A 25-year-old woman admitted to the hospital for the treatment of thyrotoxicosis suddenly becomes severely dyspneic. Her vitals are as follows: blood pressure 60/40 mm Hg, heart rate 180/minute with irregularly regular pulse, and cold peripheries. ECG reveals fast atrial fibrillation; her ECG done one day earlier had only revealed tachycardia. Chest x-ray reveals frank pulmonary edema. Which of the following is the most appropriate treatment?