Comparison of decreases in ventilation caused by enflurane and fentanyl during anaesthesia

Two groups of six patients were studied during light general anaesthesia using 2% enflurane and 66% nitrous oxide in oxygen, combined with regional anaesthesia, for hernia and varicose vein surgery. The effects of 3% enflurane were compared with those of fentanyl 0.3 microgram kg-1 i.v., by measuring inspired flow, tidal volume, the timing of inspiration and expiration, and occlusion pressure. Three per cent enflurane decreased ventilation by 12%. Tidal volume, mean inspiratory flow and occlusion pressure were decreased in approximately equal proportions (14, 12 and 8%, respectively). The timing of breathing did not change significantly. Fentanyl did not influence tidal volume. Ventilation was decreased by 28% as a result of a 10% decrease in inspiratory flow and a marked increase in the duration of expiration by 45%. The pattern of activation of the inspiratory muscles, as indicated by occlusion pressure, was changed by fentanyl. During enflurane and nitrous oxide anaesthesia, depression of ventilation by fentanyl or increases in enflurane concentration was not by a common central depressant mechanism.