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

Abstract

Observations on heat-acclimatized men by As-mussen (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 decre-ments in cardiac output below normal responses if moderate work in the heat (32 ° C, 80 % rela-tive 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 com-pared with identical measurements on these men at 25.60 C (780 F). The failure to observe a significant increment in blood lactate concentra-tion at 43.3 ° C suggested that there was minimal diversion of blood away from working skeletal * Submitted for publication December 16, 1965; ac-cepted August 11, 1966. Supported by grant-in-aid HE-00908-C14 from the