The most widely available drawover vaporizers are the EMO (Epstein, Macintosh, Oxford), OMV (Oxford Miniature vaporizer) and the TEC series (previously known as the PAC series). A few details of the vaporizers are set out below; an article in the next issue of Update will describe their use more fully.

The EMO (figure 2) is a temperature compensated vaporizer which produces an accurate output of 0 to 20% ether. It is usually used in conjunction with the Oxford Inflating Bellows (OIB) which is incorporated as a part of the EMO system. Manufacturer Penlon (UK) Ltd.

The OMV (figure 4) is a small thermally buffered vaporizer which was originally produced to be used together with the EMO in order to speed the induction of anaesthesia. Original models contained only 20mls of volatile agent, more modern ones 50mls. A variety of volatile agents may be used with the OMV including halothane, trichlorethylene, enflurane, methoxyflurane and isoflurane. Different scales are available for each agent so that after draining the vaporizer the anaesthetist may use a different volatile agent. Manufacturer Penlon (UK) Ltd.

The TEC or PAC (figure 5) vaporizers consist of a range of thermo-compensated drawover vaporizers with different models available for ether, halothane, methoxyfiurane and trichlorethylene. Manufacturer-Ohmeda (UK) Ltd.

 
Self-inflating bags or bellows allow controlled ventilation of the patient during anaesthesia or resuscitation. They should be the correct size for the patient to allow for an adequate tidal volume. Bellows and self-inflating bags incorporate a non-return valve through which they fill ensuring that fresh gas is always delivered to the patient. When there is an oxygen port on the bag or bellows this should be occluded, and oxygen added through a separate T piece (figure 2). Self-inflating bags and bellows are used with a non-rebreathing expiratory valve at the patient end to allow inspiration from the bag and expiration to atmosphere.

The Oxford inflating bellows (OIB) is popular with many anaesthetists using drawover anaesthesia. Unlike the self- inflating bags the OIB can be seen to move during spontaneous respiration. Two non-return flap valves are contained in the base of the OIB. The distal flap valve needs to be immobilised when the OIB is used with one of the non-rebreathing valves mentioned below. A magnet is supplied with the bellows for this purpose and its use will be fully described in a future article Practical drawover anaesthesia.

Connecting tubing should be of the antistatic type when ether is used and connections conform to the international standards of 22mm and 15mm tapered connections.

Patient expiratory valve. This should be a non-rebreathing valve such as an AMBU El, Laerdel or Rubin's valve (figure 6). These valves allow either spontaneous or controlled respiration without adjustment. They need regular cleaning to prevent them becoming sticky and should be resterilised if used with a patient with chest infection.

 
Oxygen T attachment. To add oxygen to a drawover system a standard T piece is mounted on the intake side of the vaporizer (figure 2). If it is mounted on the output side of the vaporizer a dilution of the volatile agent will occur. A reservoir tube (at least a metre in length) allows oxygen to accumulate during the expiratory phase. An oxygen flow of one litre/minute results in an inspired oxygen concentration of around 30-40% and a flow of four litres/minute a concentration of 60-80%.

This article contained links to the following additional information:

Issue 3 - Draw-over Anaesthesia Part 2 - Practical Application
Issue 3 - Draw-over Anaesthesia Part 3 - Looking After Your Own Apparatus

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