A 72-year-old man with hypertension and type 2 diabetes presents to the emergency department with 12 hours of palpitations and mild shortness of breath. He denies chest pain, syncope, or neurologic symptoms. Medications include lisinopril and metformin. Temperature is 36.8°C (98.2°F), blood pressure is 128/76 mm Hg, pulse is 142/min and irregularly irregular, and respirations are 18/min. ECG shows an irregularly irregular rhythm with no distinct P waves and a narrow QRS complex. Troponin is negative and basic metabolic panel is normal. The physician plans electrical cardioversion because symptoms persist despite initial observation.

Which of the following is the most appropriate next step to reduce the most important complication of the planned intervention?