A 72-year-old woman is found to have an irregular pulse rate at a routine clinic visit. She is experiencing no new symptoms at rest or on exertion. Her past medical history includes hypertension, osteoarthritis, and dyslipidemia.On physical examination, the blood pressure is 135/85 mm Hg, heart rate is approximately 72/min and irregular. The heart sounds reveal an irregular S1 and normal S2 with no audible murmurs. The jugular venous pressure is normal and the lungs are clear. On the ECG, there are no P waves and an irregular RR interval at a rate of 70/min. (See Figure below) On her previous ECG from 4 years ago she was in sinus rhythm. Which of the following is the most appropriate next step in management?