Effect of airway occlusion on respiratory timing during anaesthesia with enflurane or halothane.

Eighteen patients were studied, during operations under subarachnoid or extradural anaesthesia combined with general anaesthesia, to assess the influence of occlusion of inspiration on the duration of inspiration, and the likelihood of a relationship between lung volume and duration of inspiration. Ten patients breathed 2% enflurane and eight breathed 1% halothane, using 67% nitrous oxide in oxygen as carrier gases. The durations of inspiration and of expiration were significantly longer during enflurane administration than during halothane. Occlusion of inspiration reduced significantly the duration of the inspiratory attempt in both groups of patients, but the duration of a complete respiratory cycle was not changed. In patients receiving enflurane, but not in those given halothane, the relative duration of occluded inspiration was positively correlated with the duration of the unoccluded inspiration. There was no evidence with either agent of a reflex relationship between lung volume and inspiratory duration which would have suggested an active Hering-Breuer reflex.