Decreased microvascular nitric oxide-dependent vasodilation in postural tachycardia syndrome

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 alpha-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.