Alteration of the relationship between ventricular repolarization and heart rate induced by 60-day head-down bed rest

Microgravity exposure produces cardiovascular deconditioning, with alterations in the ventricular repolarization phase and in its relationship with heart rate. The aim of this study is to assess the changes induced in the temporal and morphological characteristics of ventricular repolarization induced by microgravity simulated with 60-day head- down tilt (HDT) bed rest. Twenty volunteers were enrolled, ten of which performed the HDT only as a control (CTRL), and ten received a nutritional countermeasure (CM) during the bed rest. 12-lead 24-h Holter ECGs (1000 Hz, Mortara Instrument Inc.) were acquired at specific epochs 9 days before HDT (BDC- 9), the 5th (HDT5), 21st (HDT21) and 58th (HDT58) day of HDT, and the first (R+0) and 8th (R+7) day after HDT conclusion. The vectorcardiogram of the night period was computed, and selective beat averaging was applied to the beats within the range of 900 to 1200 ms. The RTapex and RTend intervals were calculated, as well as the maximum T wave amplitude (Tmax), its area (Tarea), the magnitude of the ventricular gradient (VG) and its spatial orientation (QRS- 2 Tangle). Also, the linear correlation (r , slope) of these parameters with corresponding heart rate was evaluated. Statistical analysis was applied to test the effect of the HDT (Friedman test, post-hoc Wilcoxon Signed Rank test with Hochberg correction), as well as to compare CTRL and CM groups at each epoch (Mann-Whitney test). The RTend and RTapex intervals reduced during the first half of the HDT, then recovering towards HDT conclusion. At R+0 a further decrease was observed, without completely recovering at R+7. T-wave morphology was additionally affected by simulated microgravity, with reduced Tmax, Tarea and VG at the beginning of the HDT, as well as up to 8 days after HDT conclusion. The strength of the relationship of the computed QT parameters with heart rate 2 particularly decreased immediately after HDT discontinuation, showing decreased r . In conclusion, prolonged exposure to microgravity, simulated by HDT bed rest experiments, produces significant changes in the temporal and morphological characteristics of ventricular repolarization. Also, its relationship with heart rate is compromised, leading to an increased risk of developing life-threatening cardiac arrhythmias. Major changes are elicited at the beginning of the HDT, as well as after bed rest discontinuation, lasting even 8 days after a 60-day HDT bed rest. The application of a vitamin and antioxidant nutritional countermeasure did not positively contribute in preventing or reducing such alterations.