Loss of the cardiovagal baroreflex (CVB), thoracic hypovolemia, and hyperpnea contribute to the nonlinear time-dependent hemodynamic instability of vasovagal syncope We used a nonlinear phase synchronization index (PhSI) to describe the extent of coupling between cardiorespiratory parameters, systolic blood pressure (SBP) or arterial pressure (AP), RR-interval (RR), and ventilation, and a directional index (DI) measuring the direction of coupling. We also examined phase differences directly. We hypothesized that AP-RR PhSI would be normal during early upright tilt, indicating intact CVB, but would progressively decrease as faint approached and CVB failed. Continuous measurements of AP, RR, respiratory plethysomography, and EtCO2 were recorded supine and during 70 degree head-up tilt in 15 control subjects (C) and 15 fainters (F). Data was evaluated during 5 distinct times: baseline, early tilt, late tilt, faint, and recovery. During late tilt to faint, fainters exhibited a biphasic change in SBP-RR PhSI. Initially in fainters during late tilt, SBP-RR PhSI decreased (F: from 0.65±0.04 to 0.24±0.03 vs. C: from 0.51±0.03 to 0.48±0.03, P<0.01) but then increased at the time of faint (F=0.80±0.03 vs. C=0.42±0.04, P<0.001) coinciding with a change in phase difference from positive to negative. Starting in late tilt and continuing through faint, F exhibited increasing phase coupling between respiration and AP PhSI (F=0.54±0.06 vs. C=0.27±0.03, P<0.001) and between respiration and RR (F=0.54±0.05 vs. C=0.37±0.04, P<0.01). DI indicated respiratory driven AP (F=0.84 ±0.04 vs. C=0.39±0.09, P<0.01) and RR (F=0.73±0.10 vs.C=0.23±0.11, P<0.001) in fainters. The initial drop in the SBP-RR PhSI and directional change of phase difference at late tilt indicates loss of cardiovagal baroreflex. The subsequent increase in SBP-RR PhSI is due to a respiratory synchronization and drive on both AP and RR. Cardiovagal baroreflex is lost prior to syncope and supplanted by respiratory reflexes, producing hypotension and bradycardia.
- phase synchronization
- cardiovagal baroreflex
- Copyright © 2010, American Journal of Physiology - Heart and Circulatory Physiology