At CPP values less than 50 mm Hg, the brain may not be able to compensate adequately, and CBF falls passively with CPP. Through the normal regulatory process called pressure autoregulation, the brain is able to maintain a normal cerebral blood flow (CBF) with a CPP ranging from 50 to 150 mm Hg. Sustained ICP values of greater than 40 mm Hg indicate severe, life-threatening intracranial hypertension.ĬPP = MAP − ICP where MAP = ( 1 / 3 systolic BP ) + ( 2 / 3 diastolic BP )Īs a result, CPP can be reduced from an increase in ICP, a decrease in blood pressure, or a combination of both factors. ICP values greater than 20 to 25 mm Hg require treatment in most circumstances. ICP values of 20 to 30 mm Hg represent mild intracranial hypertension however, when a temporal mass lesion is present, herniation can occur with ICP values less than 20 mm Hg. For the purpose of this article, normal adult ICP is defined as 5 to 15 mm Hg (7.5–20 cm H 2O). Normal values are less than 10 to 15 mm Hg for adults and older children, 3 to 7 mm Hg for young children, and 1.5 to 6 mm Hg for term infants. Values for pediatric subjects are not as well established. The normal range for ICP varies with age. An increase in pressure caused by an expanding intracranial volume is distributed evenly throughout the intracranial cavity. The Monroe-Kellie hypothesis states the sum of the intracranial volumes of blood, brain, CSF, and other components is constant, and that an increase in any one of these must be offset by an equal decrease in another, or else pressure increases. In the average adult, the skull encloses a total volume of 1450 mL: 1300 mL of brain, 65 mL of CSF, and 110 mL of blood. There is a small amount of capacitance in the system provided by the intervertebral spaces.
In normal individuals with closed cranial fontanelles, central nervous system contents, including brain, spinal cord, blood, and cerebrospinal fluid (CSF), are encased in a noncompliant skull and vertebral canal, constituting a nearly incompressible system.
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