Beta 3 Agonist Treatment in Heart Failure

Objective: The objective of the study is to assess the structural and functional cardiac effects of treatment with the beta 3 AR agonist Mirabegron in patients with chronic heart failure. Design: The investigators are planning a study aiming at establishing proof of concept that treatment of patients with HF with Mirabegron has significant positive effects, as assessed by clinical and biochemistry measurements, but not by hard endpoints. The investigators are performing a combined dose-finding - chronic efficacy study. The study is a randomized, placebo-controlled, double-blinded trial. The follow-up period is 6 months. 70 patients with chronic heart failure will be included. Specific aims 1. Determine safety of administration of Mirabegron to patients with heart failure. 2. Determine if treatment with Mirabegron for 6 months induces beneficial cardiac structural remodelling in patients with heart failure. 3. Determine if Mirabegron improves symptoms and exercise capacity as indicated by questionnaires and 6 min walk test in patients with heart failure. 4. Determine effects of Mirabegron on cardiac conduction, repolarisation and rhythms and arrhythmias in patients with heart failure. 5. Determine effects of Mirabegron on circulating biomarkers in patients with heart failure.