Cardiovascular responses to exercise when increasing skin temperature with narrowing of the core-to-skin temperature gradient

The decline in stroke volume (SV) during exercise in the heat has been attributed to either an increase in cutaneous blood flow (CBF) that reduces venous return or an increase in heart rate (HR) that reduces cardiac filling time. However, the evidence supporting each mechanism arises under experimental conditions with different skin temperatures (T-sk; e.g., >= 38 degrees C vs. <= 36 degrees C, respectively). We systematically studied cardiovascular responses to progressively increased T-sk (32 degrees C-39 degrees C) with narrowing of the core-to-skin gradient during moderate intensity exercise. Eight men cycled at 63 +/- 1% peak oxygen consumption for 20-30 min. T-sk was manipulated by having subjects wear a water-perfused suit that covered most of the body and maintained T-sk that was significantly different between trials and averaged 32.4 +/- 0.2, 35.5 +/- 0.1, 37.5 +/- 0.1, and 39.5 +/- 0.1 degrees C, respectively. The graded heating of T-sk ultimately produced a graded elevation of esophageal temperature (T-es) at the end of exercise. Incrementally increasing T-sk resulted in a graded increase in HR and a graded decrease in SV. CBF reached a similar average plateau value in all trials when T-es was above similar to 38 degrees C, independent of T-sk. T-sk had no apparent effect on forearm venous volume (FVV). In conclusion, the CBF and FVV responses suggest no further pooling of blood in the skin when T-sk is increased from 32.4 degrees C to 39.5 degrees C. The decrease in SV during moderate intensity exercise when heating the skin to high levels appears related to an increase in HR and not an increase in CBF. NEW & NOTEWORTHY This study systematically investigated the effect of increasing skin temperature (T-sk) to high levels on cardiovascular responses during moderate intensity exercise. We conclude that the declines in stroke volume were related to the increases in heart rate but not the changes in cutaneous blood flow (CBF) and forearm venous volume (FVV) during moderate intensity exercise when T-sk increased from similar to 32 degrees C to similar to 39 degrees C. High T-sk (similar to 38 degrees C) did not further elevate CBF and FVV compared with lower T-sk during moderate intensity exercise.