Effect of Heart Rate on Ventricular Repolarization in Healthy Individuals Applying Vectorcardiographic T Vector and T Vector Loop Analysis

Abstract

Ventricular repolarization (VR) is strongly influenced by heart rate (HR) and autonomic nervous activity, both of which also are important for arrhythmogenesis. Their relative influence on VR is difficult to separate, but might be crucial for understanding while some but not other individuals are at risk for life-threatening arrhythmias at a certain HR. This study was therefore designed to assess the "pure" effect of HR increase by atrial pacing on the ventricular gradient (VG) and other vectorcardiographically (VCG) derived VR parameters during an otherwise unchanged condition. In 19 patients with structurally normal hearts, a protocol with stepwise increased atrial pacing was performed after successful arrhythmia ablation. Conduction intervals were measured on averaged three-dimensional (3D) QRST complexes. In addition, various VCG parameters were measured from the QRS and T vectors as well as from the T loop. All measurements were performed after at least 3 minutes of rate adaptation of VR. VR changes at HR from 80 to 120 bpm were assessed. The QRS and QT intervals, VG, QRSarea, Tarea, and Tamplitude were markedly rate dependent. In contrast, the Tp-e/QT ratio was rate independent as well as the T-loop morphology parameters Tavplan and Teigenvalue describing the bulginess and circularity of the loop. In healthy individuals, the response to increased HR within the specified range suggests a decreased heterogeneity of depolarization instants, action potential morphology, and consequently of the global VR.