A pregnant woman with no significant medical history presented to an outpatient cardiology clinic for evaluation of intermittent, transient but rapid palpitations, some with momentary light-headedness. None lasted more than about 20 seconds, and there were no syncopal events. At no time did she note chest pain or dyspnea. There was no family history of dysrhythmia. There was no evidence of structural heart disease on physical examination, during which blood pressure was normal, and her pulse rate was 78/min. Baseline laboratory values were unremarkable, as was an echocardiogram. She was referred for a 24-hour Holter monitor. She was taking no medications, except prenatal vitamins. She had no history of smoking or recreational drug abuse, but drank 3 cups of coffee per day. Findings from the monitor are shown (Figure, A, B, and C).