INFLUENCE OF 0.2 MINIMUM ALVEOLAR CONCENTRATION OF ENFLURANE ON THE VENTILATORY RESPONSE TO SUSTAINED HYPOXIA IN HUMANS

To determine the influence of 0.2 minimum alveolar concentration (MAC) of enflurane on the time course of ventilation during sustained hypox ia, we studied 10 healthy adult volunteers with and without enflurane. The following design was used: end-tidal PO2 was maintained at 13.3 k Pa for 8 min, at 6.7 kPa for 20 min and at 13.3 kPa for 8 min. End-tid al PCO2 was held constant throughout at 0.67 kPa above the subject's n atural value. Control experiments were conducted with no hypoxia impos ed. During the experiment subjects breathed via a mouthpiece from an a utomated gas mixing system which controlled end-tidal values. Enfluran e reduced baseline (euoxic) ventilation from 20.9 (SEM 2.0) litre min( -1) to 10.1 (1.0) litre min(-1) (ANOVA, P < 0.001). Enflurane reduced the acute ventilatory response to hypoxia (AHVR) from 20.1 (3.3) litre min(-1) to 5.0 (1.3) litre min(-1) (ANOVA, P < 0.01), and the ventila tory off-response at cessation of hypoxia from 11.7 (2.4) litre min(-1 ) to 1.8 (0.5) litre min(-1) (ANOVA, P < 0.02). There was no significa nt difference in hypoxic ventilatory decline (HVD) without and with en flurane (8.9 (2.4) litre min(-1) vs 5.5 (1.1) litre min(-1); ANOVA, ns ). These results confirm that 0.2 MAC of enflurane suppressed the acut e ventilatory response to hypoxia, but had no significant effect on th e subsequent ventilatory decline during sustained hypoxia.