Depends on the frequency. It only takes about 100 Joules to stop the heart, but like I said the source of the EMP would be a bigger concern for you than the EMP itself. The EMP would have a larger effect on non-hardend electronics.
Actually it takes from ~15J to 35 or 40J to stop the heart (possibly less depending upon each person’s physiology). I work on implantable grade defibrillators and that’s the range that we function in. Of course that’s internal to the heart, but generally external defibrillators aren’t all that much more powerful and are started lower than 40J to begin with.
The real problem becomes starting the heart up again if the patient’s heart can’t do that on its own. What we do is pace the heart until it is functioning on its own.
Me, I’m an expert electronics technician and have been in the medical industry working on these for 7 years.