A standard VT ablation will often involve us controlling the heart-beat by pacing the heart through 1 of the investigators catheters within the heart. The electrical response to pacing at different heart rates can often provide your doctor with information to help the ablation. This study will involve an additional period of pacing at different heart rates, during which the electrical response is measured in different areas around the ventricle. This will allow us to calculate areas of the ventricle, which the investigators new measure FF would predict to be the source of the VT. In the future this may then allow us to better identify patients who are at risk of VT, and to better locate the area that needs to be ablated.