The first ECG is from an active, otherwise healthy 66-year-old man who experienced a sudden onset of symptomatic tachycardia. He presented to the Emergency Dept. feeling (understandably) very anxious, with poor perfusion to his skin, chest discomfort, and palpitations. As the ED staff prepared to electrically cardiovert him, he spontaneously converted to normal sinus rhythm at a rate of 93 / min. and a QRS duration of 90 ms.