Research questions: The value of primary prevention implantable cardioverter-defibrillator implantation (ICD) therapy in patients with non-ischemic cardiomyopathy (NICM) is under debate. Improved risk stratification is needed to select patients at highest risk.

Hypotheses: 1. In NICM patients with CMR detected myocardial fibrosis, ICD implantation reduces all-cause mortality compared to guideline-directed medical therapy (GDMT) only. 2. Myocardial fibrosis assessed by cardiac MRI (CMR) can be used to stratify patients according to risk for sudden cardiac death.

Study design: 1. Patients with myocardial fibrosis: Randomized controlled trial (RCT). 2. Patients without myocardial fibrosis: Prospective registry.

Study population: Patients with non-ischemic cardiomyopathy with LVEF <35% after at least 3 months of guideline-directed medical therapy (GDMT).

Intervention: ICD implantation. Main study parameters/endpoints: primary endpoint: all-cause mortality. Secondary endpoints include: patient clinical status, quality of life, sudden cardiac death, ventricular arrhythmias, ICD complications and ICD therapy Nature and extent of the burden and risks associated with participation, benefit and group relatedness: All ICDs that are implanted in the study are standard devices that are used in daily clinical practice. Patients who are randomized to ICD implantation will be subjected to the risk of perioperative and long-term complications but will be partly protected against death from ventricular arrhythmias. Patients randomized to no ICD implantation will not be protected against the residual risk of sudden cardiac death but are not subjected to complications from ICD implantation and possible subsequent complications. The only additional burden for patients is completing quality-of-life questionnaires, all hospital visits are for routine follow-up.

Inclusion Criteria:

• Non-ischemic cardiomyopathy
• Left ventricular late gadolinium enhancement on MRI, but not hinge-point fibrosis
• Left ventricular ejection fraction <35% on any modality
• Functional class NYHA I-III
• At least 3 months optimal medical treatment for heart failure

Exclusion Criteria:

• Indication for cardiac resynchronization therapy
• Functional class NYHA IV
• Typical ischemic scar tissue on CMR
• Amyloidosis, sarcoidosis, hypertrophic cardiomyopathy, complex congenital heart disease
• high-risk mutations causing DCM
• Patient on waiting list for heart transplantation
• Left ventricular assist device present
• Severe valve disease
• Secondary prevention indication for ICD
• Untreated cardiac ischemia
• High competing risk of death (>35% in 1 year according to HFmetascore)
• Active chemotherapy for cancer
• Severe renal failure with dialysis expected within 3 years