PO-04-046 AUTOMATIC CAPTURE THRESHOLD TESTING IN LEFT BUNDLE BRANCH PACING: A RARE FAILURE

Abstract

Automatic capture management (ACM) algorithms are extremely reliable and routinely used in right atrial, right ventricular and left ventricular leads. In patients with His bundle pacing (HBP), ACM is generally not reliable and is rarely used. In left bundle branch pacing (LBBP), ACM is expected to function reliably due to minimal differences in myocardial and conduction system capture thresholds. The present case highlights a unique case of LBBP in whom ACM falsely demonstrated high thresholds despite low conduction system capture thresholds. A 83-year-old man with coronary artery disease, atrial fibrillation, sinus pauses and RBBB underwent dual chamber pacemaker utilizing LBBP lead in RV port. During LBBP lead implantation, anodal, nonselective and selective thresholds were observed at 4.0V, 1.75V and 1.0V @ 0.6 ms (Panel A). Due to longer stimulus to LB electrogram during selective LBBP transition, ACM was set to monitor only. RV (LBBP) pacing output was set chronically at 2.5v @ 0.4 ms. During multiple in-office follow-up over the next 3 years, thresholds were noted to stable at 1.75V for nonselective LBB capture and 1V @0.6 ms for selective capture. Bipolar R wave amplitudes (8 – 12 mV) and pacing impedances (480-500 Ohms) remained stable. ACM set to monitor only, demonstrated consistently high capture thresholds of 2.375 to 2.5V @ 0.4 ms (Panel B) with recommended output of 5V for 2X safety margin. Device electrograms demonstrating the transition from nonselective to selective LBBP and loss of capture are shown in Panel C. Unusually long stimulus to local electrogram and higher thresholds differences between nonselective to selective LBBP led to this rare failure of ACM to detect loss of LBB capture. While we generally program ACM to ON for all patients with LBBP, this case was a rare exception. We present an unusual case of failure of ACM to accurately detect LBB capture thresholds due a larger output difference between nonselective and selective LBBP (often observed with HBP). ACM should be used with caution in patients with conduction system pacing, using individual patient considerations.