Fluctuations in T-wave morphology and susceptibility to ventricular fibrillation

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.