Budapest Upgrade CRT Study (Version 009-4.1)

Inclusion Criteria: * Male or female patients over the age of 18 (expected survival time: over 1 year) * Patients with ischemic or non-ischemic cardiomyopathy; * Those implanted with a single or dual chamber ICD or implanted with a single or dual chamber pacemaker at least 6 months before inclusion; * Elective generator replacement or upgrade from pacemaker-only to ICD therapy may be indicated at the time of enrollment, but are not required; * Right ventricular pacing 20-100% in at least the last 90 days before enrollment (with an optimal AV interval based on the physician's discretion to avoid unnecessary right ventricular stimulation); * Symptomatic heart failure for at least 3 months before inclusion; * NYHA functional class II or III, or IV a; * Left ventricular ejection fraction measured by echocardiography ≤ 35%; * Paced QRS complex ≥ 150 ms; * Optimal heart failure medical therapy; * Informed consent obtained. Exclusion Criteria: * Intrinsic QRS with LBBB morphology (measured at VVI 40 bpm setting as per protocol); * CABG or PCI in the past 3 months; * Acute myocardial infarction in the past 3 months; * Unstable angina; * Planned coronary revascularization (PCI or CABG); * Planned cardiac transplantation; * Acute myocarditis; * Infiltrative myocardial disease; * Hypertrophic cardiomyopathy; * Severe primary mitral and aortic valve stenosis or regurgitation; * Women who are pregnant or plan to become pregnant or breastfeeding; * Subjects who are unable or unwilling to cooperate with the study protocol; * Tricuspid valve prosthesis; * Any serious disease likely to interfere with the conduct of the study; * Participation in other clinical trial; * Patients geographically not stable or unavailable for follow-up; * Chronic, severe renal dysfunction (creatinine value \> 200 μmol/l); * Severe RV dilatation (RV basal diameter \>50mm); * Severe tricuspid insufficiency.