Hemodynamics at rest and during exercise in comfortable, hot and cold environments.Measurement with a rebreathing technique

The hemodynamic effects of exercise during short-term exposure to a cold, hot and comfortable environment were studied in normal subjects. Seven untrained males (19 to 34 years) exercised on a bicycle while exposed to temperatures of 7 °C, 22 °C and 41 °C inside an environmental chamber. Values for oxygen consumption (VO₂), pulmonary capillary blood flow (Qc), pulmonary capillary and tissue volume (V(t+c)) and pulmonary diffusing capacity for CO (DL) were obtained using a non-invasive rebreathing technique. These parameters and heart rate (HR) were measured after 15 min of sitting exposure and during three episodes of exercise (50, 75 and 100 W) for 5 min each. Resting VO₂ at 7 °C was 31% higher than at 22 °C (p < 0.01) and 43% higher than at 41 °C (p < 0.05). During exercise, VO₂ increased independently of temperature. While all measured hemodynamic parameters were similar during exercise at 7 °C and 22 °C, HR was consistently higher and stroke index lower during exercise at 41 °C. At 95% of maximum heart rate (HRmax.) at 41 °C, Qc was 13% lower than at 22 °C (p < 0.01) and 14% lower than at 7 °C (p < 0.05). V(t+c) and DL increased during exercise and were not affected by temperature. In normal man: 1) central hemodynamic changes at rest are more pronounced during short-term cold than during similar heat exposure; 2) VO₂ during moderate and severe exercise is independent of ambient temperature; 3) Qc near HRmax. is decreased in hot environment; and 4) exercise induced increases in DL and V(t+c)/kg appear to be independent of ambient temperature.