Clinical death

Clinical death is the popular term for cessation of blood circulation and breathing.^[1] 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 been difficult to reverse. The absence of blood circulation and vital functions related to blood circulation was considered to be the definition of death. In the middle of the 20th century it became possible to often reverse cardiac arrest through cardiopulmonary resuscitation (CPR), defibrillation, epinephrine injection, and other treatments to restore normal heartbeat and circulation. Instead of death, cardiac arrest came to be called "clinical death", meaning the clinical appearance of death. Clinical death is now seen as a medical condition that precedes death rather than actually being dead.

At the onset of clinical death, consciousness is lost within several seconds. Measurable brain activity stops within 20 to 40 seconds.^[2] Irregular gasping may occur during this early time period, and is sometimes mistaken by rescuers as a sign that CPR is not necessary.^[3] During clinical death, all tissues and organs in the body steadily accumulate a type of injury called ischemic injury.

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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.^[4] 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.^[5]

The brain, however, accumulates ischemic injury faster than any other organ. Without special treatment after circulation is restarted, full recovery of the brain after more than 5 minutes of clinical death at normal body temperature is rare.^[6][7] 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 don't actually die until hours after resuscitation.^[8] This delayed death can be prevented in vitro by a simple drug treatment even after 20 minutes without oxygen.^[9] In other areas of the brain, viable human neurons have been recovered and grown in culture hours after clinical death.^[10] Brain failure after clinical death is now known to be due to a complex series of processes that occur after blood circulation is restored, especially processes that interfere with blood circulation during the recovery period.^[11] Control of these processes is the subject of ongoing research.

In 1990, the laboratory of resuscitation pioneer Peter Safar discovered that reducing body temperature by three degrees Celsius after restarting blood circulation could double the time window of recovery from clinical death without brain damage from 5 minutes to 10 minutes. This induced hypothermia technique is beginning to be used in emergency medicine.^[12][13] The combination of mildly reducing body temperature, reducing blood cell concentration, and increasing blood pressure after resuscitation was found to be especially effective. It allowed recovery of dogs after 12 minutes of clinical death at normal body temperature with practically no brain injury.^[14][15] The addition of a drug treatment protocol has been reported to allow recovery of dogs after 16 minutes of clinical death at normal body temperature with no lasting brain injury.^[16] Cooling treatment alone has permitted recovery after 17 minutes of clinical death at normal temperature, but with brain injury.^[17]

Under laboratory conditions at normal body temperature, the longest period of clinical death (complete circulatory arrest) survived with eventual return of brain function is one hour.^[18][19]

Hypothermia during clinical death

Reduced body temperature, or hypothermia, during clinical death slows the rate of injury accumulation, and extends the time period during which clinical death can be survived. The decrease in the rate of injury can be approximated by the Q10 rule, which states that the rate of biochemical reactions decreases by a factor of two for every 10°C reduction in temperature. As a result, humans can sometimes survive periods of clinical death exceeding one hour at temperatures below +20°C.^[20] The prognosis is improved if clinical death is caused by hypothermia rather than occurring prior to it. It is said in emergency medicine that "nobody is dead until they are warm and dead."^[21] In animal studies, up to three hours of clinical death can be survived at temperatures near 0°C.^[22][23]

Life support during clinical death

The purpose of cardiopulmonary resuscitation (CPR) during cardiac arrest is ideally reversal of the clinically dead state by restoration of blood circulation and breathing. However there is great variation in the effectiveness of CPR for this purpose. Blood pressure is very low during manual CPR,^[24] resulting in only a ten minute average extension of survival.^[25] Yet there are cases of patients regaining consciousness during CPR while still in full cardiac arrest.^[26] In absence of cerebral function monitoring or frank return to consciousness, the neurological status of patients undergoing CPR is intrinsically uncertain. It is somewhere between the state of clinical death and a normal functioning state.

Patients supported by methods that certainly maintain enough blood circulation and oxygenation for sustaining life during stopped heartbeat and breathing, such as cardiopulmonary bypass, are not customarily considered to be clinically dead. All parts of the body except the heart and lungs continue to function normally. Clinical death occurs only if machines providing sole circulatory support are turned off.

Controlled clinical death

Certain surgeries for cerebral aneurysms or aortic arch defects require that blood circulation be stopped while repairs are performed. This deliberate temporary induction of clinical death is called circulatory arrest. It is typically performed by lowering body temperature to +18°C (+64°F), stopping the heart, stopping the brain with drugs to conserve energy, turning off the heart lung machine, and draining blood to eliminate all blood pressure. At such low temperatures the clinically dead state can be sustained without serious brain injury for up to one hour. Longer durations are possible at lower temperatures, but the usefulness of longer procedures has not been established yet.^[27]

Controlled clinical death has also been proposed as a treatment for exsanguinating trauma to create time for surgical repair.^[28]

Clinical death and the determination of death

Death was historically believed to be an event that coincided with the onset of clinical death. It is now understood that death is a process, not an event.^[29] Where in this process a dividing line is drawn between life and death depends on factors beyond the presence or absence of vital signs. In general, clinical death is neither necessary nor sufficient for a determination of legal death.

If clinical death occurs unexpectedly, it will be treated as a medical emergency. CPR will be initiated. In a hospital, a Code Blue will be declared and Advanced Cardiac Life Support procedures used to attempt to restart a normal heartbeat. This effort will continue until either the heart is restarted, or a physician determines that the heart is too damaged to be restarted. If this determination is made, the physician will pronounce legal death and resuscitation efforts will stop.

If clinical death is expected due to terminal illness or withdrawal of supportive care, typically a Do Not Resuscitate (DNR) or "no code" order will be in place. This means that no resuscitation efforts will be made, and a physician or nurse may pronounce legal death at the onset of clinical death.

If a patient with working heart and lungs is determined to be brain dead, they can be pronounced legally dead without clinical death occurring.

See also

• Brain death
• Cardiac arrest
• Death
• Information-theoretic death
• Lazarus phenomenon
• Near-death experience

References