Central circulatory responses to work in dry heat before and after acclimatization

Abstract

Cardiac output, central blood volume (CBV), aortic blood pressure, heart rate, VO2, and skin (Ts) and rectal temperatures (Tr) were measured repeatedly during prolonged exercise (70 min) in 6 normal young men before and after 11-12 days of acclimatization to work in dry heat-(48.4[degree] C dry bulb - 25.6[degree] C wet bulb). Heart rate, Ts, Tr, and total sweat loss followed the usual course with acclimatization. Work V02 was unaffected. In 5 men cardiac output and CBV changed minimally. Stroke volume increased markedly in 4 men and was unchanged in 1. A 6th subject showed very high cardiac output, CBV, and stroke volume before and decreased to normal after acclimatization. Results from 5 men indicate that decreased heart rate during acclimatization usually attends increased stroke volume, not decreased cardiac output. Increased stroke volume did not result from increased CBV via thoracic redistribution of peripheral blood but by decreased heart rate attending lower surface and "core" temperatures and increased sweating.