Inclusion Criteria:
- Participants must be 6 months to <18 years old at the time when the informed consent/assent is signed.
- Left ventricular systolic dysfunction (LVSD) with left ventricular ejection fraction (LVEF) ≤ 50% at screening assessed by echocardiography.
- Elevated NT-pro BNP levels
- >500 ng/l for children ≥ 6 months to < 2 years of age
- >300 ng/l, for children ≥ 2 years to <18 years
- Heart failure etiologies including congenital heart defects (CHD) with biventricular physiology and systemic LV; idiopathic cardiomyopathy (CM); familial/inherited and/or genetic CM; history of myocarditis (diagnosis of an acute episode was at least 3 months prior to randomization); neuromuscular disorder (eg, duchenne muscular dystrophy); inborn error of metabolism; mitochondrial disorder; acquired (chemotherapy, iatrogenic, infection, rheumatic, or nutritional); ischemic (eg, Kawasaki disease and postoperative heart failure [HF]); LV noncompaction.
- Receiving standard of care (SoC) treatment for heart failure according to local guidelines or investigator´s discretion and being on a stable regimen for 30 days prior to randomization.
- Study participants must have a body weight ≥ 4.0 kg at Visit 1.
Exclusion Criteria:
- Serum potassium:
- > 5.0 mmol/L for children ≥ 2 years of age at either screening or randomization visit
- > 5.3 mmol/L for children ≥ 6 months to < 2 years of age at either screening or randomization visit (if estimated glomerular filtration rate [eGFR] < 60 mL/min/1.73m², threshold of > 5.0 mmol/L will be used)
- Severe renal dysfunction with eGFR < 30 ml/min/1.73m² at screening or randomization visit.
- Systolic blood pressure (SBP) < 5th percentile for age, sex and height at screening or randomization.
- Sustained or symptomatic arrhythmias not controlled by drug or device therapy within 30 days prior to randomization.
- Treatment with a mineralocorticoid receptor antagonist (e.g., spironolactone, eplerenone) within 30 days of randomization.
- Requirement of any intravenous (IV) vasoactive agents, mechanical ventilation, mechanical circulatory support within 30 days prior to randomization.
- Recent surgical procedure or other intervention to correct or palliate CHD within 3 months prior to randomization or anticipated to undergo cardiac surgery during the 3 months after randomization.