Metabolic and thermal responses of the rat to a helium-oxygen environment

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Abstract
Rats (avg [average] 160 g) were exposed to a 79% helium-21% oxygen mixture (He-O2) at ambient temperatures between 23 and 33 C, either acutely (3 hr) or chronically (10 days). At the lower ambient temperatures, the major effects of He were increases in food intake (avg 18%), urine osmolarity (avg 10%), O2 uptake (avg 6 ml/ kg/min, STPD), heart rate (avg 35 beats/min), respiratory rate (avg 14 breaths/min), and a decrease in subcutaneous temperature (avg -1 C). These changes were essentially absent at the higher ambient temperatures. The increase in O2 consumption was strongly correlated with the decrease in subcutaneous temperature (r = 0.8). Rectal temperature was depressed slightly (avg -0.2 C) over the ambient temperature range. The effects occurred and disappeared abruptly upon exposure to or removal from He-O2. They were similar in acute and chronic rats, indicating little adaptive response to He-O2. The results are consistent with the indirect or physical hypothesis for the He effect on metabolism; i.e., in the presence of a temperature gradient between skin and gas, the high heat conductivity of He promotes heat loss from the body surface, lowering body temperature and stimulating O2 uptake, heart rate, respiratory rate, and food intake.

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