Termination of re-entrant ventricular tachycardia by subthreshold stimulus applied to the zone of slow conduction.

A 52-year-old female patient developed recurrent sustained ventricular tachycardia during the first week after left ventricular aneurysmectomy. The patient had no history of ventricular tachycardia preoperatively. As her tachycardias proved to be resistant to several antiarrhythmic drugs, catheter ablation was considered. To define the site of origin of ventricular tachycardia, endocardial catheter mapping and pace-mapping were performed. In addition, the response to single premature stimuli applied during ventricular tachycardia was assessed. At a site in the basal portion of the antero-septal area of the left ventricle, early presystolic endocardial activity during ventricular tachycardia was found. Continuous pacing as well as premature stimulation from that site showed a marked delay between the stimulus artefacts and the induced QRS complexes. The stimulus-induced QRS complexes were identical to QRS complexes of spontaneous and induced ventricular tachycardia. At very short critical coupling intervals of single premature stimuli that did not capture the ventricles (non-propagated stimuli), ventricular tachycardia was reproducibly terminated. These findings are explained by assuming that the catheter was located within the zone of slow conduction of the re-entrant circuit, possibly in its proximal portion.