I don't think it's a simple answer.
The heart is -essentially- electrically controlled, in a complex pattern. Timing is everything.
There's also a world of difference between lab-bench experimentation and the real world, where the body, through mechanisms still not completely understood, provides the heart with a great deal of protection.
To further muddy the waters, I was present on one job, where the heart was shut down as the result of a shock to the brain. Again, the exact biological mechanisms are not nearly as well understood as we would like.
Information presented in electrician classes usually refer to 70mA as a level of shock that can cause heart problems and death. However, one needs to pay a great deal of attention to the 'fine print' that accompanies that statement. There are plenty of instances where far lower amounts have caused serious injury.
