The system should be checked for proper functioning at least every 2 to 4 hours, and any time there is a change in the ICP, neurologic examination, and CSF output. The disadvantages are difficulties with insertion into compressed or displaced ventricles, inaccuracies of the pressure measurements because of obstruction of the fluid column, and the need to maintain the transducer at a fixed reference point relative to the patient’s head.
The advantages of the ventriculostomy are its relatively low cost, the option to use it for therapeutic CSF drainage, and its ability to recalibrate to minimize errors owing to measurement drift. An intraventricular catheter is connected to an external pressure transducer via fluid-filled tubing. The ventriculostomy catheter is the preferred device for monitoring ICP and the standard against which all newer monitors are compared. When pressure autoregulation is impaired or absent, ICP decreases and increases with changes in CPP. Likewise, an increase in CPP results in vasoconstriction of cerebral vessels and may reduce ICP. This response has been called the vasodilatory cascade. This vasodilation can result in an increase in ICP, which further perpetuates the decrease in CPP. When pressure autoregulation is intact, decreasing CPP results in vasodilation of cerebral vessels, which allows CBF to remain unchanged. When CPP is within the normal autoregulatory range (50–150 mmHg), this ability of the brain to pressure autoregulate also affects the response of ICP to a change in CPP. After injury, the ability of the brain to pressure autoregulate may be absent or impaired, and even with a normal CPP, CBF can passively follow changes in CPP. 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.
0 kommentar(er)
