HIS Alternative II - UK Site

Inclusion Criteria: * Patients over 18 years of age, ischemic or non-ischemic cardiomyopathy with LVEF ≤ 35% assessed by echocardiography, NYHA class II-IV heart failure symptoms despite optimal medical treatment and * Either planned new implantation of a biventricular pacing system (CRT-P or CRT-D), where the ECG is with sinus rhythm and a typical left bundle branch block (see definition below) * Or planned upgrade of an existing pacemaker or ICD to a biventricular pacing system (CRT-P or CRT-D), where the ECG is with sinus rhythm and a typical left bundle branch block or there has been\> 90% right ventricular pacing from an existing pacemaker for at least 2 months prior to enrollment * Signed informed consent * Typical left bundle branch block: * QRS width \> 130 msec for women and \> 140 msec for men QS or rS pattern in leads V1 and V2, and mid-QRS plateau phase with or without extras in at least 2 of leads V1, V2, V5, V6, I, and aVL Exclusion Criteria: * Existing biventricular pacing system * Permanent atrial fibrillation * Severe renal failure with eGFR \< 30 ml/min * AMI or CABG within the last three months * The patient does not want to participate