A 4-year-old girl is brought to the pediatrician's office. Her father reports that she suddenly became pale and stopped running while he had been playfully chasing her and her pet Chihuahua. After 30 minutes, she was no longer pale and wanted to resume the game. She has never had a previous episode and has never been cyanotic. Her physical examination was normal, as were her chest x-ray and echocardiogram. An ECG showed the pattern seen on the next page, which indicates which of the following?

The child described in the question has no cyanosis or murmur, no cardiac or pulmonary vascular abnormalities by chest x-ray, and no evidence of structural anomalies by echocardiogram, and therefore is unlikely to have an underlying gross anatomic defect. The ECG pattern in the figure shows the configuration of preexcitation, the pattern seen in the WPW syndrome. These patients have an aberrant atrioventricular conduction pathway, which causes the early ventricular depolarization appearing on the ECG as a shortened PR interval. The initial slow ventricular depolarization wave is referred to as the delta wave. Seventy percent of patients with WPW syndrome have single or repeated episodes of paroxysmal supraventricular tachycardia, which can cause the symptoms described in the question. The preexcitation ECG pattern and WPW syndrome can occur in Ebstein malformation, but this is unlikely in the absence of cyanosis and with a normal echocardiogram. If ventricular tachycardia were present, the symptoms would likely be more profound. Active play and exposure to over-the-counter medications containing sympathomimetics in a healthy 4- year-old child can cause symptoms such as those described in the question in children with WPW syndrome by precipitating paroxysmal supraventricular tachycardia.