Spontaneous termination of a tachycardia: what is the tachycardia mechanism?--VI.

Case Presentation A 55-year-old man underwent an electrophysiologic procedure for evaluation and treatment of recurrent episodes of paroxysmal tachycardia that had been occurring sporadically for 10 years. After placement of electrode catheters in the high right atrium and right ventricular apex., but before catheter placement in the His-bundle po.sition or coronary sinus, premature ventricular depolarizations triggered <in episode of tachycjirdia that terminated spontaneously in 10 seconds. The tachycardia cycie length varied between 390 and 410 msec. The last 3 seconds of the tachycardia are shown in Figure 1. What is the tachycardia mechanism?