Cardiac resynchronization therapy resulting from atrial pacing: An unusual case of intraventricular conduction delay.

Cardiac resynchronization therapy device implantation is complicated by a significant rate of failure to place a left ventricular lead via the coronary sinus. The present case describes one such failure ironically leading to resynchronization. The patient's QRS narrowing subsequent to postimplant atrial pacing alone suggests that bradycardia-dependent phase 4 block was the indirect but exclusive cause of the patient's intraventricular conduction delay. Thus, phase 4 block should be considered when atrial pacing at a faster rate resolves a wide QRS interval.