Clinical worsening due to inappropriate automatic mode switch during biventricular pacing: What is the mechanism?

We describe a case of a 65-year-old gentleman with nonischemic cardiomyopathy and left bundle branch block who underwent cardiac resynchronization therapy device. After becoming a responder initially, he experienced significant clinical worsening on follow-up. Device interrogation revealed several long episodes of inappropriate automatic mode switch (AMS) entry due to far-field R wave oversensing resulting in loss of atrioventricular synchrony. Moreover, pacing in VVI mode with consistent VA conduction taking place during the AMS episodes was also found to be detrimental, which helped in sustaining the episodes and produced pacemaker syndrome like phenomenon. Attempts made to resolve the issue by prolonging the post-ventricular atrial blanking period was unsuccessful, hence we adjusted the atrial channel sensitivity to troubleshoot the problem.