Alterations of calf venous and arterial compliance following acclimation to heat administered at a fixed daily time in humans.

We investigated the effects of heat acclimation on venous and arterial compliance in humans. Four male and four female volunteers were exposed to an ambient temperature of 40 degrees C and relative humidity of 40% for 4 h (1330-1730 hours) per day for 9-10 consecutive days. The calf venous compliance (CV) was estimated using venous occlusion plethysmography with a mercury-in-silastic strain gauge placed around the right calf at its maximum girth. The compliance of the small (CSA) and large (CLA) arteries were assessed by reflective and capacitance compliance by analyzing the radial artery blood pressure waveforms, basing on the use of a modified Windkessel model. The calf CV, CSA, CLA, systolic and diastolic blood pressures, heart rate and core temperature were determined twice a day, 0930-1100 hours (AM test) and 1500-1630 hours (PM test), in both heat-acclimated and non-heat-acclimated (control) conditions. Heat acclimation appeared to decrease blood pressures, heart rate and significantly lowered core temperature only in the PM test. In the control condition, the calf CV was not affected by the time of day and the CSA was significantly depressed in the PM test. After acclimation to heat, the calf CV significantly increased and the CSA did not decrease in the PM test. The results presented suggest that repeated heat exposure in humans, for 4 h at a fixed time daily, increases the calf CV and the CSA particularly during the period when the subjects were previously exposed to heat.