For CRT, which QRS duration and morphology predict benefit?

• A. QRS duration < 100 ms
• B. QRS duration ≥ 120–150 ms with RBBB pattern
• C. QRS duration ≥ 120–150 ms with left bundle branch block pattern
• D. CRT benefit is independent of QRS morphology.

Answer: (C)

The concept here is electrical dyssynchrony and how CRT targets it. CRT works best when there is a clearly wide QRS complex, indicating the ventricles are not contracting in sync, and the pattern of that dyssynchrony is specifically left bundle branch block. In LBBB, the left ventricle is activated later than the right ventricle, leading to inefficient, asynchronous contraction. CRT paces both ventricles to restore timing, which improves coordination, cardiac output, and symptoms.

A QRS duration of about 120–150 ms or more signals meaningful dyssynchrony, but the morphology matters. Left bundle branch block represents a type of dyssynchrony that CRT can correct effectively, especially when the QRS is wide. Right bundle branch block or non-specific intraventricular conduction delays don’t show the same LV dyssynchrony pattern and tend to predict less robust or more variable CRT benefit. Therefore, the strongest predictor is a wide QRS duration with left bundle branch block pattern, reflecting significant and correctable electrical asynchrony.