The effect of skin temperature on cardiovascular responses during endurance exercise
These studies investigated the effect of whole body skin temperature (T [subscript sk]) on cardiovascular responses and the independent effect of heart rate on stroke volume when Tsk is hot and cold during moderate intensity endurance exercise. In study 1, we systemically evaluated the effect of graded increases in T [subscript sk] from 32°C to 39°C on cardiovascular responses during 20-30 min cycling exercise at 63% VO₂ [subscript peak]. Tsk was manipulated by wearing a water perfused suit that covered most of the body and maintained a perfused water temperature of either 20, 30, 40, and 50°C. Tsk was significantly different between each trial and averaged 32.4 ± 0.2, 35.5 ± 0.1, 37.5 ± 0.1, and 39.5 ± 0.1°C, respectively. Incremental increasing Tsk resulted in a graded increase in heart rate (HR) and a graded decrease in stroke volume (SV). Cutaneous blood flow (CBF) reached a similar average plateau value in all trials when T [sunscript c] was above ~38°C, independent of Tsk. Tsk had no apparent effects on the forearm venous volume (FVV) responses. Study 2 investigated the independent effect of increasing HR on SV by using low dose β1-blockade (βB) during exercise when Tsk was 38°C. How the rapid cooling of Tsk reverses the cardiovascular variables was also tested. When HR was lowered to the control trial level by βB when Tsk was 38°C, SV was also restored to the control even with a significantly higher CBF and cutaneous vascular conductance (CVC) and lower MAP. When Tsk was rapidly cooled at min 20 when Tsk was 38°C, CO, HR, and CBF significantly decreased while SV did not change. Taken together, our data suggest that there was no further pooling of blood in the skin when Tsk is increased from 32 to 39°C. The decrease in SV during exercise when heating the skin to high levels appears related to an increase in HR and not an increase in CBF. The restoration of SV by lowing HR even with higher CBF and CVC and lower MAP demonstrated that the increase in HR was responsible for the decrease in SV during moderate exercise when Tsk was held at 38°C.