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It is increasingly common for someone to regain a heartbeat after a cardiac arrest. During the time that the person is without a heartbeat, and so without circulation, the brain is deprived of oxygen.

Why are there long-term consequences to temporary deprivation of oxygen?

I understand that the short-term effect of loss of oxygen to the brain is to make it impossible for the cell to maintain the gradients of ions needed to initiate action potentials. As long as the cells don't die, or not too many die, why is there often long-term neurologic damage after a heart attack.

My question differs to the question of what happens during a stroke. During a stroke the deprivation is longer and more focussed.

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A stroke results in impaired blood flow. A lack of blood supply to the brain results in prolonged cortical hypoxia, cells die. Neuronal cells have a relatively high background metabolism, for example because of their reliance on ion pumping that costs energy. A lack of oxygen can kill a cell and therefore deplete the brain of thousands and thousands of neurons after a serious stroke. Depletion of certain areas of the brain of cells will reduce the output to other areas, and decrease the input to that area. Moreover, epileptic foci may appear, resulting in epileptic discharges in the brain, referred to as post-stroke seizures.

Upon recovery, gross rearrangements may occur in the brain, alongside gross changes in cellular activities throughout the affected areas, due to the brain's plasticity.

Having said that, if the oxygen depletion is short, I don't see any reason why there should be gross changes in neuronal behavior after recovery.

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Question: Why are there long-term consequences to temporary deprivation of oxygen?

Not all cases of temporary deprivation of oxygen have long-term consequences. It depends on how long the O2 deprivation period is. If it’s just no more than a few minutes, like when you hold your breath, it’s ok. But if it’s longer than that, irreversible damage may occur:

“Approximately 20% of the oxygen consumed by the human body is used by the brain. The greater part of this oxygen is used to produce the ATP required to maintain the membrane potentials necessary for electrical signaling with synaptic and action potentials Harris et al., 2012. In many vertebrates, including adult humans, interruption of the oxygen supply to the brain for more than a few minutes leads to irreversible neurological damage, including neuronal death.Larson J, et al

A lot of people who suffered cardiac arrests but had fast and effective CPRs and regained circulations and oxygenation in time recovered completely with no long-term consequences. Sadly, there are a lot of people, too, who were not revived in time and suffered long-term consequences (which can vary from mild to severe) or even brain death. It all depends on the duration of hypoxemia.

Edit: For the question of “How does oxygen deprivation affect neural activity?

There is a recent comprehensive review here: This review aims at summarizing the molecular and cellular mechanisms underlying hypoxia and hyperoxia in brain and cerebral tissue.

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