Cinétiques de la fréquence cardiaque et de la repolarisation ventriculaire durant l’effort et la récupération

During physiological heart rate variations in the course of exercise and recovery, the QT interval adapts to cardiac cycle length (RR) according to two distinct processes: QT/RR dependency which describes the steady-state relationship between QT and RR intervals, and QT/RR hysteresis which describes the time course of QT accommodation between steady-state conditions. Several approaches were suggested to quantify these two processes. It appears that both QT/RR dependency and QT/RR hysteresis are highly individual and it was put forward that certain profiles of QT/RR dependency and hysteresis could be useful risk predictors in cardiac patients. However, these descriptors of the QT/RR relationship remain largely variable among healthy subjects. While it is plausible that such inter-individual discrepancies are explained by variations of cardiac autonomic modulation, very few studies have addressed this question. The work conducted in this thesis aims to define the normal response of the QT interval during exercise and recovery and to characterize adaptations of this response in endurance-trained individuals. A systematic review of the literature addressing methods of QT/RR hysteresis estimation is reported. The review highlights the inherent limits of reported approaches and a theoretical comparison of methods suggests that one procedure, namely estimation of the memory in the modelled QT/RR relationship, is superior for comparison of QT/RR hysteresis between conditions where the range of observed RR intervals varies. A second systematic review was conducted with the aim of identifying clinical conditions in which QT/RR hysteresis was examined. It emerges from this review that an increased QT/RR hysteresis could be a predictor of severe arrhythmia, a marker of exercise-induced myocardial ischemia and a feature of the long QT syndrome. However, the proofs of such associations are rather weak and would need to be confirmed by larger studies using reliable methods that can differentiate QT/RR hysteresis from QT/RR dependency. A study was conducted in healthy men in order to determine if RR kinetics and QT/RR hysteresis varied in the same subject between exercise protocols implying different mechanisms of autonomic modulation. The results demonstrate that both RR kinetics and QT/RR hysteresis are slower during recovery compared to exercise, and that RR kinetics is slower when exercise or recovery is initiated from a higher baseline heart rate. This work also suggests that the usual assumption that QT/RR hysteresis remains invariant in a same individual is incorrect. It appears that hysteresis is larger in the transition from rest to low intensity exercise compared to other investigated conditions. In addition, our results show that quantification of individual RR and QT kinetics is more reliable when it is computed from recordings including exercise data at moderate intensities compared to estimations relying solely on data recorded during low intensity exercise. The last study aimed to compare QT/RR dependency, RR kinetics and QT/RR hysteresis between endurance-trained and moderately active young men, evaluated during exercise at identical relative work outputs. The study corroborates previous findings that RR kinetics is accelerated in endurance-trained men, both during exercise and recovery. In addition, the study reveals a small but significant rate-independent QT prolongation in athletes and an increased steady-state QT/RR slope. However, no significant modification of QT/RR hysteresis was observed in these subjects. Overall, this work identifies within-subject variations of RR and QT kinetics, likely explained by varying mechanisms of autonomic cardiac modulation across investigated conditions. In endurance-trained men, heart rate kinetics is accelerated while repolarisation is protracted and its heart rate dependency is increased.

Keyphrases: Repolarisation, Electrocardiography, Repolarization, RR kinetics, QT/RR hysteresis