from US Department of Health, Education, and Welfare: Criteria for a Recommended Standard: Occupational Exposure to Waste Anesthetic Gases and Vapors. Washington, DC, US Department of Health, Education, and Welfare, March, 1977. | |
Figure 9-27
Components of a scavenging system. APL valve, adjustable
pressure limiting valve. (Modified from Andrews JJ, Brockwell RC: Delivery
systems for inhaled anesthetics. In Barash PG, Cullen
BF, Stoelting RK [eds]: Clinical Anesthesia, 4th ed. New York, Lippincott-Raven,
2000, pp 567–594.)
The two major causes of waste gas contamination in the operating room are the anesthetic technique employed and equipment issues.[154] [155] Several factors related to the anesthetic technique cause operating room contamination: failure to turn off the gas flow control valves at the end of an anesthetic, poorly fitting masks, flushing the circuit, filling anesthetic vaporizers, use of uncuffed endotracheal tubes, and use of breathing circuits such as the Jackson-Rees, which is difficult to scavenge. Equipment failure or lack of understanding the equipment can cause operating room contamination. Leaks can occur in the high-pressure hoses, the mounting for the nitrous oxide tank, the high-pressure circuit and low-pressure circuit of the anesthesia machine, or in the circle system, particularly the carbon dioxide canister. The anesthesia care provider must be certain that the scavenging system is operational and adjusted properly to ensure adequate scavenging. If sidestream carbon dioxide or multigas analyzers are used, the analyzed gas (50 to 250 mL/min) must be directed to the scavenging system or returned to the breathing system.[154] [155]
Scavenging systems have five components ( Fig. 9-27 ): (1) the gas-collecting assembly, (2) the transfer means, (3) the scavenging interface, (4) the gas-disposal assembly tubing, and (5) an active or passive gas-disposal assembly.[153] An active system uses a central vacuum to eliminate waste gases. The pressure of the waste gas itself produces flow through a passive system.
The gas-collecting assembly captures excess anesthetic gas and delivers it to the transfer tubing.[153] Waste anesthetic gases are vented from the anesthesia system through the APL valve or through the ventilator's relief valve. All excess gas from the patient exits the breathing system through these valves, accumulates in the gas-collecting
The transfer means carries excess gas from the gas-collecting assembly to the scavenging interface. The tubing must be 19 or 30 mm, as specified by the ASTM F1343-91 standard.[153] The tubing should be sufficiently rigid to prevent kinking and as short as possible to minimize the chance of occlusion. Some manufacturers color code the transfer tubing with yellow bands to distinguish it from the 22-mm tubing of the breathing system. Many machines have separate transfer tubes for the APL valve and for the ventilator's relief valve. The two tubes frequently merge into a single hose before they enter the scavenging interface. Occlusion of the transfer means can be particularly problematic because it is upstream from the pressure-buffering scavenging interface. If the transfer means is occluded, pressure in the breathing circuit increases, and barotrauma can occur.
Figure 9-28
Each of the two open scavenging interfaces requires an
active disposal system. An open canister provides reservoir capacity. Gas enters
the system at the top of the canister and travels through a narrow inner tube to
the canister base. Gases are stored in the reservoir between breaths. Relief of
positive and negative pressure is provided by holes in the top of the canister.
A and B, The open interface
shown in A differs somewhat from the one shown in
B. The operator can regulate the vacuum by adjusting
the vacuum control valve shown in B. APL, adjustable
pressure limiting valve. (Adapted from Dorsch JA, Dorsch SE: Controlling
trace gas levels. In Dorsch JA, Dorsch SE [eds]:
Understanding Anesthesia Equipment, 4th ed. Baltimore, Williams & Wilkins,
1999, p 355.)
