For a study, researchers sought to gather patients with severe heart failure with reduced ejection fraction (HFrEF) and evaluated the drug mirabegron to see how it affected their hemodynamics. In this randomized, double-blind, placebo-controlled trial, investigators delegated patients with New York Heart Association functional class III-IV HFrEF, left ventricular ejection fraction less than 35%, and increased NT-proBNP (N-terminal pro-B-type natriuretic peptide) levels to receive either mirabegron (300 mg daily) or placebo orally for a week, as add on therapy to the standard treatment for heart failure. Invasive hemodynamic measurements were collected at baseline, 3 hours after the initial study dose, and after 1 week of treatment. The results showed that they randomly assigned 22 patients (age 66±11 years, 18 men, 16, New York Heart Association functional class III) with left ventricular ejection fractions ranging from somewhere between 20 and 7% to 1,953 ng/L on average. After 3 hours, no significant changes were seen; however, after 1 week, there was a considerably bigger increase in the cardiac index in the mirabegron group compared to the placebo group (mean difference, 0.41 [CI, 0.07–0.75] L/min/BSA; P=0.039). Compared with the placebo group, those who took mirabegron saw a substantial increase in the degree to which their pulmonary vascular resistance decreased (−1.6 [CI, −0.4 to −2.8] Wood units; P=0.02). During the activity, there was no discernible change. The study showed that individuals with moderate to severe HFrEF who received oral treatment with the β3-AR-agonist mirabegron for 1 week experienced an increase in cardiac index and a decrease in pulmonary vascular resistance.