CHRONIC ORTHOSTATIC INTOLERANCE - A DISORDER WITH DISCORDANT CARDIAC AND VASCULAR SYMPATHETIC CONTROL

Background-Chronic orthostatic intolerance (COI) is a debilitating aut onomic condition in young adults. Its neurohumoral and hemodynamic pro files suggest possible alterations of postural sympathetic function an d of baroreflex control of heart rate. (HR). Methods and Results-In 16 COI patients and 16 healthy volunteers, intra-arterial blood pressure (BP), EGG, central venous pressure (CVP), and muscle sympathetic nerv e activity (MSNA) were recorded at rest and during 75 degrees tilt. Sp ectral analysis of RR interval and systolic arterial pressure (SAP) va riabilities provided indices of sympathovagal modulation of the sinoat rial node (ratio of low-frequency to high-frequency components, LF/HF) and of sympathetic vasomotor control (LFSAP). Baroreflex mechanisms w ere assessed (1) by the slope of the regression line obtained from cha nges of RR interval and MSNA evoked by pharmacologically induced alter ations in BP and (2) by the index alpha, obtained from cross-spectral analysis of RR and SAP variabilities. At rest, HR, MSNA, LF/HF, and LF SAP were higher in COI patients, whereas BP and CVP were similar in th e two groups. During tilt, BP did not change and CVP fell by the same extent in the 2 groups; the increase of HR and LF/HF was more pronounc ed in COI patients. Conversely, the increase of MSNA was lower in COI than in control subjects. Baroreflex sensitivity was similar in COI an d control subjects at rest; tilt reduced alpha similarly in both group s. Conclusions-COI is characterized by an overall enhancement of norad renergic tone at rest and by a blunted postganglionic sympathetic resp onse to standing, with a compensatory cardiac sympathetic overactivity . Baroreflex mechanisms maintain their functional responsiveness. Thes e data suggest that in COI, the functional distribution of central sym pathetic tone to the heart and vasculature is abnormal.