Sympathetic Nerve Activity and Whole-Body Heat Stress

Abstract

We and others have shown that moderate passive whole-body heating (i.e., increased internal temperature ~0.7 °C) increases muscle (MSNA) and skin sympathetic nerve activity (SSNA). It is unknown, however, if MSNA and/or SSNA continues to increase with further increases in internal temperature. PURPOSE: Test the hypothesis that MSNA and SSNA continues to increase with more severe passive heating. METHODS: Twelve subjects, dressed in a water-perfused suit, underwent at least one passive heat stress that increased internal temperature ~1.3 °C, whilst either MSNA (N=8) or SSNA (N=6) was continuously recorded. RESULTS: Heat stress significantly increased internal temperature (Δ 1.3 ± 0.2°C, P<0.001), mean skin temperature (Δ 4.7 ± 0.9°C, P<0.001), mean body temperature (Δ 2.0 ± 0.2°C, P<0.001), heart rate (Δ 41 ± 10 beats·min−1, P<0.001) and cutaneous vascular conductance (Δ 1.1 ± 0.5 flux units·mmHg−1, P<0.001). Systolic blood pressure increased (Δ 7 ± 11 mm Hg, P=0.03), diastolic blood pressure decreased (Δ 6 ± 10 mm Hg, P=0.06), whilst mean arterial pressure was maintained (Δ 1 ± 8 mm Hg, P=0.52). Relative to baseline, MSNA increased midway through heat stress (Δ internal temperature 0.63 ± 0.01°C) when expressed as burst frequency (26 ± 14 to 45 ± 16 bursts·min−1, P=0.001), burst incidence (39 ± 13 to 48 ± 14 bursts·100 cardiac cyles−1, P=0.027) or total activity (317 ± 170 to 489 ± 150 units·min−1, P=0.022) and continued to increase until the end of heat stress when expressed as burst frequency (60 ± 15 bursts·min−1, P=0.009), burst incidence (56 ± 11 bursts·100 cardiac cyles−1, P=0.039) or total activity (648 ± 158 units·min−1, P=0.011) relative to the mid-heating stage. Similarly, SSNA (total activity) increased midway through the heat stress (Δ internal temperature 0.63 ± 0.12°C, Baseline; 1543 ± 493 to mid heat stress 3810 ± 1383 units·min−1, P=0.012) and tended to continue to increase until the end of heat stress (Δ core temperature 1.26 ± 0.18°C, 5764 ± 1761 units·min−1, P=0.115 vs. mid-heat stress). CONCLUSIONS: These results indicate that MSNA, and to some extent SSNA, continue to increase as internal temperature increases above previously reported values. Supported by NIH Grants HL61388 and HL84072