Orthostatic Tolerance, Cerebral Oxygenation, and Blood Velocity in Humans With Sympathetic Failure

Abstract

Background and Purpose —Patients with orthostatic hypotension due to sympathetic failure become symptomatic when standing, although their capability to maintain cerebral blood flow is reported to be preserved. We tested the hypothesis that in patients with sympathetic failure, orthostatic symptoms reflect 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 Doppler ultrasonography), oxygenation (oxyhemoglobin [O ₂ Hb], determined by near-infrared spectroscopy), and mean arterial pressure at brain level (MAP _MCA , determined by finger arterial pressure monitoring [Finapres]) in 9 patients (aged 37 to 70 years; 4 women) and their age- and sex-matched controls 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 μmol · L ⁻¹ ). Four patients terminated standing after 1 to 3.5 minutes. In these symptomatic patients, the orthostatic fall in V _mean was greater (45±6 versus 64±10 cm · s ⁻¹ ), and the orthostatic decrease in O ₂ Hb (−12.0±3.3 versus −7.6±3.9 μmol · L ⁻¹ ) tended to be larger. The reduction in MAP _MCA was larger after 10 seconds 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 oxygenation manifest within the first 10 seconds of standing.