Orthostatic tolerance, cerebral oxygenation, and blood velocity in humans with sympathetic failure

Background and Purpose-Patients with orthostatic hypotension due to sympath etic failure become symptomatic when standing, although their capability to maintain cerebral blood flow is reported to be preserved. We tested the hy pothesis that in patients with sympathetic failure, orthostatic symptoms re flect reduced cerebral perfusion with insufficient oxygen supply. Methods-This study addressed the relationship between orthostatic tolerance , mean cerebral artery blood velocity (V-mean, determined by transcranial D oppler ultrasonography), oxygenation (oxyhemoglobin [O(2)Hb], determined by near-infrared spectroscopy), and mean arterial pressure at brain level (MA P(MCA), determined by finger arterial pressure monitoring [Finapres]) in 9 patients (aged 37 to 70 years; 4 women) and their age- and sex-matched cont rols during 5 minutes of standing. Results-Supine MAP(MCA) (108+/-14 versus 86+/-14 mm Hg) and V-mean (84+/-21 versus 62+/-13 cm . s(-1)) were higher in the patients. After 5 minutes of standing, MAP(MCA) was lower in the patients (31+/-14 versus 72+/-14 mm Hg ), as was V-mean (51+/-8 versus 59+/-9 cm . s(-1)), with a larger reduction in O(2)Hb (-11.6+/-4 versus -6.7+/-4.5 mu mol . L-1). Pour patients termin ated standing after 1 to 3.5 minutes. In these symptomatic patients, the or thostatic fail in V-mean was greater (45+/-6 versus 64+/-10 cm . s(-1)), an d the orthostatic decrease in O(2)Hb (-12.0+/-3.3 versus -7.6+/-3.9 mu mol . L-1) tended to be larger. The reduction in MAP(MCA) was larger after 10 s econds of standing, and MAP(MCA) was lower after 1 minute (25+/-8 versus 40 +/-6 mm Hg). Conclusions-In patients with sympathetic failure, the orthostatic reduction in cerebral blood velocity and oxygenation is larger. Patients who become symptomatic within 5 minutes of standing are characterized by a pronounced orthostatic fall in blood pressure, cerebral blood velocity, and oxygenatio n manifest within the first 10 seconds of standing.