Clinical death is the medical term for cessation of blood circulation and breathing, the two necessary criteria to sustain life. It occurs when the heart stops beating in a regular rhythm, a condition called cardiac arrest. The term is also sometimes used in resuscitation research.
Stopped blood circulation has historically proven irreversible in most cases. Prior to the invention of cardiopulmonary resuscitation (CPR), defibrillation, epinephrine injection, and other treatments in the 20th century, the absence of blood circulation (and vital functions related to blood circulation) was historically considered to be the official definition of death. With the advent of these strategies, cardiac arrest came to be called "clinical death" rather than simply "death" to reflect the possibility of post-arrest resuscitation; for medical purposes, it is considered to be the final physical state before permanent death.
At the onset of clinical death, consciousness is lost within several seconds. Measurable brain activity stops within 20 to 40 seconds. Irregular gasping may occur during this early time period, and is sometimes mistaken by rescuers as a sign that CPR is not necessary. During clinical death, all tissues and organs in the body steadily accumulate a type of injury called ischemic injury.
Limits of reversal
Most tissues and organs of the body can survive clinical death for considerable periods. Blood circulation can be stopped in the entire body below the heart for at least 30 minutes, with injury to the spinal cord being a limiting factor. Detached limbs may be successfully reattached after 6 hours of no blood circulation at warm temperatures. Bone, tendon, and skin can survive as long as 8 to 12 hours.
The brain, however, appears to accumulate ischemic injury faster than any other organ. Without special treatment after circulation is restarted, full recovery of the brain after more than 3 minutes of clinical death at normal body temperature is rare. Usually brain damage or later brain death results after longer intervals of clinical death even if the heart is restarted and blood circulation is successfully restored. Brain injury is therefore the limiting factor for recovery from clinical death.
Although loss of function is almost immediate, there is no specific duration of clinical death at which the non-functioning brain clearly dies. The most vulnerable cells in the brain, CA1 neurons of the hippocampus, are fatally injured by as little as 10 minutes without oxygen. However, the injured cells do not actually die until hours after resuscitation. This delayed death can be prevented in vitro by a simple drug treatment even after 20 minutes without oxygen. In other areas of the brain, viable human neurons have been recovered and grown in culture hours after clinical death. Brain failure after clinical death is now known to be due to a complex series of processes (reperfusion injury) that occur after blood circulation is restored, especially processes that interfere with blood circulation during the recovery period. Control of these processes is the subject of ongoing research.
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