Cardiac resynchronization therapy (CRT) is a validated HFREF therapy. Unfortunately 20 to 40 % of implanted patients according to the guidelines do not have an improvement in their condition. The detection of "non responders" is imperfect. The selection of eligible patients to cardiac resynchronization is defined by symptomatic patients (NYHA class II to IV), a reduced Left Ventricle ejection fraction and by the duration of the left bundle branch block. No other parameter (clinical, electrical, echocardiographical) have been identified as positive predictive factor to cardiac resynchronization therapy.