Defining the QRS threshold for LOT-CRT benefit: a conduction pattern specific analysis

Background Conduction system pacing, particularly left bundle branch area pacing (LBBAP), has emerged as an alternative to biventricular CRT, often achieving similar or greater QRS narrowing. Left bundle optimized CRT (LOT-CRT) can provide additional benefit in selected patients, yet guidelines still favor CRT for classic indications (e.g., BLOCK-HF profiles), and when the added complexity of LOT-CRT is justified over LBBAP remains unclear. This study evaluates QRS thresholds at which LOT-CRT provides incremental benefit. Objective To identify QRS duration thresholds where LOT-CRT achieves greater QRS narrowing than LBBAP, and to examine whether baseline conduction pattern modifies this effect. Methods We performed a retrospective cohort study of consecutive patients undergoing LBBAP or LOT-CRT at a single quaternary center from June 2022 to November 2024. Baseline demographics, comorbidities, echocardiography, and ECGs were collected. Multivariable linear regression assessed associations between QRS shortening and pacing strategy, including interactions with baseline QRS duration and conduction pattern. Results Data from 131 patients were analyzed (75% male, age 74 ± 11, LVEF 46 ± 14%). Devices were 62% ICD/CRT-D and 38% PPM/CRT-P; 48% received LBBAP and 52% LOT-CRT. Baseline conduction was normal in 45.4%, RBBB in 23.1%, LBBB in 22.3%, and IVCD in 9.2%. LOT-CRT exceeded LBBAP in QRS narrowing when baseline QRS ≥133 ms, whereas LBBAP performed similarly or better below this threshold. Adjusted models showed trends toward greater LOT-CRT benefit in LBBB (~16 ms; P=0.089) and IVCD (~18 ms; P=0.058), though not statistically significant. Conclusion LOT-CRT achieved superior QRS shortening only when baseline QRS was ≥133 ms, with no advantage in narrower complexes, including patients meeting guideline-based CRT indications (e.g., BLOCK-HF–type profiles). A threshold near 133 ms may guide strategy selection between LOT-CRT and LBBAP, but outcome-focused studies are needed to determine whether these electrophysiologic differences translate into clinical benefit.