Background—This study examined the role of phase 2 early afterdepolarization (EAD) in producing a trigger to initiate torsade de pointes (TdP) with QT prolongation induced by dl-sotalol and azimilide. The contribution of transmural dispersion of repolarization (TDR) to transmural propagation of EAD and the maintenance of TdP was also evaluated.

Methods and Results—Transmembrane action potentials from epicardium, midmyocardium, and endocardium were recorded simultaneously, together with a transmural ECG, in arterially perfused canine and rabbit left ventricular preparations. dl-Sotalol preferentially prolonged action potential duration (APD) in M cells dose-dependently (1 to 100 μmol/L), leading to QT prolongation and an increase in TDR. Azimilide, however, significantly prolonged APD and QT interval at concentrations from 0.1 to 10 μmol/L but shortened them at 30 μmol/L. Unlike dl-sotalol, azimilide (>3 μmol/L) increased epicardial APD markedly, causing a diminished TDR. Although both dl-sotalol and azimilide rarely induced EADs in canine left ventricles, they produced frequent EADs in rabbits, in which more pronounced QT prolongation was seen. An increase in TDR by dl-sotalol facilitated transmural propagation of EADs that initiated multiple episodes of spontaneous TdP in 3 of 6 rabbit left ventricles. Of note, although azimilide (3 to 10 μmol/L) increased APD more than dl-sotalol, its EADs often failed to propagate transmurally, probably because of a diminished TDR.

Conclusions—This study provides the first direct evidence from intracellular action potential recordings that phase 2 EAD can be generated from intact ventricular wall and produce a trigger to initiate the onset of TdP under QT prolongation.