Review Question - QID 108912

A 67-year-old man presents to the emergency department with shortness of breath. His dyspnea began 1 hour ago while he was sitting in a chair watching television. He also endorses lightheadedness and fatigue but denies chest pain. Two weeks ago, he had a mitral valve repair for mitral insufficiency secondary to myxomatous degeneration. His medical history is otherwise significant for hypertension and diabetes mellitus, for which he takes metformin and lisinopril. His temperature is 98.8°F (37.1°C), blood pressure is 85/54 mmHg, pulse is 48/min, and respirations are 22/min. On physical exam, he appears to be in moderate distress. He has normal cardiac sounds with an irregular rhythm. His lungs are clear to auscultation bilaterally, and his electrocardiogram can be seen in Figure A. The patient is given an initial dose of atropine but this has no effect. Which of the following is the most appropriate next step in management?