The scavenging interface is the most important component of the system because it protects the breathing circuit or ventilator from excessive positive or negative pressure.[151] The interface should limit the pressures immediately downstream from the gas-collecting assembly to between -0.5 and +10 cm H2 O with normal working conditions.[153] Positive-pressure relief is mandatory, irrespective of the type of disposal system used, to vent excess gas in case of occlusion downstream from the interface. If the disposal system is active, negative-pressure relief is necessary to protect the breathing circuit or ventilator from excessive subatmospheric pressure. With active systems, a reservoir is highly desirable because it stores excess waste gas until the evacuation system can eliminate it. Interfaces can be open or closed, depending on the method used to provide positive-pressure and negative-pressure relief.[151]
An open interface contains no valves and is open to the atmosphere, allowing positive-pressure and negative-pressure relief. Open interfaces should be used only with active disposal systems that use a central vacuum system. Open interfaces require a reservoir because waste gases are intermittently discharged in surges, whereas flow to the active disposal system is continuous.[151]
Many contemporary anesthesia machines are equipped with open interfaces like those in Figure 9-28 . [156] An open canister provides reservoir capacity. The canister volume should be large enough to accommodate a variety of waste gas flow rates. Gas enters the system at the top of the canister and travels through a narrow inner tube to the canister base. Gases are stored in the reservoir
The efficiency of an open interface depends on several factors. The vacuum flow rate per minute must equal or exceed the minute volume of excess gases to prevent spillage. The volume of the reservoir and the flow characteristics within the interface are important. Spillage will occur if the volume of a single exhaled breath exceeds the capacity of the reservoir. Leakage can occur long before the volume of waste gas delivered to the reservoir equals the reservoir volume if large-scale turbulence occurs within the interface.[157]
A closed interface communicates with the atmosphere through valves. All closed interfaces must have a positive-pressure relief valve to vent excess system pressure if obstruction occurs downstream from the interface. A negative-pressure relief valve is mandatory to protect the breathing system from subatmospheric pressure if an active disposal system is used.[151] Two types of closed interfaces are commercially available. One has positive-pressure relief only; the other has positive-pressure and negative-pressure relief. Each type is discussed in the following sections.
Figure 9-29
Closed scavenging interfaces. Interface used with a
passive disposal system (left). Interface used with
an active system (right). (Left,
Adapted from Scavenger Interface for Air Conditioning: Instruction Manual. Telford,
PA, North American Dräger, October 1984; right,
Adapted from Narkomed 2A Anesthesia System: Technical Service Manual. Telford,
PA, North American Dräger, 1985.)
This interface ( Fig. 9-29, left ) has a single positive-pressure relief valve that is designed to be used only with passive disposal systems. Waste gas enters the interface at the waste gas inlets. Transfer of the waste gas from the interface to the disposal system relies on the pressure of the waste gas itself because a vacuum is not used. The positive-pressure relief valve opens at a preset value, such as 5 cm H2 O, if an obstruction between the interface and the disposal system occurs.[158] A reservoir bag is unnecessary.
This interface has a positive-pressure relief valve, at least one negative-pressure relief valve, and a reservoir bag. It is used with active disposal systems. Figure 9-29 (right) is a schematic drawing of North American Dräger's closed interface for suction systems. A variable volume of waste gas intermittently enters the interface through the waste gas inlets. The reservoir stores transient excess gas until the vacuum system eliminates it. The operator should adjust the vacuum control valve so that the reservoir bag is properly inflated (A) and not over distended (B) or completely deflated (C). Gas is vented to the atmosphere through the positive-pressure relief valve if the system pressure exceeds +5 cm H2 O. Room air is entrained through the negative-pressure relief valve if the system pressure is less than -0.5 cm H2 O. A backup negative-pressure relief valve opens at -1.8 cm H2 O if the primary negative-pressure relief valve becomes occluded.[19]
The effectiveness of a closed system in preventing spillage depends on the inflow rate of excess gas, the vacuum flow rate, and the volume of the reservoir. Leakage of waste gases into the atmosphere occurs only when the reservoir bag becomes fully inflated and the pressure increases sufficiently to open the positive-pressure relief valve. The effectiveness of an open system to prevent spillage depends on the volume of the reservoir and on the flow characteristics within the interface. [157]
The gas-disposal assembly tubing (see Fig. 9-27 ) conducts waste gas from the scavenging interface to the gas-disposal assembly. It should be collapse-proof and should run overhead, if possible, to minimize the chance of occlusion.[153]
The gas-disposal assembly ultimately eliminates excess waste gas (see Fig. 9-27 ). There are two types of disposal systems: active and passive. The most common method of gas disposal is the active assembly, which uses a central vacuum. The vacuum is a mechanical, flow-inducing device that removes the waste gases. An interface with a negative-pressure relief valve is mandatory because the pressure within the system is negative. A reservoir is very desirable, and the larger the reservoir, the lower the suction flow rate needed.[151] [157]
A passive disposal system does not use a mechanical, flow-inducing device. Instead, the pressure of the waste gas itself produces flow through the system. Positive-pressure relief is mandatory, but negative-pressure relief and a reservoir are unnecessary. Excess waste gas can be eliminated in several ways, including venting through the wall, ceiling, or floor or venting to the room exhaust grill of a non-recirculating air conditioning system.[151] [157]
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