Which CRT pattern is most likely to benefit?

• A. Left bundle branch block with QRS duration ≥ 120 ms
• B. Right bundle branch block
• C. Left bundle branch block with QRS duration ≥ 120–150 ms
• D. Non-LBBB

Answer: (C)

CRTs work best when there is electrical dyssynchrony due to left ventricular activation delay, which is most pronounced in a left bundle branch block pattern. When the QRS is widened, especially in the 120 ms range and upward toward 150 ms or more, the ventricles are out of sync enough that pacing both the right and left ventricles can realign their contraction. This resynchronization improves the timing of LV contraction, reduces mitral regurgitation, and enhances overall cardiac output, leading to better symptoms and outcomes in eligible heart failure patients.

Right bundle branch block or a non-LBBB pattern doesn’t produce the same LV dyssynchrony that CRT targets, so the likely benefit is smaller. Similarly, a shorter QRS duration suggests less electrical delay, so the potential gains from pacing are reduced. Therefore, the combination of left bundle branch block with a markedly widened QRS—especially in the 120–150 ms range and beyond—signals the pattern most likely to respond to CRT.