Drawover Vaporisers
The drawover vaporiser is driven by negative pressure developed by the patient, and must therefore have a low resistance to gas flow. Its performance depends on the minute volume of the patient: its output drops with increasing minute ventilation.
The design of the drawover vaporiser is much simpler: in general it is a simple glass reservoir mounted in the breathing attachment. Drawover vaporisers may be used with any liquid volatile agent (including older agents such as diethyl ether or chloroform, although it would be dangerous to use desflurane). Because the performance of the vaporiser is so variable, accurate calibration is impossible. However, many designs have a lever which adjusts the amount of fresh gas which enters the vaporising chamber.
The drawover vaporiser may be mounted either way round, and may be used in circuits where re-breathing takes place, or inside the circle breathing attachment.
Drawover vaporisers typically have no temperature compensating features. With prolonged use, the liquid agent may cool to the point where condensation and even frost may form on the outside of the reservoir. This cooling impairs the efficiency of the vaporiser. One way of minimising this effect is to place the vaporiser in a bowl of water.
The relative inefficiency of the drawover vaporiser contributes to its safety. A more efficient design would produce too much anaesthetic vapour. The output concentration from a drawover vaporiser may greatly exceed that produced by a plenum vaporiser, especially at low flows. For safest use, the concentration of anaesthetic vapour in the breathing attachment should be continuously monitored.
Despite its drawbacks, the drawover vaporiser is cheap to manufacture and easy to use. In addition, its portable design means that it can be used in the field or in veterinary anaesthesia.
Read more about this topic: Anaesthetic Vaporiser