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The main reason for the acute phase of ischemia-reperfusion injury is oxygen deprivation and, therefore, arrest of generation of ATP (cellular energy currency) by mitochondria oxidative phosphorylation. Tissue damage due to the general energy deficit during ischemia is followed by reperfusion (increase of oxygen level) when the injury is enhanced.
Brain injury as a result of oxygen deprivation either due to hypoxic or anoxic mechanisms is generally termed hypoxic/anoxic injury (HAI). Hypoxic ischemic encephalopathy ( HIE ) is a condition that occurs when the entire brain is deprived of an adequate oxygen supply, but the deprivation is not total.
While ischemia is not the mechanism of injury, evidence suggests [clarification needed] that the lack of perfusion through the pulmonary vasculature during an arrest reduces the alveolar–arterial gradient which creates dead space. The oxygen accumulation in the alveoli encourages ROS production which then leads to pulmonary damage.
Restoration of blood supply to ischemic tissues can cause additional damage known as reperfusion injury that can be more damaging than the initial ischemia. Reintroduction of blood flow brings oxygen back to the tissues, causing a greater production of free radicals and reactive oxygen species that damage cells.
Brain ischemia has been linked to a variety of diseases or abnormalities. Individuals with sickle cell anemia, compressed blood vessels, ventricular tachycardia, plaque buildup in the arteries, blood clots, extremely low blood pressure as a result of heart attack, and congenital heart defects have a higher predisposition to brain ischemia in comparison to the average population.
If and when the brain is reperfused, a number of factors lead to reperfusion injury. An inflammatory response is mounted, and phagocytic cells engulf damaged but still viable tissue. Harmful chemicals damage the blood–brain barrier.
Cerebral hyperperfusion syndrome, also known as reperfusion syndrome, is a dysregulated state of cerebral blood flow following the restoration of arterial blood flow to the brain, usually following treatment of carotid artery stenosis. [1]
[12] [1] When the heart stops beating, blood cannot properly circulate around the body and the blood flow to the brain and other organs is decreased. When the brain does not receive enough blood, this can cause a person to lose consciousness and brain cells can start to die due to lack of oxygen. [13]