Can the train for brain death turn back temporarily?

"the station ahead is brain death."

When a sudden cardiac arrest occurs in humans, there is no blood supply to the brain and oxygen cannot be transported, which will cause hypoxia in brain cells. Our brains are very sensitive to this, and if we continue to lack oxygen, brain death may occur in about 10 minutes.

Brain death is defined as an irreversible loss of global brain function, including brainstem reflexes. Such patients can sometimes be kept alive with the help of machines, but they have no hope of regaining consciousness and can no longer breathe independently.

At present, in many countries, brain death has become the basis for the declaration of human death, which provides an opportunity for organ donation and transplant for patients with brain death. We often hear the news that sometime somewhere someone died of an accident or illness and his / her donated organs saved many lives.

Of course, before a person is diagnosed with brain death, he needs to undergo repeated examinations to confirm the function of the nervous system according to whether the EEG is resting, whether there is spontaneous breathing, whether the pupil responds to light, and whether there is vomiting reflex in wiping the back of the throat.

Diagnostic criteria are gradually improving, but scientists do not know much about the process of brain death. After all, when a patient is in an emergency, the doctor usually has an obligation to try his best to save the patient. He cannot sit back and watch that process happen naturally, so it is difficult to obtain samples for observation. Therefore, in the past, most of the academic understanding of brain death came from animal experiments.

However, there are also some critically ill patients who wish to reduce unnecessary suffering or to walk with more dignity, so they will sign the "consent form to give up first aid" in advance. A team led by Jens Dreier, a neuroscientist at the University of Berlin, looked at patients who chose not to be first aid and found that there was a "brain tsunami" on the way to brain death, and there was a key node in the tsunami.

Scientists believe that as long as this point comes, the damage is reversible.

The Tsunami in my mind

Normally, the neurons in our brain rely on electrical signals to transmit information. When stimulated, the neuron produces an action potential to transmit the signal; after that, it relies on the sodium and potassium pump on the cell membrane to return the neuron to the resting potential in preparation for the next signal transmission. Over and over again, the EEG shows a vibrant curve.

In the case of hypoxia, the brain goes into "energy-saving mode", stopping the transmission of electrical signals, that is, stopping the communication between neurons (like a person waiting for help when trapped, not talking can reduce physical exertion). At such times, the EEG becomes calm, but the "energy saving mode" does not mean that there is no energy consumption:

The resting potential maintained by sodium potassium pump is that the concentration of sodium ion is high outside the cell membrane and low in the membrane. Therefore, even if there is no action potential to transmit the signal during hypoxia, there will still be sodium ions running into the membrane along the concentration difference. In this way, the sodium-potassium pump still has to work, sending out the sodium ions in the cell membrane to ensure the concentration difference inside and outside the membrane. But it always consumes energy to transport ions from places with low concentrations to places with high concentrations.

Once hypoxia lasts for a long time, the energy-supplying ATP molecules will be exhausted, the sodium-potassium pump will no longer work, and the difference in sodium concentration inside and outside the cell membrane will disappear. This phenomenon, called depolarization (depolarization), can spread like a tsunami in the cerebral cortex and other brain regions. Scientists observe this spread in the deathbed of some patients with severe brain injury and call it a "brain tsunami".

And the roar of the mind can trigger a large number of harmful physiological reactions. For example, because sodium ions cannot be transported out of the cell, water then enters the cell to restore balance, which in turn causes cytotoxic edema. In such a situation for a long time, a large number of nerve cells will swell and eventually lead to irreversible brain damage.

But what excites scientists is that the "brain tsunami" itself does not mean brain death. Because they found that when depolarization begins, neurons are not immediately damaged, but there is a certain delay.

There's still time to buy.

The team looked at nine patients who had suffered traumatic brain injury, all of whom had undergone treatment and signed a "first aid consent form" after family discussion.

With the patient's permission, the scientists implanted an array of electrodes in their brains and used electrocorticogram (ECoG) to monitor the "brain tsunami", the spread of depolarization in the patient's brain. In addition, they also monitored regional cerebral blood flow and hypoxia in real time.

The removal of life-sustaining treatments from each patient results in the cessation of the body's autonomic circulation (circulatory arrest) and eventually death.

Specifically, the scientists first observed in the brains of eight patients that autonomous brain electrical activity in different brain regions quieted down at the same time. This non-diffusible inhibition (nonspreading depression) occurs during a period of sharp decline in blood oxygen content in the brain.

The final diffuse depolarization (terminal spreading dipolarization), or brain tsunami, occurs 13 to 266 seconds after the occurrence of non-diffusive inhibition. At this time, the regional cerebral blood flow has also dropped to the lowest point.

In all 8 patients, the final diffuse depolarization occurred after circulatory arrest. Only one patient had the final diffuse depolarization, the brain tsunami, before the autonomic circulation stopped.

Compared with the diffuse depolarization in normal tissue, it is difficult to recover in the tissue with persistent ischemia. But scientists have found that there is a time ranging from a few minutes to more than a dozen minutes between the time of diffuse depolarization and the time when cells begin to die. So they believe that even if the brain tsunami has occurred, as long as the nerve cells do not begin to die, there is still a chance to restore the body's autonomic circulation and thus restore the supply of oxygen.

In other words, although the toxic physiological reactions caused by the roar of the mind can lead to death, it is not without the opportunity to press the pause button for the brain tsunami. If the survival time of nerve cells can be prolonged during ischemia, then the patient's chances of being saved will also be increased.

The perception of death

Since it has been found that brain tsunamis often occur after the cessation of autonomic circulation and spontaneous brain activity, scientists believe this may lead to some new controversy over the definition of death.

But even if it is not known when the brain tsunami became completely irreversible, one thing is certain: the depolarization that occurs after the cessation of the autonomic cycle does not reverse spontaneously.

The team hopes that one day the new therapy inspired by the study will buy time to save more humans who are on the brink of death because of cerebral ischemia.