The Brain During Cardiac Arrest
After the heart stops beating due to a significant reduction of blood flow, the pressures across the entire body and within the arteries and the veins reach a point in which they equalise within approximately 50 seconds. Studies have shown that due to a lack of heart beat and blood flow there is a cessation of brain electrical activity within approximately 10 seconds. This simply reflects a lack of brain function that is brought about due to a lack of blood flow into the brain. Brain oxygen levels are then depleted within approximately 2 minutes and if the blood flow is not restarted to the brain, the cells start to undergo changes which will ultimately lead to cell damage and then cell death. The first thing that happens is that the brain cells undergo a state of shock and this is brought about by a lack of oxygen.
Imagine the shock and distress of a crowd of people on a street if there was suddenly no oxygen left to breath; the people, like cells, are initially viable and can potentially function again if oxygen can be resupplied. Within the first few minutes of a cardiac arrest in which there is no blood flow to the brain, the cells in the brain start to undergo change which is brought about by the depletion of all their nutrients and the production of various toxic materials which can no longer be cleared by the circulation since it is no longer functioning. These toxic substances that are generated by the cells themselves lead the covering of the brain cells to become very loose such that there is a huge surge of calcium inside the cells that leads to significant cell damage. This process is seen across the entire brain and can continue even after attempts are made to restart the heart. After resuscitation attempts are made by hospital staff which include chest compressions and the administration of very strong and potent medications like adrenaline (epinephrine) that are designed to improve the blood pressure, studies have demonstrated that there is still very little, if any, blood flow getting into the brain. This is called the low flow state which is the initial period where there was lack of blood flow leading to changes in the brain such that much higher blood pressure was needed to allow necessary blood flow into the brain. Since even the best efforts at reviving the heart and resuscitation can only generate approximately 25% of the usual blood flow to the brain, this is insufficient. And studies using EEG, have demonstrated that electrical activity in the brain remains flat until ultimately the heart beat is restarted. For up to, or more than 24 hours after the heart is eventually restarted and there is some blood flow into the brain then what can happen is that the degree of oxygen may itself become toxic to the brain and the initial period in which there is a lack of blood flow leads to complete disorder of flow even after the heart has been restarted.