OBSERVATIONS ON THE CEREBRAL EFFECTS OF PASSIVE HYPERVENTILATION

BY 1952 there had developed a strong clinical impression that the use of controlled respiration for abdominal surgery carried out under thiopentone, relaxant, nitrous oxide anaesthesia, resulted in a reduction in the dosage of both relaxant and barbiturate. This led to the suggestion by Gray and Rees (1952) that controlled respiration, quite apart from its use to achieve adequate ventilation, supplemented in some way the anaesthetic and relaxant drugs being used. Dundee (1952) in controlled observations on patients undergoing upper abdominal operations of 2 hours duration confirmed that there was a reduction in dosage of thiopentone necessary but found less evidence of the need for a smaller dosage of relaxants. He associated these results with the lowered blood acidity which he found almost invariably accompanied controlled respiration. This work has been supported by Gray and Geddes (1959) who drew attention to the appearance of slow wave activity in the e.e.g. during nitrous oxide, oxygen relaxant, controlled respiration anaesthesia and produced some evidence that this was attributable to hyperventilation. They also pointed out that the work of Bonvallet and Dell (1956) suggested that the blood tension of carbon dioxide had an important influence on the activity of the reticular area of the mid-brain, and they suggested that depression of the reticular activating system under conditions of respiratory alkalosis might account both for the appearance of slow waves and for the increased potency of nitrous oxide which was clinically evident.