LAUSR

Background: Targets for right-sided conduction system pacing (CSP) include His bundle and right bundle branch. ECG patterns, diagnostic criteria and outcomes of right bundle branch pacing (RBBP) are not known. Objective: Our aims were to delineate electrocardiographic and electrophysiological characteristics of RBBP and to compare outcomes between RBBP and His bundle pacing (HBP). Methods: Patients with confirmed right CSP were divided according to the conduction system potential to QRS interval at the pacing lead implantation site. Six hypothesized RBBP criteria as well as pacing parameters, echocardiographic outcomes and all-cause mortality were analyzed. Results: All analyzed criteria discriminated between HBP and LBBP: double QRS transition during threshold test, selective paced QRS different from conducted QRS, stimulus to selective QRS > potential-QRS, small increase in V6RWPT during QRS transition, equal capture thresholds of CSP and myocardium, and stimulus-V6 R-wave peak time (V6RWPT) > potential-V6RWPT (adopted as diagnostic standard). Per this last criterion, RBBP was observed in 19.2% (64/326) patients who had been targeted for HBP, present mainly among patients with potential to QRS < 35 ms (90.6%, 48/53) and occasionally in the remaining patients (5.6%, 16/273). RBBP was characterized by longer QRS (by 10.5 ms), longer V6RWPT (by 11.6 ms) and better sensing (by 2.6 mV) compared to HBP. During median follow-up of 29 months, no differences in capture threshold, echocardiographic outcomes or mortality were found. Conclusions: RBBP is a distinct CSP modality that is frequently observed when the pacing lead is positioned more distally along the right conduction system.