Predictors of Favourable Response to Cardiac Resynchronization Therapy in Patients with Heart Failure with Reduced Ejection Fraction: A Narrative Review

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Abstract
Cardiac resynchronization therapy (CRT) is an effective treatment for selected patients with heart failure with reduced ejection fraction, but 30–40% of recipients do not respond. This narrative review summarizes predictors of a favourable response to CRT. Typical left bundle branch block (LBBB) and a QRS duration ≥ 150 ms remain the best-established baseline predictors. Sex, etiology, age and comorbidities affect how much benefit is gained rather than acting as yes/no predictors (the benefit is greater in women, partly because of their smaller heart size) and should not by themselves exclude an otherwise eligible patient. QRS duration indexed to left ventricular size, advanced electrocardiographic markers (QRS notching, intrinsicoid deflection, QRmax, QRS area) and echocardiographic markers of mechanical dyssynchrony (apical rocking, septal flash, the classical LBBB strain pattern, systolic stretch index, mechanical dispersion and myocardial work) have been linked to response in observational studies and may add prognostic information. Combining mechanical dyssynchrony with septal viability on cardiac magnetic resonance appears to improve selection further. Procedure-related factors, including left ventricular lead position and a high percentage of biventricular pacing, are critical for success. A multimodal approach that combines clinical, electrocardiographic, echocardiographic and imaging data, supported in the future by machine learning, appears most promising.

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