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“Wide-narrow-wide” QRS complex tachycardia: What is the mechanism?

      The tracing of Figure 1 (leads II, III, and V1 taken simultaneously) has been recorded from a 58-year-old man during the recovery phase of exercise stress testing. The first 2 beats are sinus, whereas the third QRS complex starts a tachycardia with an average rate of about 160 beats/min. At tachycardia onset, the R-R interval is relatively long but shortens over the course of the first few beats. Surprisingly, the event begins with wide QRS complexes, but suddenly beats 8–14 become relatively narrow and quite variable in duration and morphology. Finally, the tachycardia again assumes a wide complex configuration until spontaneous termination occurs a few seconds later (not reported here). What about the arrhythmia mechanism?
      Figure 1
      Figure 1Selected strip of simultaneous leads II, III, and V1. The QRS complexes are numbered from 1 to 21.

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