Anaesthetic gases are considered to be substances hazardous to health for the purposes of the Control of Substances Hazardous to Health Regulations 1988 (COSHH), except where they are administered to a patient in the course of medical treatment.
The Anaesthetic Gas Scavenging System (AGSS) is a system that actively removes anaesthetic gas mixtures from operating theatres and any other areas fitted with nitrous oxide terminal units. Gas scavenging systems are the first line of defence and the preferred method of control to protect employees from exposure to anaesthetic gases.
In the mid 1970's, human studies testing the cognitive and the motor skills of male subjects/volunteers, showed that exposure to concentrations of anaesthetic gas mixtures commonly found in the unscavenged operating room, resulted in decreased ability to perform complex tasks. The weight of the evidence regarding potential health risks from exposure to anaesthetic agents in unscavenged environments suggested that clinicians needed to be concerned. Moreover, there is biological plausibility that adds to the concern that high levels of unscavenged waste anaesthetic gases present a potential for adverse neurological effects or reproductive risk to exposed workers or developmental anomalies in their offspring.
Over the past 50 years there have been many improvements with regard to the management and control of anaesthetic gas pollution in hospital and other health facilities. Improvements have come about through the increased use of Anaesthetic Gas Scavenging Systems (AGSS), installation of more effective general ventilation systems, and increased attention to equipment maintenance and leak detection as well as to careful anaesthetic practice.
The circle anaesthesia system shown below is the breathing system most commonly used in operating theatres.
The essential components of a circle re-breathing system include a site for inflow of fresh gas (common/fresh gas inlet), a carbon dioxide absorber canister (containing soda lime or barium hydroxide lime) where exhaled carbon dioxide is absorbed; a reservoir bag; inspiratory and expiratory unidirectional valves; flexible corrugated breathing tubing; an adjustable pressure-limiting (APL) or "pop-off" valve for venting excess gas to the AGSS system; and a "Y" piece that connects the patient via a face mask, tracheal tube, laryngeal mask airway (LMA) or other airway management device.
The exhausted gases from the breathing circuit are processed by the AGSS system. This includes exhaust gases from any side-stream sampling respiratory gas analysers and/or capnographs. All exhausted gases are connected to the AGSS system because they may contain N2O or halogenated vapours.
An AGSS scavenging system normally consists of five basic components:
- A gas collection assembly such as a collection manifold, which captures excess anaesthetic gases at the site of emission, and delivers it to the transfer tubing.
- Transfer tubing, which conveys the excess anaesthetic gases to the interface.
- The interface, which provides positive (and sometimes negative) pressure relief and may provide reservoir capacity. It is designed to protect the patient's lungs from excessive positive or negative scavenging system pressure.
- Gas disposal assembly tubing, which conducts the excess anaesthetic gases from the interface to the gas disposal assembly.
- The gas disposal assembly, which conveys the excess gases to a point where they can be discharged safely into the atmosphere. Several methods in use include a nonrecirculating or recirculating ventilation system, a central vacuum system, a dedicated (single-purpose) waste gas exhaust system, a passive duct system, and an absorber.
In general, a machine-specific interface must be integrated with a facility's system for gas removal. The interface permits excess gas to be collected in a reservoir (bag or canister) and limits the pressure within the bag or canister. A facility's gas disposal system receives waste anaesthetic gases from the interface and should vent the waste gases outside the building and away from any return air ducts or open windows, thus preventing the return of the waste gases back into the facility.
Occupational exposure to waste gases may still occur. AGSS is used to enhance the safety of the environment in which operating theatre staff work, where they may be in close proximity with waste anaesthetic gases and vapours (agents).
Removal of excess anaesthetic gases from the anaesthesia circuit can be accomplished by either active or passive scavenging. When a vacuum or source of negative pressure is connected to the scavenging interface, the system is described as an active system. When a vacuum or negative pressure is not used, the system is described as a passive system. With an active system there will be a negative pressure in the gas disposal tubing. With a passive system, this pressure will be increased above atmospheric (positive) by the patient exhaling passively, or manual compression of the breathing system reservoir bag.
Use of a central vacuum system is an example of an active system: The waste anaesthetic gases are moved along by negative pressure. Venting waste anaesthetic gas via the exhaust grille or exhaust duct of a nonrecirculating ventilation system is an example of a passive system: The anaesthetic gas is initially moved along by the positive pressure from the breathing circuit until it reaches the gas disposal assembly.
Before extubating the patient's trachea or removing the mask or other airway management device, non-anaesthetic gases should be used to wash-out anaesthetic gases still in the patient circuit, that can then be removed by the scavenging system.
Regular maintenance of AGSS systems should be carried out to ensure there are no leaks and staff are not put at unnecessary risk. An AGSS system is the most effective means of controlling waste anaesthetic gases.
Edited by John Sandham.