Systemic hypoxia causes cutaneous vasodilation in healthy humans.

Hypoxia and hypercapnia represent special challenges to homeostasis because of their effects on sympathetic outflow and vascular smooth muscle. In the cutaneous vasculature, even small changes in perfusion can shift considerable blood volume to the periphery and thereby impact both blood pressure regulation and thermoregulation. However, little is known about the influence of hypoxia and hypercapnia on this circulation. In the present study, 35 healthy subjects were instrumented with two microdialysis fibers in the ventral forearm. Each site was continuously perfused with saline (control) or bretylium tosylate (10 mM) to prevent sympathetically mediated vasoconstriction. Skin blood flow was assessed at each site (laser-Doppler flowmetry), and cutaneous vascular conductance (CVC) was calculated as red blood cell flux/mean arterial pressure and normalized to baseline. In 13 subjects, isocapnic hypoxia (85 and 80% O(2) saturation) increased CVC to 120 +/- 10 and 126 +/- 7% baseline in the control site (both P < 0.05) and 113 +/- 3 (P = 0.087) and 121 +/- 4% baseline (P < 0.05) in the bretylium site. Adrenergic blockade did not affect the magnitude of this response (P > 0.05). In nine subjects, hyperpnea (matching hypoxic increases in tidal volume) caused no change in CVC in either site (both P > 0.05). In 13 subjects, hypercapnia (+5 and +9 Torr) increased CVC to 111 +/- 4 and 111 +/- 4% baseline, respectively, in the control site (both P < 0.05), whereas the bretylium site remained unchanged (both P > 0.05). Thus both hypoxia and hypercapnia cause modest vasodilation in nonacral skin. Adrenergic vasoconstriction of neural origin does not restrain hypoxic vasodilation, but may be important in hypercapnic vasodilation.