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IMPORTANCE: Current left bundle branch block (LBBB) criteria are based on animal experiments or mathematical models of cardiac tissue conduction and may misclassify patients. Improved criteria would impact referral decisions and device type for cardiac resynchronization therapy. OBJECTIVE: To develop a simple new criterion for LBBB based on electrophysiological studies of human patients, and then to validate this criterion in an independent population. DESIGN, SETTING, AND PARTICIPANTS: In this diagnostic study, the derivation cohort was from a single-center, prospective study of patients undergoing electrophysiological study from March 2016 through November 2019. The validation cohort was assembled by retrospectively reviewing medical records for patients from the same center who underwent transcatheter aortic valve replacement (TAVR) from October 2015 through May 2022. EXPOSURES: Patients were classified as having LBBB or intraventricular conduction delay (IVCD) as assessed by intracardiac recording. MAIN OUTCOMES AND MEASURES: Sensitivity and specificity of the electrocardiography (ECG) criteria assessed in patients with LBBB or IVCD. RESULTS: A total of 75 patients (median [IQR] age, 63 [53-70.5] years; 21 [28.0%] female) with baseline LBBB on 12-lead ECG underwent intracardiac recording of the left ventricular septum: 48 demonstrated complete conduction block (CCB) and 27 demonstrated intact Purkinje activation (IPA). Analysis of surface ECGs revealed that late notches in the QRS complexes of lateral leads were associated with CCB (40 of 48 patients [83.3%] with CCB vs 13 of 27 patients [48.1%] with IPA had a notch or slur in lead I; P = .003). Receiver operating characteristic curves for all septal and lateral leads were constructed, and lead I displayed the best performance with a time to notch longer than 75 milliseconds. Used in conjunction with the criteria for LBBB from the American College of Cardiology/American Heart Association/Heart Rhythm Society, this criterion had a sensitivity of 71% (95% CI, 56%-83%) and specificity of 74% (95% CI, 54%-89%) in the derivation population, contrasting with a sensitivity of 96% (95% CI, 86%-99%) and specificity of 33% (95% CI, 17%-54%) for the Strauss criteria. In an independent validation cohort of 46 patients (median [IQR] age, 78.5 [70-84] years; 21 [45.7%] female) undergoing TAVR with interval development of new LBBB, the time-to-notch criterion demonstrated a sensitivity of 87% (95% CI, 74%-95%). In the subset of 10 patients with preprocedural IVCD, the criterion correctly distinguished IVCD from LBBB in all cases. Application of the Strauss criteria performed similarly in the validation cohort. CONCLUSIONS AND RELEVANCE: The findings suggest that time to notch longer than 75 milliseconds in lead I is a simple ECG criterion that, when used in conjunction with standard LBBB criteria, may improve specificity for identifying patients with LBBB from conduction block. This may help inform patient selection for cardiac resynchronization or conduction system pacing.

This study sought to analyze the impact of the American College of Cardiology, American Heart Association, and Heart Rhythm Society (ACC/AHA/HRS) guidelines for cardiac resynchronization therapy with defibrillator (CRT-D) update on utilization and efficacy of CRT-D.

Although His bundle pacing (HBP) has been shown to improve left ventricular ejection fraction (LVEF), its impact on mitral regurgitation (MR) remains uncertain.

This study sought to describe arrhythmia characteristics using ultra-high density (UHD) mapping of macro-re-entrant left atrial flutter (LAFL) which propagate via epicardial bridging (EB), and highlight regional anatomy that poses challenges to ablation.

This study aimed to evaluate the feasibility and accuracy of using a novel grid mapping catheter during scar-related ventricular tachycardia (VT) ablation.

This study compared the efficacy and safety of the VASCADE MVP Venous Vascular Closure System (VVCS) device (Cardiva Medical, Santa Clara, California) to manual compression (MC) for closing multiple access sites after catheter-based electrophysiology procedures.

The benefit of pulmonary vein isolation (PVI) in patients with AF and heart failure with reduced ejection fraction (HFrEF) is well established; its efficacy in patients with preserved ejection fraction (HFpEF) is less clear.

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This study sought to report mechanisms of ventricular tachycardia (VT) and outcomes of VT ablation in patients with a left ventricular assist device (LVAD) as destination therapy.

Supplemental Digital Content is available in the text.