Respiration drives phase synchronization between blood pressure and R‐R interval following loss of cardiovagal baroreflex during vasovagal syncope

Abstract

Thoracic hypovolemia and hyperpnea contribute to the nonlinear time‐dependent hemodynamic instability of vasovagal syncope. We used a phase synchronization index (PhSI) to describe coupling between systolic BP (SBP), RR‐interval (RR), and ventilation, and a directional index (DI) of coupling. We hypothesized normal AP‐RR PhSI during early 70° head‐up tilt, indicating intact cardiovagal baroreflex (CVB), but decrease as faint approached. Data were recorded supine and upright in 15 control (C) and 15 fainters (F). Data were evaluated during baseline, early tilt, late tilt, faint, and recovery. During late tilt to faint, F but not C showed SBP‐RR PhSI decrease (from 0.65±0.04 to 0.24±0.03) which increased at faint (0.80±0.03) coinciding with a change in phase difference from positive to negative. Starting in late tilt and through faint, F but not C exhibited increasing phase coupling between respiration and AP and between respiration and RR. DI indicated respiratory driven AP and RR in F. 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.