The Effect of Mild Narcosis Induced by Nitrous Oxide on Thermal Balance in Humans

Clinical studies have reported that body core temperature decreases during prolonged anesthesia. Though 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 possibly interferes with the thermoregulatory system. The aim of the present study was to investigate whether mild narcosis induced by inhaling a gas mixture containing 30%N20, would affect the thermoregulatory responses at greater thermal drives for thermogenesis, as would be expected at lower skin and body core temperatures. Nine male subjects were immersed in 15OC water on two separate occasions. In one condition subjects inspired room air (20%02, 80%N2) (control), while in the other condition the inspired gas mixture contained 20%02, 30%N20, 50%N2 (N20). In both conditions, subjects were immersed to the neck for 60 minutes, or until their core temperature decreased by 2OC from the pre-immersion value. Following the cooling phase subjects rewarmed via endogenous thermogenesis while lying in a well-insulated bed for 48 minutes. Oxygen uptake ( ~ 0 ~ ) . esophageal (Tes) and rectal (Tre) temperatures, inspired minute ventilation (v~) , mean skin temperature (Tsk), mean heat flux (anean), forearm-fingertip temperature gradient (Tsk-gr) and heart rate (HR), were recorded at minute intervals. Tsk, Q, and Tsk-gr, in both conditions stabilized within 10,25 and 20 minutes of immersion, respectively, and were not significantly different between the two conditions. The cooling rate of Tes was higher under anesthesia than during air breathing. V O ~ , V* and HR increased during the immersion in both conditions and were higher in the control than in the N20 condition. In both conditions lo2 increased linearly with decreasing Tes, but at any given Tes, oxygen uptake was higher in the control than in the N20 condition. These results indicate that mild narcosis induced with the inhalation of a gas mixture containing 30%N20 attenuates the thermogenic response in humans during cold water immersion. The substantial attenuation of shivering thermogenesis occured despite a greater cooling rate of Tes. The results obtained during rewarming were similar to the ones obtained