- Background —The magnitude of QT-prolongation in response to bradycardia, rather than the bradycardia per-se, determines the risk for torsade-de-pointes (TdP) during atrio-ventricular block (AVB). However, we do not know why some patients develop more QT prolongation than others despite similar bradycardia. We hypothesized that in patients who develop significant QRS-vector changes during AVB, the effects of cardiac memory leads to excessive QT prolongation. Methods and Results —We studied 91 patients who presented with AVB and who also had an ECG predating the bradyarrhythmia for comparison. We correlated changes in QRS-morphology and axis taking place during AVB with the bradycardia-induced QT prolongation. Patients with and without QRS-morphology changes at the time of AVB were of similar age and gender. Moreover, despite similar R-R interval during AVB, cases involving a QRS-morphology change had significantly longer QT (648±84 vs. 561±84, p<0.001) than cases with no QRS-morphology change. Patients who developed a change in QRS morphology at the time of AVB had a 7-fold higher risk of developing long QT. This risk nearly doubled when the change in QRS morphology was accompanied by a change in QRS axis. Conclusions —Cardiac memory resulting from a change in QRS-morphology during AVB is independently associated with QT prolongation and may be arrhythmogenic during AVB.