EFFECTS OF 20 PERCENT NITROUS-OXIDE ON THE VENTILATORY RESPONSE TO HYPERCAPNIA AND SUSTAINED ISOCAPNIC HYPOXIA IN MAN

To examine the effects of a subanaesthetic concentration of nitrous oxide on ventilation, we have studied the ventilatory response to carbon dioxide and isocapnic hypoxia (F/(N2O) 0.2). in five subjects, we performed step decreases in PE'(O2) to 6.7 kPa for 15 min and step increases in PE'(CO2) (Delta PE'(CO2) 1.5 kPa). The carbon dioxide responses were partitioned into a fast peripheral and slow central component. All control oxygen responses were biphasic: the acute hypoxic response was 7.2 (4.6)litre min(-1) and the subsequent decline 4.2 (2.6) litre min(-1). With nitrous oxide, the acute hypoxic response was 10.3 (6.7)litre min(-1), but was followed by a nonsignificant decline of 3.8 (3.0) litre min(-1) because of a progressive increase in breathing frequency in the hypoxic period. None of the variables obtained from the carbon dioxide responses differed between groups. Our data indicate that a subanaesthetic concentration of nitrous oxide did not affect the peripheral chemoreflex loop. The hyperventilatory response to prolonged hypoxia was sustained better with nitrous oxide compared with control.