Inclusion Criteria:  The proposed inclusion criteria represent the minimum recommendations for CRT implantation according to the ESC 2021 guidelines. In addition:  Sinus rhythm and complete left bundle branch block according to Strauss criteria LVEF ≤35% NYHA class II-III Optimal medical heart failure therapy for at least 3 months before enrollment The patient is able to understand and willing to provide a written informed consent 18 years of age or older  Exclusion Criteria:  Mechanical tricuspid valve replacement More than moderate valvular disease Unstable angina, acute MI, CABG, or PCI within the past 6 months Persistent or permanent atrial fibrillation Ventricular arrhythmias (frequent PVC) which do not allow to acquire consecutive regular beats during echocardiography and electrocardiography Higher degree AV block Life expectancy of less than 12 months Pregnancy and breastfeeding Acute illness or active systemic infection