Refining Left Bundle Branch Block Activation Criteria in CRT Candidates: A Noninvasive Electrocardiographic Imaging Study.

Background/Objectives: Invasive mapping studies have shown that one-third of patients with an electrocardiographic pattern of LBBB have intact left bundle branch block conduction and therefore do not actually have a substrate for resynchronization. We aimed to evaluate the activation pattern among responders and non-responders to CRT in a relatively large patient cohort using noninvasive electrocardiographic imaging (ECGi). Methods: A cohort of 185 patients who underwent CRT implantation were enrolled. ECGi was performed at the same hospitalization during intrinsic rhythm. The earliest and the latest left ventricular (LV) activation sites were analyzed according to the 17-segment American Heart Association model. The echocardiographic response to CRT was defined as a reduction of ≥15% in LV end-systolic volume, and clinical response as an improvement of at least one New York Heart Association class at 12 months from the baseline. Results: The earliest activation in the middle inferoseptal LV segment (No.9) was significantly more prevalent among non-responders. A breakthrough in the remaining septal segments (No.2, 3, 8, or 14) was an independent predictor of both clinical (odds ratio (OR) 3.1 [95% confidence interval (CI): 1.2-8.1], p = 0.021) and echocardiographic (OR 5.1 [95% CI: 2.2-11.8], p < 0.001) responses to CRT. Other independent predictors of echocardiographic and clinical response to CRT in this study were LV pacing site in the lateral wall and QLV interval. Conclusions: The earliest endocardial activation site in segments 2, 3, 8, or 14 of the LV is an independent intrinsic activation marker associated with CRT response, complementary to intraoperative factors.