The electrical activity of the human heart during ventricular repolarization under acute normobaric hypoxia before and after interval hypoxic training

Morphological and functional differences in the heart, which have not resulted from a pathological process, do not cause specific changes in the traditional ECG, widely used for determining various functional disorders in the myocardium. The Body Surface Potential Mapping method (BSPM), known as noninvasive multichannel synchronous recording of electrical potentials of the heart on the thoracic surface from multiple unipolar leads, is a more informative method for studying the functional state of the heart, which makes it possible to obtain more data on the electrical processes in the myocardium compared to the standard electrocardiography. The aim of this research was to investigate the electrical activity of the human heart by the method of BSPM during the period of ventricular repolarization with acute normobaric hypoxia before and after a course of interval hypoxic training. The study population consisted of 14 practically healthy young men (19.7 ± 1.0 years, weight 74.4 ± 9.8 kg, height 177.2 ± 6.4 cm). All subjects gave information consent to participate in this study; the protocol of the study was approved by the Bioethics Committee of the Vil'gort Scientific Experimental Biological Station, Branch of the Federal Research Centre “Komi Science Centre”, Ural Branch of the Russian Academy of Sciences (Syktyvkar, Russia). We studied the heart's electrical activity in young men using traditional and multiple electrocardiography during the ventricle repolarization period of the heart to the acute normobaric hypoxia (gas mixture contains 12% of O₂) before and after a 19-day interval hypoxic training. BSPM with 64 unipolar leads covering the thorax was perfomed. Limb lead II was used as a reference. The electrodes located in the intercostal space on the torso with 3-5 cm distance were used. The electrodes were attached to 8 flexible strips each containing 8 electrodes. BSPM was recorded in the supine position at rest. We analyzed the amplitude characteristics of the positive and negative extrema (the amplitude of the maximum and the amplitude of the minimum, respectively) and the time they reach the maximum amplitudes at the period of the ventricular repolarization (the maximum time and the minimum time, respectively) (See Fig. 1). In the initial state, at each minute of the acute hypoxia and the recovery period - normoxia (5 min) the heart rate (HR) and hemoglobin saturation (SpO₂) were measured in each subject by an oximeter (Nonin Medical Inc., USA). Systolic and diastolic blood pressure was registered by an automatic tonometer (OMRON, Japan). At each minute of the study, unipolar ECG from 64 electrodes located on the thorax surface was recorded. In limb lead II of ECG, the QT_II, R-R_II, J-Tpeak_II and Tpeak-Tend_II intervals were determined, the corrected QT interval (QTc) was calculated using the Bazett formula. The course of interval hypoxic training (IHT) consisted of 19 days of breathing with a hypoxic mixture with 10% oxygen content in the intermittent mode. The first day included 6 cycles (one cycle - 5 minutes of breathing with a hypoxic mixture and 2 minutes of breathing with atmospheric air (normoxia)), the second day -8 cycles, from the third to the tenth day - 10 cycles. From 11 to 19 days, the training consisted of 10 cycles and normoxia was 1 minute. The normality of the distribution of values was determined by the Shapiro-Wilk test; the results are represented as mean values and their standard deviations (M±m). Statistical examinations were performed using the paired Student's t-test. The differences were considered significant at p<0.05. In this research, we revealed that hemodynamic parameters under the hypoxic influence demonstrated favorable training effect of the interval exposures on the subjects. According to the results of the analysis of hemodynamic parameters, we showed that hemoglobin oxygen saturation, heart rate, and systolic and diastolic blood pressure did not statistically differ with the same data in the initial state after the course of interval hypoxic training. In comparison with the ECG in standard leads, statistically significant changes in the temporal dynamics (before and after interval exposures) of the extrema were detected on the heart electric field (See Fig. 2 and 3). During acute normobaric hypoxia, the changes of the ventricles repolarization on the ECG limb leads were revealed: the shortening of the corrected QT interval corresponded to the decrease in durations of J-Tpeak_II and Tpeak-Tend_II intervals, the severity of correlation changed with hypoxia duration (See Fig. 4 and 5); the time required to reach maximum extrema values was shorter; the changes of temporal dynamic of the negative extremum was shown. In the initial state after the course of interval hypoxic training, the amplitudes of the maximum and the minimum differed insignificantly from the values before the interval hypoxic training. During acute hypoxia before and after the course of interval hypoxic training, a decrease in the maximum amplitude of the positive extremum was revealed. During hypoxic exposure before the course of interval hypoxic training, the maximum values of the positive and negative extrema of the heart's electrical field did not significantly differ from those in the initial state. After hypoxic training, when exposed to acute hypoxia, the amplitude maximum and minimum decreased significantly (p<0.05). After interval training under the exposure of acute hypoxia in the subjects in the period corresponding to the ST-T interval, we revealed changes in the temporal parameters of the ECG in the limb leads and of the extrema of the heart's electrical field on the torso surface in comparison with the initial state. The changes in amplitude-temporal characteristics of the extrema of the heart's electrical field were revealed using the BSPM method, that was the result of the structural changes of ventricular repolarization of the heart of the subjects (See Table). Thus, using the BSPM method during acute hypoxia after the intermittent hypoxic training we identified the initial changes in the electrical activity of the heart which were not detected using traditional methods of studying cardiac electrophysiology. The paper contains 5 Figures, 1 Table and 26 References.

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