Microgravity Exposure Alters Sympathetic Modulation of Ventricular Repolarization Quantified From the ECG via Periodic Repolarization Dynamics

Prolonged microgravity exposure induces cardiovascular deconditioning, including orthostatic intolerance due to dysregulation of autonomic modulation of the cardiovascular system. Recent studies suggest that sympathetic modulation of ventricular repolarization can be assessed by measuring the index of Periodic Repolarization Dynamics (PRD), which quantifies low-frequency oscillations in ECG T-waves. In this study PRD was analyzed in ECGs from 22 volunteers at rest and during orthostatic Tilt-Table Test (TTT) performed before and after 60-day Head-Down (-6°) Bed Rest (HDBR) experiments to simulate microgravity effects. A very notable increase was found in resting PRD values measured at POST-HDBR with respect to PRE-HDBR: 2.70[3.21] deg
²
vs 2.05[1.18] deg
²
(median[IQR]). When PRD was evaluated during TTT, an even more remarkable increase was found in POST-HDBR with respect to PRE-HDBR: 4.25[6.46] deg
²
vs 2.39[3.32] deg
²
. A jump-based countermeasure was only able to counteract microgravity-induced effects in response to TTT but not at baseline. In conclusion, prolonged exposure to simulated microgravity induces changes in ventricular re-polarization that are measurable by PRD and are more manifested when assessed following sympathetic provocation. A jump-based countermeasure is only partially effective in counteracting such effects.