Low Stroke Volume during Exercise with Hot Skin Is Due to Elevated Heart Rate

It is well known that hyperthermia lowers stroke volume (SV) during moderate-intensity prolonged exercise, yet the underlying mechanism is inconclusive, especially when skin temperature (Tsk) is hot (≥38°C). PURPOSEIn the present study, HR was independently lowered by a low dose of β1-blockade (βB) to investigate its effect on SV during exercise when skin is hot. The effect of rapid skin cooling on reversing cardiovascular responses was also examined. METHODSNine men cycled at 62% V˙O2peak wearing a water-perfused suit for 20 min during three conditions(a) moderate Tsk (~33°C) (MOD), (b) hot Tsk (~38°C) (HOT), and (c) hot Tsk (38°C) with βB (HOT-βB). Skin temperature was then rapidly cooled at 20 min in all trials by cold water (0°C–2°C) perfusion while subjects continued cycling for another 20 min. RESULTSWhen HR was lowered during HOT-βB (152 ± 4 bpm) to the same level as MOD (150 ± 4 bpm; P = 0.30), SV in HOT-βB (132 ± 8 mL) was also restored to the same level as MOD (129 ± 7 mL, P = 0.37) even with a significantly higher cutaneous blood flow (CBF) and lower mean arterial blood pressure. When Tsk was rapidly cooled, cardiac output, HR, and CBF significantly decreased while SV remained lower in HOT. Forearm venous volume was not different between trials during heating and cooling. CONCLUSIONSThe increase in HR rather than an increase in CBF or forearm venous volume was responsible for the decrease in SV during moderate-intensity exercise when Tsk was held at 38°C.