A 68-year-old man with chronic systolic heart failure is treated with digoxin 0.25 mg daily for the past 2 weeks. He presents to the emergency department with nausea, vomiting, anorexia, confusion, and visual disturbances described as yellow halos around lights. Physical examination reveals bradycardia with a heart rate of 48/min. Laboratory studies show serum potassium of 6.0 mEq/L (normal 3.5–5.0 mEq/L). ECG demonstrates PR interval prolongation, scooped ST segments, and peaked T waves. Which of the following is the most appropriate next step in management?