Unlike fingerprints, our ECG's change over a lifetime, usually not for the better. Also, from moment to moment our ECG will change reflecting our activity, emotional state, our health status at any particular time, use of drugs including caffeine, etc. So it's hard to see how a science of identification could be based on an ECG. However cardiologists can often (but not always) pick out a patient's ECG from a relatively small collection because of particular characteristics such as conduction variations or abnormalities. I've never seen any research on identifying patients in any definitive way from an ECG, although I can't rule out it's possible in theory.