Tolerance to central hypovolemia: the influence of oscillations in arterial pressure and cerebral blood velocity.

Higher oscillations of cerebral blood velocity and arterial pressure (AP) induced by breathing with inspiratory resistance are associated with delayed onset of symptoms and increased tolerance to central hypovolemia. We tested the hypothesis that subjects with high tolerance (HT) to central hypovolemia would display higher endogenous oscillations of cerebral blood velocity and AP at presyncope compared with subjects with low tolerance (LT). One-hundred thirty-five subjects were exposed to progressive lower body negative pressure (LBNP) until the presence of presyncopal symptoms. Subjects were classified as HT if they completed at least the -60-mmHg level of LBNP (93 subjects; LBNP time, 1,880 ± 259 s) and LT if they did not complete this level (42 subjects; LBNP time, 1,277 ± 199 s). Middle cerebral artery velocity (MCAv) was measured by transcranial Doppler, and AP was measured at the finger by photoplethysmography. Mean MCAv and mean arterial pressure (MAP) decreased progressively from baseline to presyncope for both LT and HT subjects (P < 0.001). However, low frequency (0.04-0.15 Hz) oscillations of mean MCAv and MAP were higher at presyncope in HT subjects compared with LT subjects (MCAv: HT, 7.2 ± 0.7 vs. LT, 5.3 ± 0.6 (cm/s)(2), P = 0.075; MAP: HT, 15.3 ± 1.4 vs. 7.9 ± 1.2 mmHg(2), P < 0.001). Consistent with our previous findings using inspiratory resistance, high oscillations of mean MCAv and MAP are associated with HT to central hypovolemia.