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1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA
2 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA; Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, USA
* To whom correspondence should be addressed. E-mail: CraigCrandall{at}texashealth.org.
Whole-body heating increases muscle sympathetic nerve activity (MSNA), however the effect of heat stress on spectral characteristics of MSNA is unknown. Such information may provide insight into mechanisms of heat stress induced MSNA activation. The purpose of the present study was to test the hypothesis that heat stress induced changes in systolic blood pressure variability parallel changes in MSNA variability. In 13 healthy subjects, MSNA, electrocardiogram, arterial blood pressure (via Finapres), and respiratory activity were recorded under both normothermic and heat stress conditions. Spectral characteristics of integrated MSNA, RR interval, systolic blood pressure variability, and respiratory excursions were assessed in the low (LF; 0.03 to 0.15 Hz) and high (HF; 0.15 to 0.45 Hz) frequency components. Whole-body heating significantly increased skin and core temperatures, MSNA burst rate, and heart rate, but not mean arterial blood pressure. Systolic blood pressure and RR interval variability were significantly reduced in both the LF and HF ranges. Compared with normothermic conditions, heat stress significantly increased the HF component of MSNA, while the LF component of MSNA was not altered. Thus, the LF/HF ratio of MSNA oscillatory components was significantly reduced. These data indicate that spectral characteristics of MSNA are altered by whole-body heating, however heat stress induced changes in MSNA do not parallel changes in systolic blood pressure variability. Moreover, the reduction in LF component of systolic blood pressure during heat stress is unlikely related to spectral changes in MSNA.
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