In which patient population is CRT most beneficial?

• A. HFrEF patients with left bundle branch block and QRS duration >130-150 ms, with persistent symptoms on GDMT.
• B. HFpEF with right bundle branch block.
• C. Any HF patient regardless of conduction status.
• D. HFrEF with narrow QRS.

Answer: (A)

CRT stands to help when the heart’s pumping is slowed not only by disease but also by dyssynchronous activation of the ventricles. This occurs most clearly in people with heart failure with reduced ejection fraction who have a left bundle branch block and a wide QRS complex. The left bundle block delays activation of the left ventricle, so the two ventricles don’t contract together efficiently; a wide QRS on the ECG is a marker of this dyssynchrony. Cardiac resynchronization therapy delivers coordinated pacing to both ventricles, realigning the timing of contraction. This improves stroke volume, can reduce functional mitral regurgitation, promotes reverse remodeling of the heart, and—when added to optimized medical therapy—has been shown to reduce hospitalizations and improve survival in trials of HFrEF with LBBB and wide QRS.

Other scenarios don’t show the same benefit: heart failure with preserved ejection fraction hasn’t demonstrated a clear advantage from CRT; right bundle branch block without left-bundle dyssynchrony hasn’t shown the same outcome gains; and in patients with a narrow QRS there isn’t meaningful dyssynchrony for CRT to correct. So the population most likely to benefit is those with HFrEF, left bundle branch block, and a wide QRS who remain symptomatic despite guideline-directed medical therapy.