Atrial fibrillation (AF) is the most common arrhythmia, accounting for one third of all hospital admissions and 1% of all emergency department visits (ED). Approximately 65% of those presenting to the ED with AF are admitted. There are also numerous reasons for patients to get AF with rapid ventricular rate (AF RVR) during hospitalization. In the acute setting these patients are often treated with diltiazem, a non-dihydropyridine calcium channel blocker (ND CCB), or metoprolol, a beta blocker (BB). Non-dihydropyridine calcium channel blocker (diltiazem and verapamil) use is considered harmful and national guidelines recommend against use in patients with decompensated heart failure (HF). This recommendation is based on studies with long-term treatment. The purpose of this study is to assess the difference between metoprolol and diltiazem for the acute treatment of AF RVR in patients with HF with reduced ejection fraction (HFrEF).