Control Of Cutaneous Vascular Conductance During Presyncope In The Heat Stressed Human

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Pronounced reductions in peripheral vascular resistance occur upon the onset of presyncope during orthostatic stress. This response contributes to the rapid fall in blood pressure associated with presyncopal symptoms. During pronounced orthostatic stress in the heat stressed individuals, cutaneous vascular conductance (CVC) remains above pre-heat stress baseline. It is unknown whether CVC increases at presyncope during combined heat and orthostatic stress. PURPOSE This project tested the hypothesis that CVC increases during orthostatic-induced presyncope in heat stressed individuals. METHODS Five healthy subjects experienced presyncope during lower body negative pressure while heat stressed (via water perfused suit). Forearm skin blood flow (SkBF), arterial blood pressure (catheterization of brachial artery), and heart rate (HR) were continuously measured. CVC was calculated from the ratio SkBF and mean arterial pressure (MAP) 1 min prior to presyncope and at presyncope. RESULTS Heat stress significantly increased skin and internal temperatures, CVC, and HR, without changing MAP. At presyncope HR (82±18 to 65±12 bpm; P=0.02) and MAP (71±11 to 54±4 mmHg; P=0.05) were reduced relative to the period prior to presyncope. However, there was no difference in CVC between these time points (0.88±0.22 to 0.91±0.31 AU/mmHg; P=0.81) despite a decrease in SkBF (62.3±16.2 to 48.6±15.2 AU; p < 0.01). CONCLUSIONS These finding do not support the hypothesis that CVC increases during presyncope, thereby suggesting that cutaneous vascular responses do not contribute to the reduction in blood pressure during vasovagal syncope in heat stressed subjects. Work supported by NIH HL67422 and HL61388