Table 2.

Electrocardiogram (ECG) Findings and Conduction Abnormalities in Candidates for Physiologic Pacing Across Heart Failure (HF) Phenotypes

LVEF category	ECG findings	Underlying conduction abnormalities	Clinical presentation	Pacing strategies
HFrEF ≤40%	LBBB with QRS ≥130 ms	LBBB	Congestive HF symptoms, decline in EF	Resynchronization (biventricular or conduction system pacing)
	Non-LBBB with QRS ≥150 ms	RBBB or nonspecific intraventricular conduction delay	Congestive HF symptoms, decline in EF	Biventricular or conduction system pacing
	LBBB and RBBB	Alternating bundle-branch block	Alternating bundle-branch block	Biventricular or conduction system pacing
	AV block, prolonged PR interval (>300 ms),a Mobitz I or II, complete heart block	AV nodal or intra-Hisian disease	Intermittent to persistent bradycardia	Conduction system pacing (preferred) or biventricular pacing
	AF with slow ventricular response	AV nodal or intra-Hisian block in AF	Bradycardia-related symptoms	Conduction system pacing (preferred) or biventricular pacing
	AF with rapid ventricular response	Iatrogenic complete AV block	AF refractory to rate control	AV node ablation and physiologic pacing (biventricular or conduction system pacing)
HFmrEF 41%-49%	LBBB with QRS ≥150 ms	LBBB	Congestive HF symptoms, decline in EF	Biventricular or conduction system pacing
	AV block, prolonged PR interval	AV node or intra-Hisian block in sinus rhythm	Bradycardia-related symptoms	Conduction system pacing preferred (LBBP)
	AF with slow ventricular response	AV node or intra-Hisian block in AF	Bradycardia-related symptoms	Conduction system pacing preferred (LBBP)
	AF with rapid ventricular response	Iatrogenic complete AV block	AF refractory to rate control	AV node ablation and conduction system pacing
Normal range ≥50%b	AV block, prolonged PR interval	AV node or intra-Hisian block in sinus rhythm	Bradycardia-related symptoms	Conduction system pacing preferred over RV pacing
	AF, QRS ≥150 ms, slow heart rate	AV node or intra-Hisian block in AF	Bradycardia-related symptoms	Conduction system pacing preferred (LBBP)
	AF with rapid ventricular response	Iatrogenic complete AV block	AF refractory to rate control	AV node ablation and conduction system pacing

Abbreviations: AF, atrial fibrillation; AV, atrioventricular; EF, ejection fraction; HFmrEF, heart failure with mildly reduced ejection fraction; HFrEF, heart failure with reduced ejection fraction; LBBB, left bundle-branch block; LBBP, left bundle-branch pacing; LVEF, left ventricular ejection fraction; RBBB, right bundle-branch block; RV, right ventricle.

^aExcessive AV delay (PR interval >300 ms) causes atrial relaxation before ventricular contraction, leading to diastolic mitral regurgitation and worse outcomes, regardless of LVEF.

^bPatients with normal LVEF requiring frequent ventricular pacing are at increased risk of pacing-induced cardiomyopathy with RV pacing; conduction system pacing is preferred to maintain systolic function.