A 14-year-old boy presents to the Emergency Department with a 14-hour history of palpitations. He has a history of one other episode of palpitations 4 months prior that lasted most of a day, but terminated spontaneously. The current episode started abruptly while he was playing baseball. He feels as though it intermittently stops, but then starts again when he stands up and walks. When he walks very quickly or climbs stairs, he has trouble catching his breath. He denies chest pain, dizziness, or syncope. He has otherwise been well, denies fever, and takes no medications. There is no history of illicit drug use. He has no known drug allergies. The family history is noncontributory for congenital heart disease and premature sudden death. Both parents are alive and well. There is a healthy younger sister. He attends ninth grade and is a good student. He plays baseball and is a trumpet player in the school band.
On physical exam, he is afebrile, anxious, but in no distress. The heart rate is irregular at approximately 104 bpm, the respiratory rate is 22 per minute, and the blood pressure is 108/64. The head is atraumatic and the mucous membranes are moist. The tympanic membranes and oropharynx are clear. The neck is supple without jugular venous distension. The lungs are clear to auscultation. The cardiac exam has a mildly tachycardic and irregularly irregular rhythm. There is a normal first heart sound and a soft physiologically split second heart sound. There is no gallop. There is no significant murmur. The pulses are full and equal. The abdomen is soft with the liver edge palpable at the right costal margin. The extremities are warm with mildly delayed capillary refill. The skin has no rash. The neurologic exam is nonfocal.
An electrocardiogram was performed (Fig. 1).