A young man with a narrow and a wide QRS tachycardia

      Figure 1 shows electrocardiograms (ECGs) of a 22-year-old man with a history of palpitations. He was known at the emergency department because of a 2-year history of occasional episodes of a rapid heart rhythm. The ECGs recorded at those times, one shown in Figure 1B, always revealed a narrow QRS tachycardia with alternating R-R intervals of 280 and 320 ms. Each QRS complex is followed by a P wave. Those tachycardias were terminated by intravenous adenosine. The sinus rhythm ECG outside tachycardia is shown in Figure 1A. He refused an electrophysiology study. However, one evening friends brought him to the hospital with the tachycardia shown in Figure 1C. That episode occurred during a tennis match on a very hot summer day.
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      Figure 1Twelve-lead electrocardiograms during (A)sinus rhythm, (B) a narrow QRS tachycardia, and (C) a wide QRS tachycardia.
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