Figure 1

ECG morphology at different AV intervals. Different ECG morphology corresponding to each programmed AV interval, including the basal ECG of the patient without ventricular pacing. With the progressive shortening of the programmed AV interval, QRS morphology (mainly in V1 lead) changes from LBBB to a RBBB-like trace at the shortest AV interval, corresponding to a complete capture of both ventricles by LV pacing. Different degrees of ventricular fusion between RV spontaneous activation and LV pacing can be seen for AV intervals programmed in the range 200–120 ms.