A mathematical model is proposed for the autonomic control of cardiovascular system, which takes into account two separated self-exciting sympathetic control loops of heart rate and peripheral vascular tone. The control loops are represented by self-exciting time-delay systems and their tone depends on activity of the aortic, carotid, and lower-body baroreceptors. The model is used to study the dynamics of the adaptive processes that manifest in a healthy cardiovascular system during the passive head-up tilt test. Computer simulation provides continuous observation of the dynamics of the indexes and variables that cannot be measured in the direct experiment, including the noradrenaline concentration in vessel wall and heart muscle, tone of the sympathetic and parasympathetic control, peripheral vascular resistance, and blood pressure. In the supine and upright positions, we estimated the spectral characteristics of the model variables, especially in the low-frequency band, and the original index of total percent of phase synchronization between the low-frequency oscillations in heart rate and blood pressure signals. The model demonstrates good quantitative agreement with the dynamics of the experimentally observed indexes of cardiovascular system that were averaged for 50 healthy subjects.

Aim of the study is to apply the analysis of phase dynamics to investigation of the coupling patterns between heart rate variability, respiration and peripheral circulation in healthy subjects at rest and after moderate physical exercises. Material and Methods ― 30-minutes electrocardiogram (ECG), photoplethysmogram (PPG) and respiration records were obtained from healthy subjects aged 22±2 (mean ± standard deviation) before and after active Martine Kushelevsky test (20 squats in 30 seconds). The coherence function was estimated between all the signals from each subject, and the phase dynamics modeling was used to detect the directional coupling in high-frequency (HF; 0.14-0.40 Hz) and low-frequency ranges (LF; 0.04-0.14 Hz). Results ― At rest (before the physical activity) no statistically significant couplings were detected between the HF rhythms of respiration and heart rate (RR intervals). For the HF rhythms in respiration and PPG, the influence respirationPPG was detected. No couplings were detected between the LF rhythms. After the moderate physical exercise, predominant direction of the influence was RR intervals  PPG in the LF range and both RR intervals  respiration and PPG  respiration in the HF range. The influence PPG  respiration was delayed by several seconds. Conclusion ― adaptation to the moderate physical activity has led to the decrease in the overall coherence and changes in patterns of directional coupling between the LF and HF rhythms in respiration, heart rate and peripheral circulation. The obtained results confirm nonlinearity and complexity of the coupling patterns in cardiovascular system.

This study involved 72 volunteers divided into two groups according to the apnea–hypopnea index (AHI): $\mathrm{AHI}>15$ episodes per hour ($\mathrm{e}\mathrm{p}/\mathrm{h}$) (main group, $n=39$, including 28 men, median AHI 44.15, median age 47), $0\le \mathrm{AHI}\le 15\mathrm{e}\mathrm{p}/\mathrm{h}$ (control group, $n=33$, including 12 men, median AHI 2, median age 28). Each participant underwent polysomnography with a recording of 19 EEG channels. Based on wavelet bicoherence (WB), the magnitude of connectivity between all pairs of EEG channels in six bands was estimated: $D{f}_1$ $\left[0.25;1\right]$, $D{f}_2$ $\left[1;4\right]$, $D{f}_3$ $\left[4;8\right]$, $D{f}_4$ $\left[8;12\right]$, $D{f}_5$ $\left[12;20\right]$, $D{f}_6$ $\left[20;30\right]$ Hz. In all six bands considered, we noted a significant decrease in symmetrical interhemispheric connections in OSA patients. Also, in the main group for slow oscillatory activity $D{f}_1$ and $D{f}_2$, we observe a decrease in connection values in the EEG channels associated with the central interhemispheric sulcus. In addition, patients with $\mathrm{AHI}>15$ show an increase in intrahemispheric connectivity, in particular, forming a left hemisphere high-degree synchronization node (connections PzT3, PzF3, PzFp1) in the $D{f}_2$ band. When considering high-frequency EEG oscillations, connectivity in OSA patients again shows a significant increase within the cerebral hemispheres. The revealed differences in functional connectivity in patients with different levels of AHI are quite stable, remaining when averaging the full nocturnal EEG recording, including both the entire sleep duration and night awakenings. The increase in the number of hypoxia episodes correlates with the violation of the symmetry of interhemispheric functional connections. Maximum absolute values of correlation between the apnea–hypopnea index, AHI, and the WB synchronization strength are observed for the $D{f}_2$ band in symmetrical EEG channels C3C4 ($-0.81$) and P3P4 ($-0.77$). The conducted studies demonstrate the possibility of developing diagnostic systems for obstructive sleep apnea syndrome without using signals from the cardiovascular system and respiratory activity.

According to the European Society of Cardiology guidelines, patients with chest pain requires the calculation of pretest probability (PTP) of coronary artery disease (CAD), followed by the identification of diagnostic tactics.Aim. To analyze the registry of patients with stable CAD in order to assess the frequency of PTP calculation, as well as the validity of diagnostic tactics choice depending on its level.Material and methods. The data of the registry of patients with stable CAD for the periods from 2012 to 2014 and from 2017 to 2019 were analyzed. We assessed the number of CAD patients, proportion of men, and distribution of patients depending on angina functional class. In addition, data on PTP calculation, as well as distribution of patients and diagnostic strategy selected depending on its value was analyzed. To assess differences (p) in pairwise comparisons, Student’s t-test was used. Differences were considered significant at p<0,05. Statistical analysis was performed using Microsoft Office Excel 2010 (Microsoft, USA) and STATISTICA 6.0 (StatSoft Inc., USA).Results. In 2017-2019, the number of detected CAD cases and proportion of men increased. In both time periods, an insufficient level of calculating PTP of CAD remains. In patients with intermediate PTP, 15-85% of priority tactics are invasive interventions, and with high PTP, the percentage of invasive methods does not reach the proper level, which does not correspond to modern guidelines for the management of CAD patients and leads to misappropriation of funds and healthcare resources.Conclusion. According to medical records, PTP can be determined in less than half of patients. In CAD patients with intermediate PTP, non-invasive studies are not carried out in full, since coronary angiography is preferred. In patients with a high PTP, invasive diagnostics is insufficient.