The effect of 30% nitrous oxide on thermoregulatory responses in humans during hypothermia

Abstract

Clinical studies have reported that body core temperature decreases during prolonged surgery and anesthesia. Although this finding has been attributed primarily to increased heat loss resulting from exposure of body cavities and infusion of cold solutions, it is generally recognized that anesthesia interferes with the thermoregulatory system. The present study examined the effects of mild narcosis induced by 30% N2O on shivering thermogenesis and cutaneous thermoregulatory vasoconstriction in humans, during exposure in a much more intense peripheral thermal stimulus than the ones often used in clinical studies. Nine male subjects were immersed in 15° C water on two separate occasions. During one occasion subjects inspired air (control condition), and during the other occasion the inspired gas mixture contained 20% O2, 30% N2O, and 50% N2 (N2O condition). On both occasions, subjects were immersed to the neck for 60 min, or until their core temperature decreased by 2° C from the preimmersion value. Following the cooling phase, subjects rewarmed via endogenous thermogenesis while lying in a well- insulated bed for 48 min. In the N2O condition, subjects continued to inspire the anesthetic gas mixture during the 48-min period of recovery. O2 uptake (V̇(O2)), esophageal temperature (T(es)), mean skin temperature (T̄(sk)), mean heat flux (Q) and forearm-fingertip temperature gradient (T(sk-gr)) were recorded at 1-min intervals. T̄(sk) and Q in both conditions stabilized within 10 and 25 min of immersion, respectively, and were not significantly different between the two conditions. The cooling rate of T(es) was greater during the N2O than the control condition. V̇(O2) increased during the immersion in both conditions and was greater in the control than in the N2O condition. In both conditions, V̇(O2) increased linearly with decreasing T(es), but at any given T(es), V̇(O2) was higher in the control than in the N2O condition. No significant difference was observed in cutaneous thermoregulatory vasoconstriction between the two experimental conditions, as indicated by the T(sk-gr) values. The estimated T(es) threshold for shivering (estimated from the O2 consumption vs. ΔT(es) regression) was reduced by 0.95 ± 0.26 (SE)° C during the immersion phase and by 0.39 ± 0.05 (SE)° C during the rewarming phase in the N2O condition compared to the control condition. Although the thermosensitivity (gain) of shivering appeared preserved during the immersion phase, it was reduced during the N2O rewarming phase. These data are consistent with the suggestion that general anesthetics markedly increase the interthreshold range of the thermoregulatory responses, and also indicate that the effect of anesthetics on shivering thermosensitivity depends on the intensity of the peripheral thermal stimulus.