Phase synchronization in a double-loop feedback model of blood pressure regulation

We studied the dynamics of blood pressure and heart rate fluctuations during a head-up tilt testing in 127 patients and healthy individuals. To assess the regulation efficiency, in addition to the conventionally measured baroreflex sensitivity BRS, we also studied the stochastic phase synchronization coefficient Sync between short-term variations of the blood pressure and the heart rate estimated by Hilbert transform. We observed either negative or insignificant correlations between BRS and Sync in healthy subjects as well as in patients with moderate autonomic disorders. We attribute this effect to the partial compensation of the reduced BRS by a more frequent activation of the feedback mechanism indicated by a higher Sync value. In contrast, in diabetes patients with confirmed autonomic neuropathy BRS and Sync exhibited pronounced positive correlation (C = 0.82; p <; 0.01) that likely indicates the breakdown of this compensation mechanism.