Reductions in cardiac output, central blood volume, and stroke volume with thermal stress in normal men during exercise.

Observations on heat-acclimatized men by Asmussen (1) and Williams and colleagues (2) showed no increase in cardiac output above the normal response during exercise in a hot, humid environment. Indeed, Asmussen found decrements in cardiac output below normal responses if moderate work in the heat (32° C, 80% relative humidity) was prolonged from 30 to 60 minutes. No systematic study has been reported of cardiac output during exercise at high ambient temperature in men unacclimatized to heat. Metabolic requirements may be prepotent in the regulation of cardiac output, since Rowell and colleagues (3) observed no decrements in oxygen uptake of unacclimatized men during moderate to heavy exercise at 43.3° C (1100 F) when compared with identical measurements on these men at 25.60 C (780 F). The failure to observe a significant increment in blood lactate concentration at 43.3° C suggested that there was minimal diversion of blood away from working skeletal