Sympathetic denervation of the epicardial border zone in the genesis of dispersion of refractoriness and arrhythmogenesis in a 7-day-old canine myocardial infarction model.

BACKGROUND

The purpose of this study was to clarify whether sympathetic denervation occurs in the infarcted heart and contributes to the dispersion of the effective refractory period (ERP) and arrhythmogenesis.

METHODS

ERP was measured at 47 epicardial sites in 13 dogs with 7-day-old infarctions after proximal ligation of the left anterior descending artery. To delineate the sympathetic innervation, the effects of ansae subclaviae stimulation (ASS), norepinephrine infusion, and prazosin infusion on ERP were tested.

RESULTS

The per cent change in ERP (delta ERP) induced by ASS was significantly lower at test sites where the surviving epicardial myocardial thickness (Th) was 2 mm or less than at those with a Th of more than 2 mm and the normal zone. Eleven out of 179 sites (6.1%) overlying the infarct showed no ERP change after ASS. ASS paradoxically prolonged ERP at 29 sites (16.2%). In contrast, norepinephrine infusion produced a greater delta ERP in the infarct zone than in the normal zone. Prazosin shortened ERP at sites where ASS prolonged it, but had no effect at sites where ASS shortened ERP. ASS increased both the degree of ERP dispersion and inducibility of ventricular tachycardias or ventricular fibrillation (VT/VF), whereas norepinephrine increased VT/VF inducibility despite a reduction in ERP dispersion.

CONCLUSIONS

We conclude that heterogeneous sympathetic denervation contributed to a prolongation and dispersion of ERP in the surviving epicardium overlying the infarct. Furthermore, a supersensitive response to norepinephrine with resultant ERP shortening and a paradoxical ERP prolongation during ASS caused by alpha-receptor mechanisms that may be related to increased electrical instability were observed.