Human Cardiovascular Adjustments to Rapid Changes in Skin Temperature during Exercise

In 11 normal men, central circulatory responses were measured while skin temperature was changed in a square-wave pattern during uninterrupted exercise (26% to 64% maximal oxygen consumption). Skin temperature was changed at 30-minute intervals, beginning at 32°C. On raising it to 38.2°C at low oxygen consumption (V · o 2 ), cardiac output increased 2.5 liters/min, and central blood volume, aortic mean pressure, and stroke volume fell (7%, 7%, and 11%, respectively). Right atrial mean pressure fell 2.2 and 2.3 mm Hg during control and heating periods, respectively. All variables returned to control levels when skin temperature was reduced toward 26.9°C. Raising it to 40°C reproduced these changes with a more clear-cut drop in right atrial mean pressure. Results indicated reduced peripheral venous tone and cutaneous pooling of blood during heating and rapid reversal on cooling. On raising skin temperature to 38.7°C at high V · o 2 , cardiac output increased 19% (3.1 liters/min), stroke volume decreased 14%, and central blood volume rose slightly. Aortic mean pressure fell during the control period and was maintained or rose during heating periods. On cooling, central blood volume and stroke volume rose, cardiac output remained elevated, and aortic mean pressure fell. Increases in cardiac output during heating were related to skin temperature and not to V · o 2 or body temperature. At high V · o 2 , circulatory adjustments favor metabolic rather than thermoregulatory demands.