Patients sometimes suffer from life-threatening abnormal heart racing that originates from the lower chamber of the heart. These patients will often need an implantable defibrillator which has the ability to shock the heart back to a normal heart rhythm, but this does not prevent them from getting frequent recurrences of the bad heart rhythm needing shocks from the device. This can be painful and potentially harmful. Medicines to prevent recurrences of shocks are not very effective and have many side effects. An alternative to medicines for this is a procedure called a catheter ablation in which a wire is passed up through the blood vessels of the leg into the heart and used to find the short circuits which cause the dangerous heart rhythm. When the spot causing the trouble is found, the investigators can burn it ("ablate" it). This procedure is challenging and methods are needed to make it more effective and easier to do. One of the main ways for finding the short circuits involves using the electrocardiogram (the "ECG"). The regular ECG is simplistic and only makes use of recordings from 10 sites (6 precordial sites and 4 sites on both upper and lower limbs) on the body surface. The investigators are testing whether making recordings from 120 sites on the chest and back and using special computerized analysis of the recordings can help make catheter ablation for dangerous heart rhythms more effective.