Delayed termination following pacing induced shifts from monomorphic to polymorphic ventricular tachycardia:...

Interactions between paced wavefronts and monomorphic ventricular tachycardia (VT) dictate antitachycardia pacing outcomes. Monomorphic VTs were initiated in isolated rabbit hearts (n=6) that were endocardially cryoablated to limit viable tissue to the visible epicardium and the ablated apex served as an anatomic anchor. Preparations were optically mapped during single and dual site pacing at 50% to 90% of VT cycle length with 8 pulses per trial. Of these trials, responses to the 48 single site pulses and to the 172 dual site pulses that captured tissue were analyzed. Overall, we found most pulses reset the VT, and a small number of pulses that abruptly terminated the VT. Of particular interest, we found 12 pulses that shifted the anatomically anchored VT to functionally reentrant wavefronts, and thereby induced polymorphic VT. Delayed termination was observed following 6 of these instances, and the underlying non-sustained polymorphic VT's presented temporal characteristics similar to those presented by delayed termination after antitachycardia pacing in ICD patients.