Cardiovascular responses to heat stress and blood volume displacements during exercise in man.

Subjects exercised in the upright position at approximately 50% of maximal oxygen consumption in four situations: in 25 degrees C air, in 45 degrees C air [mean skin temperature (Tsk) 35 degrees C], in 35 degrees C water immersed to the level of the xiphoid process, and finally wearing a suit perfused with 35 degrees C water. The water immersion prevented gravitational shifts of blood volume to the legs. In this situation the forearm blood flow (FBF) rose continually with increasing core temperature (Tes) in contrast to the attenuation in rise above 38 degrees C Tes in 45 degrees C air. The differences were significant above 38.6 degrees C Tes in experiments in eight subjects. The effects of immersion on cardiac output (CO), stroke volume (SV), and heart rate (HR) were studied in five of the subjects in relation to Tes, since the rate of rise of Tes was different in the four situations. CO and SV tended to be higher during both rest and exercise in the water than in the other three conditions, while HR rose in the same manner with increasing core temperature, except that it was lower in 25 degrees C air, where Tsk was lower. Thus, the prevention of hydrostatic shifts of peripheral venous volume permitted the maintenance of a higher SV and peripheral blood flow, and enhanced the ability of the circulation to deal with the combined exercise and heat stress.