Mr G Kamm,
Anaesthetist, Muchami Hospital, Tanzania.
Dr IH Wilson,
Consultant Anaesthetist.

Introduction		Bellows
Differences to Boyles machine		Expiratory valves
Function of components		Oxygen T attachment
Types of vaporizers

In many parts of the world a regular supply of compressed anaesthetic gases cannot be maintained. Shortages of nitrous oxide are common and in some places oxygen is also unavailable. Anaesthetists working in such environments, whether in a developing country or in a disaster situation, may still be faced with patients requiring surgery necessitating techniques of anaesthesia not dependent on a supply of compressed gases. Suitable techniques include drawover anaesthesia, local anaesthesia and ketamine anaesthesia. This article considers the theory of drawover anaesthesia. Future editions of Update will contain articles covering the use of drawover systems for both adults and children, and care of the apparatus.

 
How does drawover anaesthesia differ from anaesthesia given with a Boyles machine?

During anaesthesia using a Boyles machine (figure 1), compressed gases (oxygen and nitrous oxide or air) pass from cylinders mounted on the machine to rotameters, (a type of flow meter for gases), and then through the vaporizer where a volatile agent such as halothane is added to the gas mixture. The resulting mixture is delivered to the patient via an anaesthetic circuit, such as the Magill system. This type of anaesthesia system, known as "a continuous flow apparatus", is dependent on a supply of compressed gases. If these run out during an operation, so does the anaesthetic!

A drawover system (figure 2) is designed to provide anaesthesia without requiring a supply of compressed gases. Atmospheric air is used as the main carrier gas and is drawn by the patient's inspiratory effort through the vaporizer, where the volatile agent, normally ether or halothane, is added. The mixture is then inhaled by the patient via a non-rebreathing valve. The components of a drawover circuit are illustrated in figure 2.

Features of drawover apparatus:

1. Robust, compact and portable
2. Low purchase price and running costs
3. Straightforward maintenance
4. Not dependent on compressed gases

During drawover anaesthesia the patient moves, (or "draws"), air through the vaporizer which must have a very low resistance to the intermittent gas flow which is generated. The volume of air passing through the vaporizer is determined by the patient's tidal volume (the volume of air in a single breath) and the respiration rate. Considerable variations in flow through the vaporizer occur, depending on the type and depth of anaesthesia, the age of the patient and whether the patient is breathing spontaneously or being artificially ventilated. These conditions of gas flow require the drawover vaporizer to be specially designed.

Vaporizers designed for continuous flow anaesthesia should never be used in a drawover system as the high internal resistance to gas flow is too great. They are designed to work under a continuous high pressure and flow, and are called plenum vaporizers.

As air flows into the vaporizer it is directed either to the vaporizing chamber where it collects vapour from the volatile agent being used, or into a bypass chamber which does not come into contact with the volatile agent (figure 3). The air from the two chambers mixes as it leaves the vaporizer.

The ratio of air flow going to the different chambers determines the final concentration of volatile agent leaving the vaporizer, and is determined by the concentration control. The process of vaporisation removes heat from the volatile agent and vaporizer, due to the latent heat of vaporisation. This heat loss reduces the efficiency of vaporisation, and may result in a fall in concentration of volatile agent being delivered by the vaporizer. Some vaporizers compensate for cooling by a temperature operated valve which automatically increases the ratio of air directed through the vaporizing chamber as cooling occurs. Vaporizers with this facility are said to be thermo-compensated. Other vaporizers partially compensate for heat loss by containing a substance (such as water or copper) which delay changes in vaporizer temperature by providing a reservoir of heat. Vaporizers using this system are described as thermally buffered. Some vaporizers, such as the EMO, utilise both systems.

(Continued...)

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