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Draw-Over Apparatus

Draw-over systems refer to those breathing systems in which an inhalation anesthetic is vaporized by the patient's breathing. These include the original methods of administration of inhalation anesthetics, where the agent is poured over a pad or gauze which is then applied to the patient's face. Such methods are obsolete, with the possible exception of Cox's mask which may still be useful for field anesthesia of horses in rural areas and developing countries.

Modern Draw-Over Systems

The most simple form of modern drawover system consists of two reservoir tubes, a vaporizer and a non-rebreathing valve:

The patient (P) inspires and expires via the non-rebreathing valve (V). Air (A) enters the system from the atmosphere and may be supplemented with oxygen. The tubes provide reservoirs of oxygen (if it is being used) and anesthetic-containing gas.
Most systems incorporate a self-inflating bag (e.g. Ambu bag) so that ventilation can be controlled or assisted if necessary:

Draw-over systems are most useful in field anesthesia, developing countries and in other situations where supplies of compressed gases are not readily available. They are particularly useful if ether is used, owing to the lack of respiratory depression produced by this agent.


Comments on this article should be addressed to Dr Guy Watney
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