BACKGROUND A 37-year-old woman presents to the emergency department (ED) with a persistent feeling that her heart is racing and skipping beats. She has had similar feelings several times in the past but never for more than a few minutes and always with spontaneous recovery. This episode occurred while she was playing water polo with her community league team, and to her surprise, it did not spontaneously dissipate as it had in the past. She began to feel drained of energy and stamina and thought she would pass out. She immediately got out of the pool and was quickly rushed to the ED. On examination, the patient appears pale and diaphoretic, and it is clear that she is lethargic and has shortness of breath (dyspnea). Electrocardiographic monitoring (ECG) is quickly initiated (see Figure 1), and it reveals what appears to be ventricular tachycardia at a rate of 224 bpm, though the rhythm is somewhat irregular. Her pulse is weak and thready and does not correspond to findings on auscultation or to her palpated heart rate of 250 bpm. Her blood pressure is 80/46 mm Hg. Synchronized cardioversion is performed and successfully converts the patient’s rhythm to a normal sinus rhythm at 58 bpm. She becomes more responsive, and her vital signs stabilize. A repeat ECG is obtained (see Figure 2). What is this woman’s underlying disorder, and how should it be treated? Why did she appear to have ventricular tachycardia on her arrival to the ED?