Loss of Biventricular Capture After CRT-D Implantation: Device Malfunction or Something Else?

CASE SUMMARY: A 75-year-old woman with nonischemic cardiomyopathy and left bundle branch block underwent cardiac resynchronization therapy defibrillator implantation. Postimplant electrocardiography demonstrated successful biventricular pacing with QRS narrowing to 110 ms from a preimplant baseline of 169 ms. Three consecutive beats with an abruptly different QRS morphology raised concern for intermittent loss of biventricular pacing or device malfunction. TAKE-HOME MESSAGE: Dynamic atrioventricular-delay optimization algorithms such as SyncAV can produce transient changes in QRS morphology that may mimic loss of biventricular pacing but represent expected device behavior and recognition of this pattern prevents unnecessary device evaluation.