Functional Substrate-Only Guided VT Ablation

Inclusion Criteria: * Men and women \>18 years of age referred for clinically indicated VT ablation and experience monomorphic or polymorphic scar-based VT documented by telemetry, ICD interrogation, ECG or event monitoring. * Scar-based VT is defined as VT in patients with structural heart disease (assessed with either abnormal nuclear perfusion imaging (\>5% defect), late gadolinium uptake on cardiac MRI, wall thinning \<10mm or calcified myocardium on cardiac CT, akinesis or hypokinesis on echocardiogram, presence of Q waves on ECG, history of myocardial infarct). * Patients undergoing epicardial VT ablation and who undergo prophylactic percutaneous hemodynamic support devices will also be included. Exclusion Criteria: * Patients without structural heart disease will be excluded from the pri-mary analysis, but enrolled in a prospective registry * Patients who are pregnant * Presence of intracardiac thrombus * active acute coronary ischemia with unrevascularized coronary artery disease (CAD \>70% stenosis) * Active bacteremia * Inaccessible ventricles due to dual mechanical valves * Inability to tolerate and inability to tolerate anticoagulation during ablation and for at least 1 month after ablation