Isolated septal substrate for ventricular tachycardia in nonischemic dilated cardiomyopathy: Incidence, characterization, and implications

Background: The substrate for ventricular tachycardia (VT) in nonischemic cardiomyopathy (NICM) has a predilection for the basolateral left ventricle with right bundle branch block VT morphology. Objective: The purpose of this study was to describe a unique group of NICM patients with septal VT substrate. Methods: Between 1999 and 2010, 31 (11.6%) of 266 patients with NICM undergoing VT ablation had septal substrate and no lateral involvement. Mean age was 59 ± 12 years, and ejection fraction was 30% ± 14%. Eight patients had heart block. Results: Cardiac magnetic resonance showed septal delayed enhancement in 8 of 9 patients. Electroanatomic mapping demonstrated bipolar low voltage (<1.5 mV) extending from the basal septum in 22 of 31 patients. The remaining 9 patients had normal endocardial bipolar voltage but abnormal unipolar septal voltage (<8.3 mV) consistent with intramural abnormalities. Epicardial mapping in 14 patients showed no scar in 9 and patchy basal left ventricular summit scar in 5. VTs were mapped to the septal substrate, with 62% having right bundle branch block morphology and V ₂ precordial transition pattern break in 17% suggesting periseptal exit. After substrate and targeted VT ablation, no VT was inducible in 66% and no "clinical targeted" VT in 86%. Over a mean follow-up of 20 ± 28 months, VT recurred in 10 (32%) patients. Conclusion: Isolated septal VT substrate is uncommon in NICM. Biventricular low-voltage zones extending from the basal septum are characteristic, but septal scarring can be entirely intramural as evidenced by unipolar/bipolar electrograms and imaging. Multiple unmappable morphologies are the rule, often requiring several procedures aggressively targeting the septal substrate to achieve moderate long-term VT control.