Arrhythmic risk evaluation during exercise at high altitude in healthy subjects: Role of microvolt T-wave alternans

Background: Altitude-induced sympathetic hyperactivity can elicit rhythm disturbances in healthy subjects, in particular during exercise. Aim: To asses the real susceptibility of healthy myocardium to malignant ventricular arrhythmias during exercise at high altitude using microvolt T-wave alternans (MTWA). Methods: We evaluated eight healthy trained participants (one female, 42 ± 9 years) during a mountain climbing expedition on Gashembrum II (Pakistan, 8,150 m). MTWA and heart rate variability (HRV) were measured in each subject at sea level and at high altitude, both under rest conditions and during exercise. MTWA was determined with the modified moving average method. HRV was expressed as root mean square of successive differences. Results: Rest HRV at high altitude was significantly lower compared to rest HRV at sea level (36 ± 5 vs 56 ± 9 ms, P = 0.003). HRV during exercise was significantly lower with respect to rest condition both in normoxia (46 ± 7 vs 56 ± 9 ms, P = 0.0001) and hypoxia (27 ± 4 vs 36 ± 5 ms, P = 0.005). Moreover, HRV was significantly lower during exercise at high altitude compared to exercise at sea level (27 ± 4 vs 46 ± 7 ms, P = 0.0002) and arrhythmias were more frequent during exercise in hypoxia. Nevertheless, MTWA was absent under rest conditions both at sea level and at high altitude and minimally evoked during exercise in both conditions (22 ± 3 μV and 23 ± 3 μV, respectively, P = 0.2). Conclusions: In spite of an enhanced sympathetic activity, MTWA testing during exercise at high altitude was negative in all participants. Healthy trained subjects during exercise under hypoxia seem to be at low risk for dangerous arrhythmias.