Complexity of Spontaneous QT Variability Unrelated to RR Variations and Respiration During Graded Orthostatic Challenge

The fluctuations of the duration of the electrical activity of the heart, measured as the time distance between Q-wave onset and T-wave end (QT), is under autonomic control. We studied the complexity of the QT variability regulation via the computation of sample entropy of QT variability during sympathetic activation induced by graded head-up tilt. Sample entropy was computed over the original QT series and after factorizing it into partial processes describing QT variability related to heart period, measured as the time interval between consecutive R-wave peaks (RR), linked to respiration (R) and unrelated to RR and R. We found that QT variability complexity is high and does not vary with the intensity of the stimulus. This result was the consequence of a non-significant tendency of the complexity of the QT variability related to RR to decrease and a significant raise of the complexity of the QT variability unrelated to RR and R with the magnitude of the orthostatic challenge. We suggest that the sample entropy of the QT variability unrelated to RR and R could quantify the increased heterogeneity of the neural inputs genuinely modulating QT during a sympathetic arousal.