ACCLIMATIZATION IN A HOT, HUMID ENVIRONMENT - CARDIOVASCULAR ADJUSTMENTS

Four trained young men worked for 4 h/day at 40-50% of their maximum aerobic capacity, 1st for 3 days at 25.degree. C db [dry bulb], 18.degree. C wb [wet bulb], and then for 10 consecutive days at 45.degree. C db, 32.degree. C wb. The central circulatory adaptation to work in heat could be divided into 4 distinct phases: phase I (day 1) was characterized by a progressive fall in stroke volume (SV) during heat exposure but cardiac output (CO) was maintained above control values by high heart rates. Phase II (days 2 and 3) was marked by increases in SV and decreases in heart rate but with little change in CO from phase I. During phase III (days 4-8 of acclimatization), CO increased due to increases in SV. Phase IV (days 6-8) was associated with decreases in rectal and skin temperature towards control levels. SV and HR both decline in this phase so that CO was not elevated greatly above control levels. Central circulatory and temperature regulating events are not causally associated in acclimatization.