Which of the following represents indications for cardiac resynchronization therapy (CRT)?

• A. LVEF 50% or less
• B. LBBB with QRS 120 ms or more
• C. NYHA class I
• D. LVEF 35% or less; LBBB with QRS 150 ms or more; NYHA class II–IV; failure of RAAS inhibitor and BB therapy x 3 months

Answer: (D)

CRT is reserved for heart failure patients with clear signs of electrical dyssynchrony and persistent symptoms despite optimized medical therapy. The essential criteria are: left ventricular ejection fraction at or below 35% to show significant systolic dysfunction, a wide QRS complex with left bundle branch block pattern at least 150 ms to indicate intraventricular dyssynchrony, and symptoms that are NYHA class II to IV, meaning there is noticeable limitation from heart failure. Plus, the patient should have already received guideline-directed medical therapy (a RAAS inhibitor and a beta-blocker) for about 3 months without adequate improvement. When all four elements are present, CRT is appropriate. The other options fail because they miss one or more requirements: a higher LVEF threshold, a shorter QRS duration, or being asymptomatic (NYHA class I).