The study measure multiple neurohormones in patients with heart failure being treated with Sacubitril/Valsartan in increasing doses over an 8 week period.

Inclusion Criteria:

1. Patients must give written informed consent before any study assessment is performed.
2. Ambulatory ≥ 18 years of age, male or female, treated at Ringerike Hospital.
3. Patients with symptomatic chronic heart failure and reduced ejection fraction (≤ 40%).
4. Patients on optimized medical treatment for heart failure. -

Exclusion Criteria:

1. Patients not able to comply in the study.

2. Patients having contraindication for treatment with Entresto;

   1. Hypersensitivity to the active substances or to any of the excipients listed in section
   2. Hyperkalemia: > 5.4 mmol/L
   3. Known history of angioedema related to previous ACE inhibitor or ARB therapy.
   4. Hereditary or idiopathic angioedema.
   5. Concomitant use with Aliskiren-containing medicinal products in patients with diabetes mellitus or in patients with renal impairment (eGFR <60 ml/min/1.73m2)
   6. End-stage renal disease (<15 mL/min per 1.73m2 or treatment by dialysis).
   7. Severe hepatic impairment, biliary cirrhosis and cholestasis (Child-Pugh C classification).
   8. Pregnancy Breast-feeding-