Microgravity effects on ventricular response to heart rate changes

The effect of simulated microgravity on ventricular repolarization (VR) has been evaluated on healthy volunteers by a 5-day Head Down (-6°) Bed Rest (HDBR) maneuver. QT to RR and QT_p (measured until the peak of the T wave) to RR hystereses have been measured during a tilt table test, and differences between them have been studied to better understand possible changes in the final part of the repolarization. To characterize the hystereses, two indices have been computed: M₉₀, quantifying adaptation lag in beats, and α evaluating the slope of parabolic regression fitting. Significant differences between QT and QT_p were found before, but not after HDBR. Specifically, before HDBR was considerable lower for QT_p than for QT, while α was significantly higher. After HDBR, M₉₀ and a took essentially the same values for QT and QT_p. This fact evidenced the different effect of HDBR on QT to RR and QT_p to RR adaptations, and suggest HDBR could lead to an impairment in ventricular repolarization dispersion.