ELECTROCARDIOGRAPHIC CHANGES DURING HYPOBARIC HYPOXIA

Introduction: Acute hypobaric hypoxia during high-altitude exposure may affect cardiac autonomic regulation and, consequently, ECG intervals and morphology; however, published findings remain heterogeneous due to differences in altitude, exposure profile, and measurement methodology. The aim of this study was to assess short-term ECG changes during routine hypobaric hypoxia exposure to 5,000 m above sea level (a.s.l.) in clinically healthy young men. Materials and methods: The study included 68 healthy men (17–24 years) undergoing a standardized hypobaric-chamber (HC) protocol comprising baseline, ascent, a 30-minute stay at 5,000 m a.s.l. (six consecutive 5-minute phases), and descent. Continuous 3-lead Holter ECG monitoring and pulse oximetry were performed throughout the exposure. Heart rate (HR), PQ, QRS, QT, QTc, ST-interval, and R- and T-wave amplitudes were evaluated using repeated-measures analysis with appropriate corrections; associations between ECG parameters, HR, and peripheral capillary oxygen saturation (SpO₂) were assessed. Results: Mean SpO₂ decreased from 96.87±0.97% at baseline to 79.85±3.11% at 5,000 m a.s.l., while HR increased from 85.64±14.22 bpm to 98.20±8.03 bpm. HR showed significant inverse associations with SpO₂ within each study stage (p<0.001). No rhythm disorders were observed during any stage of chamber exposure. Across successive 5-minute phases at 5,000 m a.s.l., PQ, QRS, and QT intervals showed progressive shortening, whereas QTc demonstrated only a small, non-significant tendency toward prolongation. No measurable ST-interval deviation was detected. Changes in R-wave amplitude and T-wave morphology were non-significant. Conclusions: There are significant negative correlations between oxygen saturation and heart rate. The durations of the PQ interval, QT interval, and the QRS complex shortened significantly during ongoing hypoxia, including the 30-minute stay at the simulated altitude of 5,000 m a.s.l. in the HC (while remaining within the normal range). These changes may reflect integrated chemoreflex-mediated autonomic–chronotropic adaptation to hypoxemia. In contrast, R- and T-wave amplitudes, QTc duration, and the ST interval did not change significantly. A routine altitude-hypoxia tolerance test performed at a simulated altitude of 5,000 m a.s.l. in the HC in this cohort did not reveal any clinically relevant changes in ECG morphology. Accordingly, continuous rhythm and heart rate monitoring may be sufficient for surveillance in similar low-risk settings. However, extrapolation to other populations, longer exposures, or higher-risk individuals should be approached with caution.