Researchers at the University of Pennsylvania have discovered that cells do not expire promptly when the heart stops but instead take several hours to die once cut off from their blood supply. However, cells die quickly if oxygen is reintroduced.
Researchers don't know why cells die when oxygen is reintroduced. They speculate that the body can't tell the difference between a diseased cell and one that is being "reperfused" with oxygen and so destroys the reviving cell in self-defense.
These findings may radically change how cardiac arrest patients are treated. "Suspended animation" would be induced, followed by slow oxygenation. A test on 34 subjects had an 80% success rate. Currently the success rate is apparently 15%.