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

Abstract

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°C vs. ≤36°C, respectively). We systematically studied cardiovascular responses to progressively increased T_sk (32°C-39°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°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 ~38°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°C to 39.5°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 ~32°C to ~39°C. High T_sk (≥38°C) did not further elevate CBF and FVV compared with lower T_sk during moderate intensity exercise.

Keywords: cardiovascular drift; cutaneous (skin) blood flow; heart rate; hyperthermia; stroke volume.