Synchronization of low-frequency oscillations in the human cardiovascular system

We investigate synchronization between the low-frequency oscillations of heart rate and blood pressure having in humans a basic frequency close to 0.1 Hz. A method is proposed for quantitative estimation of synchronization between these oscillating processes based on calculation of relative time of phase synchronization of oscillations. It is shown that healthy subjects exhibit on average substantially longer epochs of internal synchronization between the low-frequency oscillations in heart rate and blood pressure than patients after acute myocardial infarction. The human cardiovascular system (CVS) is one of the most important physiological systems whose operation is governed by several rhythmic processes interacting with each other. The most significant among them are the main heart rhythm, respiration, and low-frequency (LF) oscillations in heart rate and blood pressure with a basic frequency close to 0.1 Hz having a great importance for maintaining cardiovascular homeostasis. The origin of these LF oscillations is still a subject of controversy. According to one hypothesis, the 0.1 Hz oscillations in heart rate and blood pressure are largely an index of baroreflex gain. On another hypothesis, these oscillations have a central origin and represent an intrinsic property of autonomous neural network. We have investigated interaction between the 0.1 Hz cardiovascular oscillations in healthy subjects and patients after acute myocardial infarction (AMI). Peculiarities of interaction of the elements of CVS reflect its state and may contain useful information for medical diagnostics. The interaction between the rhythms is studied using a phase synchronization measure based on the Hilbert transform. A method is proposed for quantitative estimation of synchronization by using an algorithm of automated detection of phase synchronization epochs. The method is based on a linear approximation of instantaneous phase difference of analyzed signals in a moving window. Recommendations on a choice of the method parameters ensuring its high efficiency are given. A statistical significance of the calculated synchronization measure is analyzed using surrogate data. It is shown that healthy subjects exhibit on average substantially longer epochs of synchronization between the LF oscillations in heart rate and blood pressure than patients after AMI. Arguments are adduced in favor of the concept that oscillations of heart rate and blood pressure with a frequency of about 0.1 Hz have a central origin. Moreover, these oscillations may be considered as different processes that exhibit a comparatively high internal synchronization between themselves in healthy subjects ensuring a high adaptability of the CVS.