Conventionally, mapping and ablation of ventricular tachycardia is done from the endocardial aspect with ablation catheters within the heart. Sometimes the focus is sub epicardial situated and needs epicardial ablation either through the coronary sinus or from the true epicardial aspect. Earlier approach to epicardial ablation was per operative mapping and ablation using a thoracotomy. In 1996 Sosa and colleagues  described percutaneous technique for pericardial puncture from the subxiphoid route and epicardial mapping. It can be done either under local anaesthesia or general anaesthesia. Initial attempts where to target sub epicardial ventricular tachycardia circuits in Chagas disease. Later the same group used the epicardial approach to treat ventricular tachycardia occurring late after myocardial infarction . The percutaneous pericardial puncture needle was same as that is being used for epidural anaesthesia. It is introduced at an angle of 45 degrees, towards the left scapula from the subxiphoid region, under local anaesthesia. It is gently advanced, guided by fluoroscopy till it is close to the cardiac silhouette when a slight negative pressure is felt. Most often the region chosen is the medial third of the right ventricle, noted on coronary angiography to be free of major vessels. Electrode catheters placed at the right ventricular apex and in the coronary sinus also serve as reference points while puncturing the pericardium. Two milliliters of iodinated radiocontrast is injected to confirm the intra pericardial position of the needle tip. If in the correct plane, the contrast collects around the cardiac silhouette. If outside, it collects in the mediastinum. Once the position is confirmed, a floppy tipped guidewire is introduced into the pericardial space. The wire should easily slip into the pericardial space without any resistance if it is in the right plane. Guide wire position is checked by fluoroscopy and a 8F introducer sheath is threaded over it. After this a 4mm tipped ablation catheter is introduced and mapping done from various regions of interest depending on the clinical ventricular tachycardia.
Epicardial approach has a higher risk of complications and hence is often considered only after an endocardial approach fails. Sometimes it is the first line approach when the ECG pattern or the underlying heart disease like dilated cardiomyopathy makes the possibility of epicardial circuits high. An intracardiac thrombus or prosthetic mechanical valves in aortic or mitral position may also mandate a primary epicardial approach.