We studied the properties of low-frequency (LF) heart rate variability (HRV) and photoplethysmographic waveform variability (PPGV) and their interaction under conditions where the hemodynamic connection between them is obviously absent, as well as the LF regulation of PPGV in the absence of heart function. The parameters of HRV and finger PPGV were evaluated in 10 patients during cardiac surgery under cardiopulmonary bypass (on-pump cardiac surgery) with or without cardioplegia. The following spectral indices of PPGV and HRV were ertimated: the total spectral power (TP), the high-frequency (HF) and the LF ranges of TP in percents (HF% and LF%), and the LF/HF ratio. We assessed also the index S of synchronization between the LF oscillations in finger photoplethysmogram (PPG) and heart rate (HR) signals. The analysis of directional couplings was carried out using the methods of phase dynamics modeling. It is shown that the mechanisms leading to the occurrence of oscillations in the LF range of PPGV are independent of the mechanisms causing oscillations in the LF range of HRV. At the same time, the both above-mentioned LF oscillations retain their activity under conditions of artificial blood circulation and cardioplegia (the latter case applies only to LF oscillations in PPG). In artificial blood circulation, there was a coupling from the LF oscillations in PPG to those in HR, whereas the coupling in the opposite direction was absent. The coupling from the LF oscillations in PPG to those in HR has probably a neurogenic nature, whereas the opposite coupling has a hemodynamic nature (due to cardiac output).

This study aims to investigate the strength of synchronization between the autonomic control loops of the cardiovascular system (CVS) in patients with COronaVIrus Disease 2019 (COVID-19). Methods — We calculated the total percent of phase synchronization index (S index) between the loops of autonomic control of heart rate and vascular tone in two sample groups: healthy individuals and COVID-19 patients. Results — The group-average value of the S index in COVID-19 patients is lower comparing to healthy individuals, the Mann-Whitney U-test confirmed that the differences are statistically significant. Conclusion — Obtained results suggest that the decreased value of the S index reflects the presence of a viral disease, and the S index is a promising basis for non-invasive screening diagnostics of viral diseases, particularly of COVID-19.

Background — Analysis of the state of complex biological systems requires the use of sensitive methods for diagnosing interactions using experimental time series. Objective — To evaluate the possibility of using the technique of cross-recurrence quantification (CRQ) in studying the health status of newborns using low-frequency components of RR-interval signals and photoplethysmograms, reflecting the dynamics of the circuits in autonomic regulation of blood circulation. Methods — The study included two groups of neonates: 10 full-term newborns and 10 preterm neonates. We carried out simultaneous recording of electrocardiographic and photoplethysmographic signals. CRQ analysis was employed as the primary tool. Results — We established that some indices of CRQ analysis, characterizing the degree of interaction of the studied circuits, act as sensitive markers. They make it possible to distinguish the dynamics of the studied contours between healthy newborns and preterm neonates. Conclusion — The results of our study confirmed that CRQ is a promising tool in creating methods for diagnosing health conditions, including in newborns.

Many authors emphasize the fundamental and practical importance of investigation of the low-frequency rhythm in the brain and cardiac signals that correlate with the activity of the autonomic control of circulation. However complexity, nonstationarity and high nonlinearity of the system complicate the research and require application of the specialized methods. This study aims to compare the methods of coupling detection that were adapted to the detection of coupling between the loops of autonomic control of circulation. The analysis of the data from healthy subject demonstrated that the methods based on the analysis of phase dynamics and coherence function calculations are adequate for the investigation of coupling patterns in the autonomic control of circulation. However the results of both methods require the control of statistical significance.

Heart rate variability values are potentially important for assessing vegetative dysfunction in children.The purpose of the research: a comparative study of spectral heart rate variability parameters and synchronization of low-frequency oscillations, characterizing baroreflex in the vegetative regulation of the cardiovascular system in healthy newborns and adults.15 healthy newborns and 60 healthy individuals in the age of 18-34 years old were included in the study. We performed synchronous recording of electrocardiograms and photoplethysmograms with 10 minutes duration, at rest. Were evaluated spectral heart rate variability indices and the index of synchronization of low-frequency oscillations in heart rate variability and photoplethysmograms (S index).Results. In newborns, the peak of low-frequency oscillations was predominantly in the range 0.07–0.09 Hz, and high frequency 0.40–0.50 Hz. Newborns had lower LF% values than adults: 22.8 (14.1, 29.4) vs 32.9 (25.1, 41.9) (p=0.009). The S index in newborns was 20.1 (16.9, 26.5)%, and 33.2 (21.2, 45.4)% in healthy adults (p=0.023).Conclusion. We have first demonstrated that the interaction of baroreflex regulation of heart rhythm and peripheral blood filling in healthy newborns is characterized by lower values of S index, than in healthy adults, which can be explained by the immaturity of the vegetative regulatory elements of the cardiovascular system.

The purpose of this work is to study the interaction between the autonomic regulatory loops of blood circulation from long time series. Methods ― We simultaneously recorded four-hour signals of electrocardiogram and photoplethysmogram from the ear and finger of ten healthy adults. We determined the intervals of phase synchronization of the studied regulatory loops and analyzed the dependence of their length on the recording time. The deviations of the total percentage of phase synchronization (index S) from its mean value were estimated in moving non-overlapping windows. Results ― For studied signals we found no significant correlation between the length of synchronization epoch and the time of its beginning. A sharp increase in the deviation of the index S from its mean was shown at the end of the experiment. Conclusion ― The increase in the deviation from the mean at the end of our records is most likely associated more with psychosomatic influences than with hormonal regulation or immobilization stress.