As someone with both a pacemaker and formally an ICD (now a wearable defibrillator) I doubt that the taser would do much damage to the heart judging merely by the position of the probes which are rarely more than skin deep and therefore nowhere near any of the internal organs. Now how electricity behaves in the body (presumably merely going between the probes) is another question, as I have heard of many tales of ICD's delivering shocks to other people who are in contact with the "shockee".