Oscillatory Properties of Skin Sympathetic Nerve Activity in Heat Stressed Humans

Abstract

2141 Heat stress increases sympathetic outflow to muscle and skin vascular beds and sweat glands. These responses are evidenced by increases in both muscle (MSNA) and skin (SSNA) sympathetic nerve activities during heat stress. In a previous study, we found that heat stress significantly increased the high frequency (HF) component of MSNA without affecting the low frequently (LF) component, when compared with normothermic conditions. A previous study showed that SSNA has LF and HF spectral components. However, the effect of heat stress on spectral characteristics of SSNA is unknown. Such information may provide insight into whether SSNA and MSNA are similarly modulated during heat stress. PURPOSE: The purpose of the present study was to test the hypothesis that heat stress induces parallel changes in SSNA and MSNA variability. METHODS: In 11 healthy subjects SSNA, RR interval, arterial blood pressure (via Finapres), respiration, and forearm skin blood flow (SkBF) were recorded under normothermic and heat stress conditions. Spectral characteristics of these variables were assessed in the LF (0.03 to 0.15 Hz) and HF (0.15 to 0.45 Hz) ranges. RESULTS: Whole-body heating significantly increased skin and core temperatures, heart rate, SSNA total activity, and SkBF, but did not change mean arterial blood pressure. After whole body heating, systolic blood pressure and RR interval variability were significantly reduced in both frequency ranges, while SkBF variability increased significantly in both frequency ranges. Compared with normothermic conditions, heat stress significantly increased the LF and HF components of SSNA, while the LF/HF ratio of SSNA variability was not altered. This finding is in contrast to MSNA in which the LF/HF ratio is reduced by heat stress. CONCLUSIONS: Differences in the change in spectral power between MSNA and SSNA suggest these responses are differentially modulated during a heat stress. It is possible that differential modulation of SSNA relative to MSNA is related to the effects of SSNA in causing sweating and cutaneous vasodilation during a heat stress. Project funded in part by NIH HL61388, HL67422, GM68865 & AHA 0225036Y