is from just about anything that will stop oxygen
from going to the brain-- a blood clot or a bursted vessel in the brain. The internal defibrillator will stop the heart rhythm that causes sudden death, but it doesn't typically stop the heart rhythm that throws the blood clots
to the brain.
So, strokes can be from a bunch of stuff and those defibrillators are not really for preventing strokes, just sudden death from a heart beating like crazy.