Upon enrollment, patients were randomly assigned, in a 1:1 ratio, to receive diltiazem administered parenterally at a dose of 0.25 mg/kg (to a maximum dose of 30 mg) or metoprolol administered at a dose of 0.15 mg/kg (to a maximum dose of 10 mg). Randomization was performed through the use of a computer-generated randomization list. Pharmacy released the study drug in a locked tackle box coded in number sequence to correspond to that of the computer-generated randomization list, upon which the pharmacist also prepared the study drug in blinded fashion. The study medications were packaged in identical-appearing dispensing kits. Patients who were randomly assigned to diltiazem received a bolus injection in a syringe that appeared identical to that of metoprolol. Admixture and labeling were performed by the pharmacist in the ED and dispensed to the treating nurse for administration. Doses of each study medication were adjusted with normal saline to a total of 10 ml in each syringe to prevent un-blinding. The time at which the first dose was administered was denoted as time zero (baseline). If the primary endpoint was not achieved at time 15 minutes, then a second escalation dose was administered. If the patient had been enrolled in the diltiazem group, the escalation dose was 0.35 mg/kg (to a maximum dose of 30 mg), and for patients enrolled in the metoprolol group, the escalation dose was 0.25 mg/kg (to a maximum dose of 10 mg). As with the initial dose, the escalation dose was prepared by the pharmacist and given to the treating nurse for patient administration in a blinded fashion.