A 19-year-old college swimmer suffered syncope during an intercollegiate race. She described losing consciousness “for two seconds,” and coming to the surface gasping for air and choking. Over the prior month she had also had 2–3 episodes of palpitations, one associated with lightheadedness all while training or competing. During a stress test she exercised 14 min, but during recovery developed a wide complex tachycardia at 300 beats/min lasting 15–20 s without syncope (Fig. 146.1). The past medical history, family history and physical examination were unremarkable. A urine drug screen was negative. An ECG taken in the EP clinic is shown (Fig. 146.2). What is the differential diagnosis and what further evaluation and management should be considered for this patient?