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Effect of premature activation in analyzing QT dynamics instability using QT-RR model for ventricular fibrillation and healthy subjects

Perturbations in the normal heart rate are generally represented by the presence of premature activation (PA) beats in the surface electrocardiogram (ECG). The presence of PA is one of the main reasons of instability in QT dynamics which could initiate arrhythmia. Analyzing Boundary-Input Boundary-Output (BIBO) stability of the short term linear autoregressive QT-RR model is a way of detecting instability in QT dynamics from the ECG. The aim of this paper is to investigate if PA is the only reason for instability in the ventricular repolarisation process, which is denoted by QT interval of surface ECG. Ten healthy subjects with normal sinus rhythm and seven patients with sustained ventricular tachycardia (VT) were analyzed in this study. 10 min long ECG data were collected from each subject of the healthy group and 10 min ECG before the start of VT were taken for each subject of the VT group. Autoregressive QT-RR model was derived for each non-overlapping 1 min long ECG segment of the 10 min long ECG data. Instability in QT dynamics was quantified by measuring the numbers of unstable segments in ECG data for each subject (Nus). Results of this study revealed that like the VT group subjects, QT instability detected by QT-RR model is also found in healthy subjects whose ECG segments are mostly free from PA beats. This finding indicates that BIBO unstable QT characteristics might arise from other inherent factors of cardiovascular system in addition to PA.