Effects of whole-body and local thermal stress on hydrostatic volume changes in the human calf.

In the present study, we examined the effect of thermal stress on the magnitude and pattern of change in leg volume during orthostatic stress and thigh occlusion in humans. Ten healthy volunteers underwent whole-body thermal stress produced by a cool- or hot-water-perfused suit and local heat stress of the calf. During whole-body thermal stress, changes of calf circumference during head-up tilt (HUT) at angles of 15 degrees and 60 degrees for 2 min each were monitored. In the supine position, the distensibility of the calf veins was evaluated from the magnitude and half-time of the change in calf circumference during thigh cuff occlusions at 20, 30, 50, 70, and 80 mmHg. Skin blood flow in the calf was measured by laser-Doppler flowmetry. Skin blood flow increased by [mean (SEM)] 295 (71)% with whole-body heating and decreased by 31 (4)% with whole-body cooling. Local heating of the calf increased skin blood flow in the heated area by 263 (25)%. During HUT, the calf cross-sectional area, calculated from the circumference, increased rapidly during whole-body heating and slowly during the cooling. The magnitude of the increase in calf area with HUT did not alter during whole-body heating, whereas it was reduced during cooling compared with the normothermic control. Whole-body and local heating did not alter the magnitude of change in calf area at a given cuff pressure, whereas whole-body cooling decreased it. The time to the half-maximal response of the change in calf circumference was shortened by 35 (17)% during whole-body heating and by 44 (4)% during local heating, whereas it was prolonged by 31 (16)% during whole-body cooling compared with the normothermic control. These results indicate that the magnitude and the pattern of change in calf volume during the early phase of orthostatic stress are modulated due to the changes in venous distensibility and blood flow in the skin during thermal loading.