Influences of skin and core temperature on cardiovascular responses during exercise
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The cardiovascular effects of whole body heat stress during exercise are well established. However the independent contribution of elevated skin temperature (Tsk) or core temperature (Tc) on these responses remains unclear. The purpose of this study was to determine how increases in Tsk and Tc alone and in combination, impact cardiovascular responses during moderate intensity exercise. To accomplish this goal, eight healthy, recreationally active males were immersed to the neck in a cold (14 - 17°C) or hot (40 - 42.5°C) water bath for 20 to 25 min to alter Tc immediately prior to exercise with either cool Tsk (i.e. fans) or warm Tsk (i.e. heaters). Conditions during exercise were cool skin and cool core (CC), warm skin and cool core (WC), cool skin and warm core (CW), and warm skin and warm core (WW), and were conducted in a randomized crossover design. When data was combined (n=16), warm core conditions (CW and WW) were associated with significantly higher average heart rate (HR) and lower stroke volume (SV) during exercise compared to cool core conditions (CC and WC); 168.1 ± 3.2 vs. 152.2 ± 4.0 beats/min and 139.2 ± 7.3 vs. 147.7 ± 9.4 mL/beat, respectively. The approximate 9 mL/beat decline in SV and 16 beat/min increase in HR in warm core conditions tended to increase cardiac output (Q), 23.2 ± 0.6 vs. 22.2 ± 0.7 L/min, P=0.078. Similarly, warm Tsk conditions (WC and WW) were associated with significantly higher average HR and lower SV during exercise compared to cool Tsk conditions (CC and CW); 165.2 ± 3.3 vs. 155.1 ± 3.4 beats/min and 140.8 ± 7.8 vs. 146.0 ± 8.7 mL/beat, respectively. Additionally, there was also a trend for Q to be elevated with warm skin (23.0 ± 0.6 vs. 22.4 ± 0.6, P=0.075). Although combined data indicated that warm Tsk conditions significantly lowered average SV by ~6 mL/beat, there was no reduction in SV during exercise by warm Tsk, when Tes was cool (i.e. <37.0°C), as evidenced by identical values for SV in CC and WC, 147.7 ± 9.8 vs. 147.7 ± 9.0 mL/beat, respectively. In contrast, SV was significantly lower in WW compared with CW, 133.9 ± 7.0 vs. 144.4 ± 7.8 mL/beat, respectively. Therefore, the major reduction in SV by warm Tsk occurred during WW, when Tes was elevated (i.e. >38.0°C). Analyzing data independently for precooling and preheating conditions revealed that warm Tsk was associated with greater HR drift from 5 to 20 min of exercise, compared to cool Tsk, when esophageal temperature (Tes) was both cool or warm (23.9 ± 2.2 vs. 17.5 ± 2.3 and 12.3 ± 1.3 vs. 4.6 ± 1.7 beats/min, respectively). These observations demonstrate that both Tes and Tsk can directly influence cardiovascular responses during exercise, as indicated by elevations in HR during exercise with warm Tsk, with both warm and cool Tes. However SV is not compromised by warm Tsk if Tes is below 37.5°C. Furthermore, when both Tes and Tsk are elevated simultaneously, cardiovascular strain (i.e. increased HR and reduced SV) is much greater than when either is elevated alone. This is demonstrated by the finding that average HR was 175.8 ± 3.2 beats/min in WW, compared to 149.8 ± 4.0, 154.7 ± 4.1, and 160.3 ± 3.5 beats/min, in CC, WC, and CW, respectively, and the fact that SV was lowest during exercise in WW. In conclusion, individuals exercising in the heat should take measures to keep skin cool, especially when Tes is 39°C or greater to attenuate the cardiovascular strain that occurs with warm Tsk, when Tes is elevated.