Oxford Miniature Vaporiser for halothane in ventilated asthmatics

Introduction

Critically ill asthmatics that require mechanical ventilation may benefit from halothane and other inhalational agents. Various methods of administration of halothane have been tried. Anaesthetic machines are commonly used but are resource-expensive.

Methods

We used a simple in-line Oxford Miniature Vaporiser (OMV), as part of the inspiratory limb of a Servo 300 (Siemens) mechanical ventilator. We employed this device in three patients for a total duration of 120 hours without adverse effects.

Results

The OMV is a small and portable thermally buffered vaporiser used to speed the induction of anaesthesia (Fig. 1). 'Draw over anaesthesia' is simple in concept and entails drawing a carrier gas over a volatile liquid, thus entraining its vapour to the gaseous carrier. 'Draw over' systems operate at less than, or at, ambient pressure, and flow through the system is intermittent, varying with different phases of inspiration, and ceasing in expiration. A one-way valve prevents reverse flow in the circuit. This is different to 'plenum anaesthesia' in which a carrier gas is pushed through the vaporiser at a constant rate. In 'draw over' systems the carrier gas is drawn through the vaporiser either by the patient's own respiratory efforts or by a self-inflating bag or manual bellows with a one-way valve placed downstream from the vaporiser.

Figure 1

(abstract P62)

Conclusion

We used the OMV as part of a regular positive pressure ventilatory circuit. The OMV was specially calibrated for halothane and was robust and reliable. Halothane delivery was controlled and in steady fashion; a simple wall-suction unit scavenged the exhaled halothane. Staff acceptance and user-friendliness were high. We recommend the OMV for regular ICU use for halothane in asthma.