Effects of 20% nitrous oxide on the ventilatory response to hypercapnia and sustained isocapnic hypoxia in man

Abstract

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 (FlN2O). In five subjects, we performed step decreases in PE′O2 to 6.7 kPa for 15 min and step increases in PE′CO2 (δPE′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. (Br. J. Anaesth. 1994; 72: 17-20) © 1994 British Journal of Anaesthesia.