Indications for cardiac resynchronization therapy (CRT) include:
a) Severe heart failure (NYHA class III or ambulatory class IV) despite optimal medical therapy
b) Left ventricular ejection fraction (LVEF) ≤ 35%
c) QRS duration ≥ 130 ms
d) Presence of left bundle branch block (LBBB) or non-LBBB morphology with prolonged QRS duration