Exercise-induced atrioventricular block in a patient with a pacemaker: a functional consequence of AV search...

Algorithms designed to promote intrinsic atrioventricular (AV) conduction in dual-chamber pacemakers aim to reduce unnecessary right ventricular pacing and its long-term adverse effects. The AV Search+ algorithm periodically prolongs the AV delay to facilitate native conduction and is particularly useful in non-pacemaker-dependent patients. However, under certain conditions, such algorithms may lead to unexpected disturbances in AV synchrony, especially during exercise.

A 65-year-old woman with symptomatic second-degree AV block (Mobitz II) underwent dual-chamber pacemaker implantation. During follow-up, she developed progressive exertional dyspnoea. Exercise stress testing revealed a paradoxical decrease in heart rate during increasing workload with reproduction of symptoms. Programmer-monitored exercise testing demonstrated a functional 2:1 AV block caused by atrial undersensing, as atrial depolarizations occurred within the post-ventricular atrial refractory period during AV Search+-mediated AV delay prolongation. Deactivation of the AV Search+ algorithm restored stable 1:1 AV conduction during exercise and resulted in complete symptom resolution.

This case highlights a rare but clinically relevant complication of AV conduction search algorithms in dual-chamber pacemakers. Although these algorithms effectively reduce ventricular pacing, inappropriate interactions between prolonged AV delays and atrial refractory periods may result in symptomatic functional AV block, particularly during exercise and in patients with advanced AV conduction disease. Exercise testing with device interrogation is a key diagnostic tool in pacemaker patients with exertion-related symptoms, and individualized device programming is essential to ensure optimal clinical outcomes.