Susceptibility of the ventricles to fibrillation has been related to the degree of spatial inhomogeneity in the repolarization process. We studied the pattern of beat-to-beat fluctuations in ventricular repolarization processes in order to determine whether a relationship also exists between the temporal variability of ventricular repolarization and susceptibility to ventricular fibrillation. We used the morphology of the T-wave recorded in surface and epicardial leads as a measure of the ventricular repolarization process. The Ventricular Fibrillation Threshold (VFT) was used as the standard measure of cardiac susceptibility to fibrillation. In dog experiments, T-wave morphologic indices were computed on 1,024 sequential beats. Histogram, autocorrelation and power spectrum analyses were performed on the sequence of T-wave morphologic indices. A series of 27 experiments were performed on 20 dogs in which VFT was reduced by several different interventions - hypothermia, tachycardia and coronary artery ligation. For all three interventions we observed the same characteristic change in the pattern of T-wave morphology fluctuations. In particular, we found that as the VFT was reduced, a pattern of T-wave alternans developed. This pattern was generally not detectable by visual inspection of the ECG. It was, on the other hand, easily quantified in terms of a T-wave alternans index (TWAI) which we computed from the power spectrum of the T-wave fluctuations. In 26 of the 27 experiments, measured VFT decreased (p<.001); in 20 of these experiments the TWAI computed from the surface ECG increased (decreased) when VFT decreased (increased) (p<.01). In 17 experiments epicardial electrograms were, recorded. In 16 of these experiments VFT decreased (p<.001). In 16 of these 17 experiments TWAI computed from the epicardial ECG increased (decreased) when the VFT decreased (increased) (p<.001). We conclude that statistical analysis of fluctuations in ECG complex morphology may provide a sensitive probe of ventricular vulnerability to fibrillation.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine