A reservoir bag for the Triservice anaesthetic apparatus

Authors


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Surgeon Commander Birt has made some useful suggestions concerning the Triservice apparatus [1]. The original Triservice anaesthetic apparatus [2] was supplied with the Houtonox oxygen regulator (Penlon Ltd, Abingdon, UK). This was a compact and rugged flow regulator with a strong metal casing that was accurate and independent of position. It had two preset flow-rates; 1 l.min−1 minute and 4 l.min−1 being the values chosen for production purposes to conserve precious supplies of oxygen. Houghton showed that supplementation with oxygen at 4 l.min−1 would be likely to raise the inspired oxygen concentration to about 60%. This was over 30 years ago, well before the introduction of the pulse oximeter. Later, Wilson, van Heerden and Leigh [3] demonstrated that higher levels of inspired oxygen are required for effective pre-oxygenation, and Lowe and McFadzean [4] quantified this for the Triservice anaesthetic apparatus (44 s of apnoea to 95% saturation after pre-oxygenation using 4 l.min−1 with the Triservice anaesthetic apparatus and 173 s using the Bain breathing attachment). For efficient pre-oxygenation the inspired oxygen concentration should be increased to 100% and Birt has suggested one way to do this. An alternative was proposed earlier by Johnson [5] who prefilled a 40-litre plastic bag attached to the end of the oxygen reservoir of the Triservice anaesthetic apparatus for pre-oxygenation. This method is effective when the oxygen flow rate is limited as with the Houtonox regulator. However, other pre-oxygenation studies [6–8] have demonstrated that three maximal deep-inspirations are as effective as 3–4 min of normal respiration using pure oxygen. By using a 20-litre bag prefilled with oxygen, effective pre-oxygenation can be achieved using three maximal deep-inspirations assuming a vital capacity of 6 litres.