I guess it depends on the definition of death. Obviously, full death doesn't occur the moment heart stops pumping. Brain starts gradually decaying when oxygen supply stops, past the point when it's unrecoverable, and to some more distant point when all the tissue completely decays. But in between there is some unknown area.
Sometimes we need to step back and try to question what we perceive as meaningful information. Without recalling the hilarious and very smart article about a dead fish that was shown as apparently being able to read journals (if we trust fMRI a little bit too much), an ECG signal does not always mean that there is a brain activity.<p>(edited)