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

Abstract

Experiments were designed to assess how cardiac output adjusts to competitive metabolic and thermal requirements of exercise and heat stress. In 6 normal, non-heat-acclimatized young men cardiac output was determined 7 times per man during each of 3 to 4 intensities of exercise at 78[degree]F and 110[degree]F. Exercise (duration, 15 rain.) required 50, 58, 63 and 73% of maximal O2 intake at both temperatures. Indocyanine green was injected into right atrium and sampled from aortic arch. At 50 and 58% of maximal O2 intake cardiac output averaged 15.0 and 16.4 1/min. at 78[degree]F and 14.8 and 16.0 1/min. at 110[degree]F (168 measurements). Stroke volume and CBV [Central blood volume] were reduced 16%. Cardiac output was maintained by increased heart rate and decreased mean circulation time (mean transit time [MTT] decreased 16%). At higher work loads CBV and stroke volume remained depressed at 110[degree]F. Inability to further increase heart rate (maximal heart rates were approached at the 3rd work load) or to decrease MTT prevented normal increments in cardiac output at 110[degree]F. Nevertheless, normal increments in O2 intake were achieved by widened A-V [Atrio-ventricular]O2 difference. Thus in response to heat stresses during exercise blood shifts from the core to the periphery. Added demands for heat dissipation are met by repartitioning cardiac output rather than by increasing it.