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===[[TBI]] Pathophysiology===
====Primary injury====
*Contusions
**Bruises to brain parenchyma
*Hematomas
**[[Epidural hematoma]]
**[[Subdural hematoma]]
**Intraparenchymal
**Intraventricular
**[[Subarachnoid hemorrhage]]
*Diffuse axonal injury
*Direct cellular damage
**Neurons
**Axons
*Tearing and shearing of tissues

====Secondary injury====
''Brain swelling causes [[increased ICP]] which compresses the tissue causing ischemia with direct compression of the vasculature causing brain tissue herniation and brain death''
*Leads to expansion of the original injury (predominantly metabolic insult)
**Calcium and sodium shifts
**Mitochondrial damage
**Production of free radicals
*Ultimately leads to damage to axonal integrity and axonal transport
**Enzyme activity leads to apoptosis
*Microscopic structural injury is often unidentifiable on CT or MRI

====Cerebral Blood Flow and Autoregulation====
*vasoconstriction
**HTN, Hypocarbia, alkalosis
*No good way to measure cerebral blood flow
**Use CPP as surrogate
***CPP is amount of pressure needed to perfuse the brain
***CPP=MAP-ICP
****When ICP elevates, CPP decreases
****Normal ICP 
*****15 in adults
*****<10 to 15 in children
*****1.5 to 6.0 in infants
*Autoregulation allows the body to control the cerebral blood flow
**Autoregulatory mechanism is damaged in most TBI patients