Although aerobic exercise has consistently been shown to induce postexercise hypotension (PEH), the underlying physiological mechanisms are still unclear, especially when exercise is performed in hot environment.
PURPOSE: To investigate the effect of heat stress on PEH induced by isocaloric cycling bouts.
METHODS: Seven normotensive and prehypertensive men, aged 21-33 yr, visited the laboratory on four occasions to perform: a) a maximal cardiopulmonary exercise test in a temperate environment; b) a non-exercise control session in a temperate environment (CON); and c) two bouts of isocaloric (300 kcal) continuous cycling at 60% VO2 reserve in environmental temperatures of 21°C (TEMP) and 35°C (HOT) in a randomized, counter-balanced order. Systolic (SBP) and diastolic (DBP) blood pressures, cardiac output, systemic vascular resistance, mean skin temperature, rectal temperature, plasma volume, and autonomic function (spontaneous baroreflex sensitivity and heart rate variability) were determined for 60 min after the non-exercise control session and after the two exercise bouts. SBP and DBP also were determined during the subsequent 21 hr recovery period using an ambulatory BP monitor.
RESULTS: During the 60 min postexercise recovery period SBP was 8.6 (P = 0.003) and 3.7 (P = 0.004) mmHg lower in HOT compared to CON and TEMP, respectively. DBP was 10.0 (P < 0.001) and 5.3 (P = 0.04) mmHg lower. Compared to TEMP, rectal temperature was 0.6 °C higher (P = 0.001), mean skin temperature was 1.8 °C higher (P = 0.013), plasma volume was 2.6 percentage points lower (P = 0.005), and systemic vascular resistance was 1.9 mmHg•L•min-1 lower (P = 0.014) in HOT during the 60 min post-exercise recovery period, whereas no significant differences between TEMP and HOT were observed for cardiac output, baroreflex sensitivity, or heart rate variability. During the subsequent 21 hr recovery period SBP was 4.5 and 3.1 mmHg lower in HOT than CON (P < 0.001) and TEMP (P = 0.003), respectively, whereas no significant effect was observed for DBP (P = 0.92).
CONCLUSION: Exercise in the heat increases the hypotensive effects of physical exercise for at least 22 hr. The underlying mechanisms (at least for the first 60-min recovery period) appear to be increased body temperature, reduced plasma volume, and decreased systemic vascular resistance.