The linear dependence of ventricular repolarization variability on heart rate variability in head-down bed rest studies

Head down bed rest as Earth-based study for simulating microgravity affects cardiovascular system. However the consequences for the health of this experiment during and at recovery are still under research and in particular ventricular repolarization (VR) effects are not well known. VR dysfunctions could lead to cardiac arrhythmias and eventually to sudden cardiac death. Interactions of VR variability with heart rate variability (HRV) was used as proarrhythmic marker. In this study, three VR beat-to-beat indices extracted from ECG signals as QT, QT_p (QRS on-set to T wave peak) and T_pe (peak to end of T wave) were measured and their variabilities studied. ARARX modeling was the tool used to estimate the VR variability fraction driven by HRV at different time periods: PRE, during head down bed rest (5-day & 21-day periods) and POST. We found significant differences (p-values <; 0.01) comparing the repolarization variability content driven by heart rate in all VR series at PRE versus during BR microgravity conditions and after in both cases of study. Head down bed rest test increases the amount of linear dependency of VR variability and HRV. Results tend to evidence a slow recovery capacity for all VR variability after simulated microgravity exposition, being more notable for 21-day HDBR.