The present study is a multicenter interventional non randomised study in patients requiring an implantable device for cardiac resynchronization therapy. Its primary objective is to investigate whether the presence of a specific echocardiographic contraction pattern before implantation is associated with increased rates of acute improvement in myocardial function (as measured by an ultrasound) and to compare the improvement in two groups of patients based on the type of pacing (biventricular or left-sided pacing)

There is now sufficient evidence that the presence of typical LBBB mechanical stimulation on 2D Strain Echocardiography - and not simply the presence of electrocardiographic criteria - is associated with increased rates of adequate response to resynchronization therapy with implantable CRT relative to patients who met electrocardiographic criteria for LBBB without however, the typical stimulation pattern documented echocardiographically.

Identification criteria of LBBB typical mechanical stimulation in 2DSE are :1) early shortening of at least 1 basal or mid-ventricular segment in the septal wall and early stretching in at least 1 basal or midventricular segment in the lateral wall, 2) early septal peak shortening (within the first 70% of the ejection phase), and 3) lateral wall peak shortening after aortic valve closure.

The absence of one of the three criteria categorizes the patient in the group of atypical LBBB stimulation pattern.

Along with the development of the newest non-invasive imaging methods using ultrasound, the calculation of myocardial work (Myocardial Work) as well as the calculation of GWW (Global Wasted Work), GCW (Global Constructive Work) and GWE (Global Work Efficiency) is possible and can be used as a way of assessing myocardial function, the response to administered treatment (e.g. resynchronization therapy) as well as in the search for patients with the most likely response to a possible treatment.

The purpose of the study is to investigate which patients are most likely to benefit from LBBP pacing. Classic biventricular pacing will be compared with LBB pacing in patients requiring resynchronization with LVEF < 35%. At the same time, the presence or not of a typical LBBB- pattern of mechanical stimulation will be evaluated - documented through 2DSE (Two-dimensional strain echocardiography). - The immediate improvement of GWE in patients with succeeded LBB pacing - as documented by measurements in the electrophysiological laboratory- will be compared to the group of patients with the classical method of biventricular pacing (BiVp) as well as the correlation of the presence of the sonographic pattern of LBBB stimulation compared to the group of patients who do not present this mechanical activation pattern in ultrasound.

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