Electrocardiographic Changes During Exercise in Acute Hypoxia and Susceptibility to Severe High-Altitude Illnesses

BACKGROUND: The goals of this study were to compare ECG at moderate exercise in normoxia and hypoxia at the same heart rate, to provide evidence of independent predictors of hypoxia-induced ECG changes, and to evaluate ECG risk factors of severe high-altitude illness. METHODS AND RESULTS: A total of 456 subjects performed a 20-minute hypoxia exercise test with continuous recording of ECG and physiological measurements before a sojourn above 4000 m. Hypoxia did not induce any conduction disorder, arrhythmias, or change in QRS axis. The amplitude of the P wave in V1 was lower in hypoxia than in normoxia. The amplitudes of the R, S, and T waves and the Sokolow index decreased in hypoxia. Under hypoxia, the amplitude of the ST segment decreased in II and V6 and increased in V1, the ST slope rose in V5 and V6, and the J point was lower in II, V5, and V6. Multivariate regression of hypoxic/normoxic ratios of electrophysiological parameters and clinical characteristics showed a correlation between the decrease in Sokolow index and T-wave amplitude in V5 with desaturation at exercise. Trained status and low body mass index were associated with a smaller decrease in T-wave amplitude in V5 and V6. Comparison of ECG between subjects suffering or not suffering from severe high-altitude illness failed to show any difference. CONCLUSIONS: During a hypoxia exercise test, a dose-dependent hypoxia-induced decrease in the amplitude of the P/QRS/T waves was observed. No standard ECG characteristic predicted the risk of developing severe high-altitude illness. Further studies are required to clarify the cause of these electric changes and their potential predictive role in cardiac events.