Chronic exposure to traces of nitrous oxide and/or halogenated agents constitutes a health hazard. There exists maximum levels for allowed exposure but they are different in different countries. As an example (UK, 1998) a mean exposure during 8 hours should not be higher than 10 ppm for halothane, 50 ppm for isoflurane or 100 ppm for nitrous oxide. The allowed exposure is less in the USA.

Classic sources of pollution are the anaesthetic machine itself, the patient circuit, disconnections, filling of vaporizers, improper fitting tubes, and use of the mask. The exhalation of agent by the patient in the recovery phase is not to be forgotten and can reach relatively high levels for instance in a small recovery box of a horse in absence of air conditioning. Nowadays simple hand held apparatus to measure halogenated pollution and indicator-badges have been developed but not yet very popular in veterinary clinics.

Therefore all the equipment should be controlled, the use of a mask as connection of the patient with the anaesthetic circuit avoided, vaporisers should have the key-filler option only, and low-flow or closed system anaesthesia should be considered when appropriate. A good scavenging policy is to be put in place together with sufficient air conditioning and education/sensibilisation of the staff. A possibility is to switch over to total intravenous anaesthesia in stead of inhalation anaesthesia. An additional option is to avoid the use of nitrous oxide.

Pollution of the environment by anaesthetic gas is a common problem because the amount of gas used to anaesthetise patients commonly far exceeds the patient's needs. Scavenging is the collection and the subsequent removal of vented gases from the operation room.

Scavenging systems have in principle five components: 1) the gas collecting assembly; 2) the transfer tubing; 3) the scavenging interface; 4) the gas disposal tubing; 5) an active or passive gas disposal assembly.

An active system uses a central vacuum to eliminate the gases whereas in a passive system the pressure of the waste gas itself produces flow.

The gas collecting assembly

Excess gas containing anaesthetic leaves the breathing circuit via the pop-off valve into the gas collection assembly. Modern pop-off valves are equipped with a side-port to collect excess gas. Another source of vented gas is the pop-off valve of the ventilator. One needs to locate the evacuation port for these gases on the ventilator. When half-open anaesthesia systems are used without a pop-off valve the scavenging is more difficult to organise.

Scavenging can be done without physical connection with the anaesthetic breathing circuit and only with an active system acting as a vacuum cleaner but this is much less efficient.

Transfer tubing

This tube carries excess gas from the gas collecting assembly to the scavenging interface.

Scavenging Interface

This is the most important component of the system because it protects the breathing circuit or ventilator from excessive positive or negative pressure. An open interface contains no valves and is open to the atmosphere allowing both positive and negative pressure. 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 also mandatory when an active system is used.

Gas disposal tubing

This tubing conducts vented gas to the gas disposal assembly.

Gas disposal assembly

When an active system is used a reservoir is very desirable and the larger the reservoir the lower the suction flow need to be. With a passive disposal assembly negative pressure relief and a reservoir are unnecessary. They vent through the wall, ceiling, floor or to the grill of a non-recirculating air conditioning system. Another possibility is the venting through a charcoal canister. Charcoal will absorb the halogenated agents but not nitrous oxide. Exhaustion of the absorption capacity for halogenated agents is indicated by the weight of the canister.

Hazards of scavenging systems

Obstruction of the scavenging pathway can cause positive pressure to be transmitted to the breathing system because excess patient gas cannot be vented. Excessive negative pressure can be applied to the breathing system if an active disposal system is used. The consequence depends on the actual design of the pop-off valve. With some valve/scavenging system combinations the patient part of the anaesthetic circuit will be emptied from anaesthetic gas mixture and the breathing bag will collapse. Depending on the design a positive pressure can build up in the patient circuit because the pop-off valve is blocked in the closed position. Some scavenging systems have several built-in safety options including fine tuning of flow and negative pressure in the scavenging system and a "back-up system consisting of an integrated charcoal canister in case of scavenging system failure.

Summary of measures to avoid theatre anaesthetic pollution

The presence and use of a scavenging system is not a guarantee for a clean area. Therefore: avoid face mask-anaesthesia and induction chambers when possible; b) use correctly inflated cuffed endotracheal tubes; c) check the scavenging system function before you start; d) do not prefill your circuit with anaesthetic or nitrous oxide unless the system is kept completely closed (including pop-off valve and y-piece); e) flush the circuit with oxygen before disconnecting; f) use key fillers by preference and if not available, fill vaporisers at a moment when the room can be well ventilated; g) redirect the outlet flow of an anaesthetic gas analyser again in the system or directly in the scavenging system; h) organise regularly a check of the situation by measuring of pollution levels during a normal working day.

References

1.  Umgang mit Anasthesiegassen Gefahrdung, Schutzmassnahmen. M. Rilegger, M Jost, A. Meier, R Knutti, C. Schlatter. Schweizerische Unfallversicherungsanstalt, Abteilung Arbeitzmedizin, 6002 Luzern, Switzerland Dokument 2869/29.d

2.  British Standard Specifications for Active Scavenging Systems, BS 6834 London BSI 1987

3.  Scavenging Systems. Lumb and Jones' Veterinary Anesthesia 3rd Edition, 1996, p 401-404.