 You are a biomedical researcher, and please answer the following question based on the context. 
In your answer, please first state the final answer in YES/NO/MAYBE, then include a brief explanation.

### Question: Catheter ablation for ventricular tachycardia after failed endocardial ablation: epicardial substrate or inappropriate endocardial ablation?

### Context: 1: The substrate of myocardial ventricular tachycardia (VT) may involve the subepicardial myocardium.
2: The purpose of this study was to assess the incidence of epicardial substrates in patients with a previously failed endocardial ablation attempt for VT as well as the safety and effectiveness of epicardial ablation.
3: Using an electroanatomic mapping system, endocardial and epicardial maps were acquired. Irrigated radiofrequency current ablations of all inducible VTs were performed.
4: Between 2005 and 2009, 59 patients with or without structural heart disease underwent epicardial VT ablation. Pericardial access failed in 3 (5%) of these patients. Of the remaining 56 patients, an epicardial substrate was found in 41 (73%). Overall, acute success was achieved in 46 (78%) of 59 patients, with complete VT abolition in 27 (46%) and partial abolition in 19 (32%). Successful outcomes were the result of endocardial ablation only in 14 (24%) patients, epicardial ablation in 21 (36%), and endocardial/epicardial in 11 (19%). Ablation failed to prevent reinduction in 8 (13%) patients, and VTs were noninducible prior to ablation in 5 (8%). Two periprocedural deaths occurred, one after right ventricular perforation and one due to electromechanical dissociation. Hepatic bleeding occurred in two patients. Recurrence of any VT occurred in 27 (47%) of 57 surviving patients during median follow-up of 362 days (q1-q3; 180-468 days). Repeat epicardial mapping was not feasible due to adhesions in 3 (25%) of 12 patients.


###Response: In patients with a previously failed endocardial VT ablation, epicardial mapping reveals a VT substrate in nearly three fourths of all patients, and epicardial ablation is required for successful VT abolition in more than half of patients. However, life-threatening complications may occur. Repeat epicardial access was not possible in 25% due to local pericardial adhesions.
 