Identification of the Slow Conduction Zone in Idiopathic Left Ventricular Tachycardia

The mechanism of verapamil sensitive idiopathic left ventricular tachycardia (ILVT) is considered to be reentry. However, the nature of the reentry circuit, including the location of the slow conduction zone, is unclear. We sought the local electrical activity that would reflect slow conduction by precise mapping around the tachycardia exit (TE) in nine patients with ILVT (mean age, 28 ± 10 years) undergoing radiofrequency catheter ablation (RFCA). The TE was defined as the earliest discrete spiky potential (SP) recorded during the tachycardia, or as a complete configuration-matched pacemap 12-lead electrocardiogram (ECG). In all patients, the TE was located at the mid or inferior distal portion of the septum. The SP at the TE preceded the surface QRS by 20 ± 9 ms. The pacemap score at the TE was 11.4 ± 0.6 points. In three patients, fractionated potentials (FP) were recorded during the tachycardia. The onset of the FP preceded the surface QRS by 47 ± 8 ms and was earlier than the SP at the TE (P < 0.01). The sites where an FP was detectable were restricted to a small area, and were at a distance of 14 ±4 mm from the TE. The direction of the FP site from the TE was more basal in two patients and inferior in one, Pacemap ECGs at the sites with an FP showed poor matching (9 ± 1 points), presumably because of predominant capture of the local ventricular muscle rather than an electrically isolated reentry circuit. Successful RFCA was achieved at the site of the FP in all three patients in which one was recorded, and at the TE in the other six patients. The FP, which has been shown to reflect the slow conduction of the ventricular tachycardia circuit in structural heart disease, was also detected in ILVT in the present study, and it is likely to reflect electrical excitation of the distal rim of the slow conduction zone.