Metabolic and thermal responses of man in various He-O-2 and air environments.

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Abstract
Metabolic and thermal responses of resting man were studied for 2 hr. while breathing and/or immersed in a He-02 mixture (70-76% He, 21% O2, 3-9% N2) or air in a comfortable thermal environment (29.4 C DB, 19.0 C WB). The gaseous combinations were: (1) air breathing, air immersion; (2) He-02 breathing, air immersion; (3) air breathing, He-O2 immersion; and (4) He-O2 breathing, He-02 immersion. The immersion gases were contained in a vinyl isolator while the subjects breathed from a separate gas source through a mouthpiece equipped with 2 one-way valves. Changes in mean skin temperature (Tg) and rectal temperature (Tr) were found only between the trials in which the subjects were immersed in air or He-O2, and were independent of the gases breathed. These changes were lower Tg (P< .01) and Tr (P< .001) in the He-02-immersion trials implying that He promotes body cooling solely because of its relatively high thermal conductivity. Oxygen consumption was independent of the immersion gases and those breathed as were heart rate, pulmonary ventilation, and respiratory exchange ratio. These results indicate that in man in a comfortable thermal environment He neither induces metabolic alterations at the cellular level, nor does the accelerated heat loss stimulate heat production measurably in 2 hr. of exposure.