I would tend to discount some of the war stories about low voltage electrocution. I suspect many are urban legends (invented or embelished but passed around often and long enough to give some air of authority), and others have not been given due investigation for such problems as higher voltage leakage currents, internal catheters and similar side issues. Sufficient current density at the heart (which is the significant point) is hard to achieve at low voltages, even with good surface electrodes.
There is a significant biological effect at low voltages though. You only need to "taste" the terminals of a 9v battery to give a good illustration of that. One thing that needs to be considered at low voltage is electrolysis. Tissue damage from electrolysis is possible from low currents over extended periods. There have been cases of third degree burns caused by dc currents at ecg electrodes and electrocautery electrodes which had insufficient dc blocking. These developed because of the extended exposure (probably hours to days) to the current. Bob johnsonemc-p...@ieee.org emc-p...@ieee.org emc-p...@ieee.org emc-p...@ieee.org
