Hi!
I think the easiest way to do it is just to place a resistor in series with each lead, and to limit voltage by a diode limiter. To widen the common-mode range you need to limit the voltage to some volts. You also have to care about safety - first-fault has to be safe.
It depends what you are after, it is quite costly but check the isolation amplifiers from Texas.
Here are one article: https://focus.ti.com/lit/an/sboa062/sboa062.pdf
Search for Defibrillator at TI and you find much more. Also try google.
If you need more PM me and I try to find.