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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; Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, USA
* To whom correspondence should be addressed. E-mail: CraigCrandall{at}texashealth.org.
Skin sympathetic nerve activity (SSNA) exhibits low and high frequency spectral components when subjects are normothermic. However, spectral characteristics of SSNA under heat stress conditions are unknown. Since the main components of the integrated SSNA during heat stress conditions (sudomotor/vasodilator activities) are different from that in normothermic and cooling conditions (vasoconstrictor activity), we hypothesize that spectral characteristics of SSNA in heat stressed subjects will be different relative to when subjects are under normothermic or cooling conditions. In 17 healthy subjects, SSNA, electrocardiogram, arterial blood pressure (via Finapres), respiratory activity and skin blood flow were recorded under both normothermic and heat stress conditions. In 7 of the 17 subjects, these variables were also recorded during cooling. Spectral characteristics of integrated SSNA, RR interval, beat-by-beat mean blood pressure, skin blood flow variability, and respiratory excursions were assessed. Both heat stress and cooling significantly increased total SSNA. The SSNA spectral power in the low (0.03 to 0.15 Hz), high (0.15 to 0.45 Hz), and very high (0.45 to 2.5 Hz) frequency regions were significantly elevated by both heat stress and cooling. Interestingly, heat stress caused a greater relative increase of SSNA spectral power within the 0.45 to 2.5 Hz region, when compared to the other spectral ranges. In contrast, cooling did not show this similar effect. Differences in the SSNA spectral distribution between normothermic/cooling and heat stress conditions may reflect different characteristics of central modulation of vasoconstrictor and sudomotor/vasodiator activities.
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