Beta3 Agonist Treatment in Chronic Pulmonary Hypertension Secondary to Heart Failure

Inclusion Criteria: * Written inform consent; * \>18 years-old; * HF with reduced or preserved ejection fraction, according to the definition of the European Society of Cardiology guidelines. * Severe PH and/or combined postcapillary and precapillary PH (also knows as reactive or out-of-proportion PH) determined by RHC showing the following: * Pulmonary arterial wedge pressure or end-diastolic left ventricular pressures ≥15 mmHg; * Mean PAP≥25, and: * PVR≥3 UW and/or diastolic gradient≥7 mmHg or * Transpulmonary gradient≥12. * NYHA functional class II-IV; * On optimized evidence-based pharmacological treatment; * Stable clinical condition defined as no changes in therapeutic regimen or hospitalization in the 30 days preceding recruitment and no current plan for changing therapy. Exclusion Criteria: * Non-coronary cardiac surgery or non-coronary percutaneous procedure within the 12 months preceding recruitment or programmed; * Myocardial infarction or coronary revascularization during the last 3 months, * Myocardial resynchronization therapy initiated during the last 6 months; * Sinus tachycardia or atrial fibrillation with uncontrolled heart rate (\>100 bpm); * Uncontrolled hypertension (PAS\>180 or PAD\>110 mmHg) or symptomatic hypotension (PAS\<90 mmHg). * Infiltrative myocardial disease. * Expected survival \<1 year due to a disease other than PH; * Severe renal failure (GFR \<30 mL/min/1.73 m2 or haemodialysis); * Severe hepatic impairment (serum aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) \>3x the upper limit of normality at screening; * cQT interval on the ECG \>430 ms in male or \>450 ms in female; * Concomitant use of specific pulmonary vasodilator therapy (i.e. endothelin receptor antagonists, phosphodiesterase -5 inhibitors, guanylate cyclase stimulators). * Concomitant use of digoxin, flecainide, propafenone, dabigatran, tricycle antidepressants, or another strong inhibitors of CYP2D6 (with the exception of betablockers). * Significant obstructive lung disease (FEV1/FVC\<0.7 associated with FEV1\<50% of predicted value). * Significant restrictive lung disease (TLC\<60%). * Participation in another clinical trial. * Female with childbearing potential. * Known hypersensitivity to mirabegron or to any of its excipients.