Microgravity exposure for long periods of time lead to body deconditioning and it could drive in increased risk of suffering arrhythmias which can be life-threatening. The study of cardiac ventricular repolarization (VR) ac-tivity has been used to evaluate the arrhythmic risk. VR is reflected in the electrocardiographic signal (ECG), so it is possible to be analysed with a non-invasive technique. Some risk markers have been proposed, but in particular in this study, it is taken the relationship of QT p interval (measure from QRS onset to T wave apex) to heart rate (HR). VR has a dependece with previous HR ocurrence times. Therefore, if HR changes, it will be expected an adaptation on VR. This adaptation is not necessarily in-stantaneous. If it is an abrupt change it will exist a lag which has been related with arrhthymic risk. This lag is characterized by M 90 such as the time to reach the 90% of the adaptation. The QT p to HR relationship is also evaluated. It is modeled considering a set of re-gression functions to cover a wide range of physiology possibilities. The study was performed with healthy sub-jects (male) under simulated microgravity conditions by 5-day head-down bed rest (HDBR) experiment and the abrupt changes in HR were produced by Tilt Table Test (TTT) when subjects cross over to standing position. As the TTT is used as a test for orthostatic intolerance (OI), we divided the subjects in two groups with respect to OI time: G Short and G Long , excluding those who had OI (less than 4 minutes). It was found different behavior for both groups. G Short group presented a significant de-crease in the adaptation lag that it may be produced by an extra deconditioning in the sympathovagal response, whereas in G Long group was found a significant increase of the adaptation lag and a significant reduction of the slope, α, correlating with an increase of suffering arrhy-timic risk.