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Anesthesia Service and Equipment


Scavenging of waste anesthetic gases

All operating rooms and areas in which inhalation anesthesia is carried out should be equipped with a scavenging system in order to reduce the exposure of operating room personnel to waste anesthetic agents vented from the breathing system.

Breathing system outlet

Excess gas is usually (except in the case of Mapleson E and F systems) vented from the breathing system via an adjustable pressure-limiting (or pop-off) valve.

The valve disc is held in place by a weak spring, the tension on which can be adjusted by a screw machanism, thereby adjusting the pressure required to open the valve.
     
The outlet of the breathing system must be connected to the scavenging system and not simply vented into the operating room. Modern adjustable pressure relief valves incorporate a 19 mm outlet for this purpose:

 
Old expiratory valve with no scavenging outlet.   Modern expiratory valve with 19 mm scavenging outlet.

Types of scavenging system

Charcoal canisters

A canister containing activated charcoal is connected to the outlet of the breathing system and removes halogenated anesthetics by filtration. It is replaced after every 12 hours of use.

Advantages
  No set-up cost
Mobile - moves with the machine
Disadvantages
  Continuing cost of replacement. The canisters can be refilled with fresh charcoal, but this tends to be a rather messy undertaking
Does not remove nitrous oxide

Passive systems
The waste gases can be ducted out of the building via:
 

An open window
A pipe passing though an outside wall
An extractor fan vented to the outside air (N.B. not into the building's air-conditioning system)

Advantages
  Inexpensive to set up
Simple to operate
Disadvantages
  May be impractical in some buildings

Active systems

These systems connect the exhaust of the breathing system to the Hospital vacuum system via an interface controlled by a needle valve.

The Ohio scavenging interface has connections for the outlets from the breathing system and ventilator (A), one or two reservoir bags (B), and the vacuum line (V). The suction is controlled by the needle valve (N). There are both positive (P1) and negative (P2) pressure relief valves in case the reservoir bag becomes empty or too full.
      A home-built interface can be constructed quite easily
and inexpensively from parts obtained at any hardware store:

This consists of a reservoir made of plastic pipe of around 3" in diameter and 2 feet in length, capped at both ends and with a needle valve fitted at the bottom. Two holes are drilled in the top cap, one to allow the hose from the outlet of the anesthetic machine to be inserted and the other to allow passage of atmospheric air. The valve is adjusted so that air is slowly sucked into the pipe when the anesthetic machine is in use: this will prevent waste gases escaping into the atmosphere.

Advantages
  Convenient in large hospitals where many machines are in use in different locations
Disadvantages
 

Vacuum system and pipework is a major expense
Needle valve may need continual adjustment

Comments on this article should be addressed to Dr Guy Watney
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