The Independent Effect of Heart Rate on Stroke Volume When Skin Temperature is Hot and Cool During Exercise

Abstract

There are two hypotheses of how stroke volume (SV) is affected during hyperthermic exercise. The traditional hypothesis is that an increase in cutaneous blood flow (CBF) is thought to lead to a decline in SV. An alternate hypothesis is that a decline in SV is due to an increase in heart rate (HR). However, these two hypotheses have not been tested under the same thermal stress. PURPOSE: 1) To determine the independent effect of HR on SV by using low dose β 1-blockade (BB) when skin temperature (Tsk) is high (>38°C) during exercise. 2) To see how the rapid lowering of Tsk reverses the changes in cardiovascular variables. METHODS: Tsk was manipulated by wearing a water perfused suit that covered the whole body, except head, hands, and feet and maintained a perfused water temperature of 30 or 50°C. Subjects (n=8, active men 24 ± 4 y) cycled at 60% VO2peak for 20 min in three conditions: 30°C water with placebo (30-PL), 50°C water with PL (50-PL), and 50-BB. Tsk was rapidly cooled at 20 min of exercise in all trials by perfusing cold water through the suit (0°C) plus fans. Subjects continued to cycle for another 20 min with cool Tsk. Esophageal temperature (Tes), Tsk, VO2, cardiac output (CO), HR, mean arterial pressure (MAP), cutaneous blood flow (CBF), and forearm venous volume (FVV) were measured during exercise. RESULTS: Mean Tsk during the first 20 min of exercise were 33.4 ± 0.2, 38.1 ± 0.2, and 38.2 ± 0.2 °C, for 30-PL, 50-PL, and 50-BB, respectively. Mean Tsk during the last 20 min of exercise were 29.5 ± 0.2, 29.8 ± 0.3, and 30.0 ± 0.3 °C, for 30-PL, 50-PL, and 50-BB, respectively. When HR was lowered to the same level as 30-PL (147.9 ± 3.8 bpm) by BB in 50-BB (151.7 ± 4.0 bpm), SV was also restored to the same level as 30-PL (132.0 ± 7.3 ml) in 50-BB (135.6 ± 7.5 ml). SV was restored even with a significantly higher CBF (77.1 ± 3.4% vs 55.9 ± 3.6% for 50-BB vs 30-PL; p<0.05) and lower MAP (98.5 ± 2.6 mmHg vs 105.4 ± 3.3 mmHg for 50-BB vs 30-PL; p<0.05). When Tsk was rapidly cooled, HR and CBF were significantly decreased while SV was maintained in 50-PL. There was no apparent effect of Tsk on FVV responses. CONCLUSION: The increase in HR was responsible for the decrease in SV when Tsk was above 38°C. Rapidly cooling Tsk while Tes remains elevated decreased CBF and HR.