Published Online
on October 17, 2005

A more recent version of this article appeared on October 25, 2005

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Submitted on November 30, 2004
Revised on July 23, 2005
Accepted on August 2, 2005

Decreased Microvascular Nitric Oxide-Dependent Vasodilation in Postural Tachycardia Syndrome

Marvin S. Medow PhD, Christopher T. Minson PhD, and Julian M. Stewart MD, PhD^*

From New York Medical College, Valhalla (M.S.M., J.M.S.), and University of Oregon, Eugene (C.T.M.).

^* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.

Background--One variant of postural tachycardia syndrome (POTS), designated low-flow POTS, is associated with decreased peripheral blood flow related to impaired local vascular regulation.

Methods and Results--To investigate the hypothesis that microvascular endothelial dysfunction produces decreased peripheral blood flow in low-flow POTS, we performed experiments using laser-Doppler flowmetry (LDF) combined with iontophoresis in 15 low-flow POTS patients, 17 normal-flow POTS patients, and 13 healthy reference volunteers varying in age from 14 to 22 years. We tested whether -adrenergic vasoregulation was impaired using iontophoretic delivery of tyramine, phentolamine, and bretylium followed by a norepinephrine dose response. We tested endothelial-dependent and -independent receptor-mediated vasodilation by measuring acetylcholine and sodium nitroprusside dose responses. We tested whether nitric oxide-dependent vasodilation was different in these groups by testing the local thermal hyperemic response to saline used as a reference compared with the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (L-NAME). Adrenergic and receptor-dependent cutaneous vasoregulation was similar for low-flow POTS, normal-flow POTS, and reference subjects. Thermal hyperemia produced distinctly different findings: there was marked attenuation of the nitric oxide-sensitive plateau during prolonged heating, which was insensitive to L-NAME in low-flow POTS subjects. The pattern of thermal hyperemia response in low-flow POTS subjects during saline administration resembled the pattern in reference subjects during L-NAME administration and was minimally affected by L-NAME.

Conclusions--The data suggest that flow-dependent nitric oxide release is reduced in low-flow POTS. This may account for local flow regulation abnormalities.

Key words: blood flow • lasers • nitric oxide synthase • syncope • tachycardia

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