Electrocardiographic changes during exercise in acute hypoxia and susceptibility to severe high-altitude illnesses

Abstract

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 V 1 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 V 6 and increased in V 1, the ST slope rose in V 5 and V 6, and the J point was lower in II, V 5, and V 6 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 V 5 with desaturation at exercise. Trained status and low body mass index were associated with a smaller decrease in T-wave amplitude in V 5 and V 6 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.