Intracranial pressure in an adult is normally below 15 mmHg. Sustained elevations in intracranial pressure above 20 mmHg are not well tolerated by the injured brain and have been associated with increased mortality. Sustained elevation in intracranial pressure may result in cerebral ischemia if cerebral perfusion is hampered and, if severe, can result in brain herniation. Brain herniation may occur across the falx and involve the cingulate gyrus, across the tentorial hiatus and involve the uncus, or through the foramen magnum and involve the cerebellar tonsils.
Cerebral ischemia results from inadequate cerebral perfusion. The ideal degree of cerebral perfusion in the injured brain is unknown. In the uninjured brain, cerebral blood flow is tightly regulated by myogenic, humoral, and neural mechanisms to maintain a constant flow despite changes in systemic arterial pressure and intracranial pressure. This autoregulation functions over a broad range of systemic arterial pressures from a lower limit of 65 mmHg to an upper limit of 140 mmHg and maintains cerebral blood flow at approximately 50 ml/100 g/min. If blood flow falls below 25 ml/100 g/min, electrical activity is lost and the electroencephalogram is flat, below 12 ml/100 g/min brainstem-evoked responses are lost, and below 10 ml/100 g/min brain failure results with loss of ion homeostasis and eventual cell death.
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