Minimum oxygen requirements during anaesthesia with the Triservice anaesthetic apparatus. A study of drawover anaesthesia in the young adult.

Thirty-six servicemen were anaesthetised using the Triservice anaesthetic apparatus. They were allocated randomly into one of two groups, to breathe spontaneously or to receive artificial ventilation, and into subgroups who were given air alone, or air supplemented with 1 or 4 litres/minute of oxygen. A further 12 subjects were studied subsequently using 0.5 litres/minute of added oxygen. Intra-operative blood gases were compared with those of awake premedicated controls. Artificial ventilation was associated with an unchanged arterial oxygen tension with air alone; in the other subgroups arterial oxygen tension was higher than with spontaneous respiration when related to inspired oxygen fraction (p less than 0.05). Air anaesthesia caused significant hypoxaemia with spontaneous ventilation (p less than 0.05), and 50% of the subjects required assisted ventilation. There was also a significant respiratory acidosis (p less than 0.05). Intermittent positive pressure ventilation is the method of choice for field anaesthesia when oxygen is unavailable. Spontaneous respiration must be supplemented with at least 0.5 litres minute of oxygen.