RESUMEN
Previous findings of a narcosis-induced reduction in heat production during cold water immersion, as reflected in oxygen uptake (VO2), have been attributed to the attenuation of the shivering response. The possibility of reduced oxygen utilization (VO2) by the muscles could not, however, be excluded. Accordingly, the present study tested the hypothesis that mild narcosis, induced by inhalation of a normoxic gas mixture containing 30% nitrous oxide (N2O), would affect VO2. Nine male subjects participated in both maximal and submaximal exercise trials, inspiring either room air (AIR) or a normoxic mixture containing 30% N2O. In the submaximal trials, the subjects exercised at 50% of maximal exercise intensity (Wmax) as determined in the maximal AIR trial. Though the subjects attained the same Wmax in the AIR and N2O trials, maximal VO2 was significantly higher (P < 0.05) during the N2O condition [58.9 (SEM 3.1) ml x kg(-1) x min(-1)] compared to the AIR condition [55.0 (SEM 2.4) ml x kg(-1) x min(-1)]. However, the VO2-relative exercise intensity relationship was similar during both maximal AIR and maximal N2O at submaximal exercise intensities. There were no significant differences in the responses of oesophageal temperature, sweating rate, heart rate and ventilation between AIR and N2O in the maximal and submaximal tests. It was concluded that the previously reported narcosis-induced reductions in VO2 observed during cold water immersion can be attributed solely to a reduction in the shivering response rather than to decreased oxygen utilization by the muscles.