Changing QRS duration and morphology with A-paced and A-sensed rhythm in postcardiac resynchronization therapy (CRT) patients.

Meticulous interpretation of the ECGs of patients with cardiac resynchronization defibrillator therapy (CRT-D) device is important to ensure optimal synchronization of both the chambers. We present the ECGs of such a patient who was noted to have different morphologies of the biventricular-paced QRS depending in atrial-sensed (A-sensed) or atrial-paced (A-paced) rhythm.