A 22-year-old man is seen in the Emergency Department with rapid palpitation, dyspnea and weakness of 1 hour duration. He gives a history of occasional milder episodes of palpitation since his early teens, that subsided spontaneously in 15-20 minutes. He otherwise enjoys good health and plays vigorous sports without undue symptoms:

• Atrial fibrillation in a patient with Wolff-Parkinson-White syndrome
• Ventricular rate 148/min

In most cases of atrial fibrillation in patients with Wolff-Parkinson-White the QRS  pattern varies from very wide (conduction through the bypass tract)  to narrow (conduction through the AV node) and in-between (conduction through both, with fusion).
The patient underwent DC cardioversion and sinus rhythm was restored. The ECG showed a WPW pattern. EP studies demonstrated a left lateral bypass tract that was successfully ablated.