The effect of nitrous oxide-induced narcosis on aerobic work performance

Abstract

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.

Concentration-related metabolic rate and behavioral thermoregulatory adaptations to serial administrations of nitrous oxide in rats

“…Whether our findings are relevant to human use of this drug is an important but largely unanswered question. Work involving single cool water immersion sessions during inhalation of 25–30% N 2 O indicated that the drug augmented hypothermia development by lowering the core temperature threshold for activation of shivering thermogenesis [ 55 – 57 ]. However, to our knowledge, the effects of repeated N 2 O administrations on thermoregulatory outcomes in humans simply have not been the subjects of published research.…”

Nitrous oxide consistently attenuates thermogenic and thermoperceptual responses to repetitive cold stress in humans

“…The present experimental design does not allow us to identify which component(s) of the neural network controlling heat-producing thermoeffectors was (were) affected by N 2 O. Previous studies have shown that N 2 O affects insignificantly cutaneous thermoreception and axonal conduction ( 26 ) and probably does not impair the oxidative capacity of skeletal muscle (i.e., the specific effector end-organ; 27 ). Considering that N 2 O disrupts the functioning of synaptic transmission ( 28 ), primarily via the inhibition of the glutamate N -methyl- d -aspartate (NMDA) receptors ( 29 – 32 ), it is reasonable to assume that the N 2 O-evoked metabolic downregulation was determined largely by a diminished thermoafferent input ( 33 , 34 ) and/or a modified central integration of the input ( 35 , 36 ).…”

“…Two subjects, however, withdrew from the study, due to adverse effects (e.g., severe nausea and discomfort) encountered during the first 30 min of breathing N 2 O-in both cases, the trial was terminated immediately after reporting the symptoms, and subjects were fully recovered within $5 min after commencing to breathe room air. We, therefore, included 14 subjects, with the following characteristics: mean (range) age 23 (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) yr, height 179 (164-192) cm, weight 71.4 (55.7-93.6) kg, body surface area 1.90 (1.10-2.18) m 2 , body fat 8.3 (4.1-12.6)%, and total skinfold thickness 64 mm. Considering that autonomic thermoregulation (16) and thermal perception (17) may be influenced by female reproductive hormones, we did not recruit any woman for this study.…”