A 68-year-old woman presents to the emergency department (ED) with palpitations and dyspnea. She was feeling well until 3 hours ago, when she had a spontaneous sensation in her chest of an irregular heart rhythm, followed shortly thereafter by a sensation of breathlessness that she describes as mild. She has not had similar symptoms in the past.

She says she has no chest pain, dizziness, or peripheral edema. She has a known history of moderately controlled hypertension and typically has blood pressure (BP) readings of 154/90 mm Hg at home. She has been taking chlorthalidone 12.5 mg/d and enalapril 25 mg/d for the past 3 years but has not seen her regular physician for the past year.

Her physical examination reveals an irregularly irregular heart rate of 110 beats/min (bpm), a respiratory rate of 22 breaths/min and unlabored, and a BP of 164/98 mm Hg. Her examination is otherwise normal.