Correlation of Typical LBBB Mechanical Activation Pattern by 2D Strain Echocardiography With Acute GWE Improvement in Patients Receiving LBBp or Conventional BiVp for Cardiac Resynchronization Therapy (Echo LBBp)

Inclusion Criteria: * \>18 years \< 90 years * Patients with a documented indication for resynchronization therapy \[symptomatic patients despite optimal medication, HFrEF (EF\<35%), LBBB QRS morphology\] * COMPLETE LBBB (LBBB defined as QRS\>130msec, wide "notched or slurred" R wave in leads I, aVL, V5, V6 and occasional RS pattern in V5, V6, absence of Q waves in leads I, V5 and V6 but in lead aVL narrow Q wave may be present in the absence of myocardial pathology, R peak time \>60ms in leads V5 and V6 but normal in leads V1, V2, V3 when small R's are discernible in precardial leads, ST and T usually opposite to QRS direction) * Patients with ntraventricular septum diameter \>8 mm * Written informed consent Exclusion Criteria: * Patients with RBBB or atypical LBBB QRS morphology * Patients eligible for an upgrade procedure (already carring a PM or ICD) * Patients with hypertrophic cardiomyopathy