EFFECT OF LOW-DOSE ENFLURANE ON THE VENTILATORY RESPONSE TO HYPOXIA IN HUMANS

To investigate the effects of enflurane on the control of breathing we have studied the ventilatory responses to isocapnic hypoxia in 12 adults with and without sedation with enflurane. Design 1 consisted of three steps into hypoxia (PE'(O2) = 6.7 kPa), each lasting 3 min, separated by periods of euoxia lasting 5 min (PE'(O2), = 13.3 kPa). Design 1 was repeated four times in each subject on the same day in random order. with carrier gas (control) and with 0.04 MAC, 0.07 MAC and 0.13 MAC of end-tidal enflurane concentrations. Design 2 consisted of 20-min exposures to hypoxia with and without 0.07 MAC of enflurane. Each exposure was preceded and followed by 5 min of euoxia. End-tidal PCO2 was held constant at 0.13-0.27kPa greater than the resting level throughout both designs. Mean (SEM) ventilatory responses to hypoxia for design 1 were: 8.2 (1.3) litre min(-1) (control), 6.6 (1.4) litre min(-1) (0.04 MAC), 5.7 (1.1) litre min(-1) (0.07 MAC) and 3.7 (0.5) litre min(-1) (0.13 MAC) (P < 0.001). For design 2, enflurane produced a 15% reduction in resting ventilation (P < 0.01), a 40% decrease in the acute ventilatory response to hypoxia (P < 0.01) and a 32% reduction in ventilatory decline (ns) which occurred during sustained hypoxia.