Modification of a draw-over vaporizer for use with sevoflurane

Draw-over anaesthesia is widely used throughout the developing world, in disaster areas and in military anaesthesia when the supply of pressurized oxygen is unreliable. To date, no draw-over vaporizer has been able to deliver sufficient concentrations of sevoflurane for use in inhalation induction of anaesthesia. A laboratory study to assess the performance of a new vaporizer (DDV2) to deliver sevoflurane in a wide range of situations is described.

In this study, the concentration of sevoflurane delivered at different dial settings (1-4%) and at different temperatures (20-40°C) in a draw-over mode was measured. The concentration of sevoflurane delivered at different dial settings with continuous flow (6 and 8 litre min(-1)) at 20°C was measured. The maximum possible concentration of sevoflurane that can be delivered by the DDV2 was measured at a continuous flow rate of 8 litre min(-1) at 20, 30, and 40°C.

Concentrations of sevoflurane delivered in the draw-over mode were within 0.5% of dialled setting up to 30°C. Above this temperature, higher levels of vapour were delivered. With continuous flow, concentrations of sevoflurane at 20°C were within 0.5% of dialled setting and were stable throughout the duration of the experiment. On the 'induction' setting, concentrations of sevoflurane of between 6.4% and 10.1% could be delivered with continuous flow.

The modifications to the DDV2 allow stable concentrations of sevoflurane to be delivered in draw-over and continuous flow modes over a range of temperatures. With continuous flow, concentrations of sevoflurane sufficient for induction of anaesthesia can be achieved.