Although we often treat death as if it were a singular event, a moment in time separating human existence from What Comes After, in truth it is a complex terrain marked by vague milestones. Perhaps that's why death has so many definitions, from clinical death -- when heartbeat, breathing and circulation cease -- to the biological death that occurs sometime later, as brain cells begin to suffocate and we move beyond doctors' power to resuscitate.
The twilit country between those two borders is not a friendly one. In fact, it's under perpetual travel advisory, and with good reason. Take cardiac arrest. When it happens outside of a hospital, it kills 94 percent of the time, racking up an estimated 250,000 American deaths annually [source: Deckard and Ebright]. Those fortunate enough to survive face another grim prospect: a significant risk of developing neurological problems born not only of ischemia but also of reperfusion, the sudden return of blood flow following resuscitation [sources: Adler at al.; Deckard and Ebright]. Those injuries add to the biochemical, structural and functional problems that happen before and during the arrest. Thus, far from merely affecting the heart, cardiac arrest can entail a chain of cell destruction afflicting multiple organs, programmed cell death (apoptosis) in neurons and, possibly, bodily death [sources: Adler at al.; Deckard and Ebright].
Fortunately, many of these damaging processes are susceptible to low temperatures [sources: Adler at al.; Deckard and Ebright]. By cooling cardiac patients with signs of potential neurological issues after ROSC, the medical community has improved the odds of a positive neurological outcome from 20 percent to closer to 75 percent, with some patients reporting a full return to normalcy [sources: Deckard and Ebright; Winslow]. We've gone from a world in which 6-10 minutes with no pulse meant inevitable brain death to one in which that time can be stretched to 20 minutes [source: Winslow]. More radical experimental approaches hold the promise of extending the resuscitation window even longer (see sidebar).
Resuscitation. That's an important word. Because, chilled or not, patents with no pulse, breathing or circulation are dead in a way that will trigger most Do Not Resuscitate orders.
So what's happening here? How do a few degrees of lowered body temperature measure the gap between bad and much, much worse? To understand that, we need to see what goes on when your heart shuts off.