Draw-over or push over during manual ventilation with the Tri-Service Anaesthetics Apparatus?

Abstract

The Tri-Service Anaesthetics Apparatus (TSAA) is a simple anaesthetic system designed by the UK military to be used in resource-poor environments 1. It was originally conceived as a draw-over system (gas pulled through the vaporiser by the patient or a self-inflating bag), but is increasingly used in a push-over mode (gas pushed through the vaporiser by a self-inflating bag or ventilator) for ergonomic reasons 2. It has been recognised that the push-over configuration can lead to agent over-delivery 3. In the traditional draw-over mode, the vaporiser is isolated from the self-inflating bag by the bag's inlet valve, which prevents retrograde gas flow. In the push-over mode, the position of the bag and vaporiser are reversed. Re-inflation of the bag draws the gas/agent mixture in a retrograde manner back through the vaporiser, as there is no outlet valve on the self-inflating bag. This leads to agent being added to the carrier gas mixture, and over-delivery with subsequent ventilation. Manufacturer's changes to the inlet port of the standard Laerdal Resuscitator bag (Laerdal Medical, Stavanger, Norway) means that it is no longer possible to assemble the system in the draw-over mode. We attempted to modify the push-over system to improve its accuracy by placing additional one-way valves proximally, distally or on both sides of the vaporiser (Oxford Miniature Vaporiser 50, Penlon, Abingdon, UK) to reduce reciprocal gas movement (Fig. 3). We manually ventilated a Maquet Test Lung 190 (Maquet Ltd, Sunderland, UK) targeting 500 ml tidal volumes at a frequency of 10 breaths per minute. The vaporiser was placed in a 20 °C water bath, filled with isoflurane and set to deliver an inspired fraction of 1%. Volatile concentration and gas volumes were measured using a G.E. Datex Ohmeda E sCAiOV module (General Electrical Healthcare, Milwaukee, USA). All push-over configurations over-delivered when compared with the draw-over setup. The push-over configuration that gave the most accurate performance had additional valves on both sides of the vaporiser, delivering 1.5% isoflurane, compared with 1.4% in the draw-over configuration and 1.8% in the standard push-over configuration. However, although the modified push-over system had similar accuracy to the draw-over system, the additional valves significantly increased resistance to ventilation, to the extent the system would not be practical for spontaneous ventilation. This configuration therefore cannot be recommended, and users of the TSAA in the push over configuration should be aware that volatile concentration is likely to be higher than expected.