The purpose of this study is to assess whether the burden of untreated non-sustained ventricular tachycardias (VTs), or episodes treated with anti-tachycardia pacing, correlates with appropriate implantable cardiac defibrillator (ICD) shock therapies and to evaluate if the timing of radiofrequency VT ablation affects the prognosis of ICD recipients.

Enrolled patients will remain in a first phase of the study until the first appropriate ICD shock will be delivered.

The objective of this first stage is to assess whether the burden of untreated non sustained VTs or episodes treated with anti-tachycardia pacing is predictive of appropriate ICD shocks.

The second phase of the study will start after the first appropriate ICD shock delivered for VT.

Patients will be then randomized to immediate VT ablation or to standard treatment, meaning waiting until next arrhythmic storm to perform a VT ablation procedure.

The objective of this phase is compare the rate of worsening heart failure hospitalizations and deaths from any cause between the two groups.

Inclusion Criteria:

• Patients already implanted with ICD (single-/dual -chamber, with or without cardiac resynchronization therapy) for primary or secondary prevention of sudden cardiac death

Exclusion Criteria:

Phase A exclusion Criteria:

• General contraindication to transcatheter ablation
• Contraindication to antithrombotic therapy
• Patients chronically treated with class I and III antiarrhythmic drugs

Phase B exclusion criteria:

• Patients developing first occurrence of incessant VTs
• Patients receiving a shock for first occurrence of clinically verified ventricular fibrillation
• Patients with aetiology differing from ischemic heart disease for coronary artery disease and from non-ischemic dilatative cardiomyopathy