Increased intracranial pressure

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Increased intracranial pressure

C. Thomas Wass, MD

Intracranial pressure (ICP) is determined by the relationship of the volumes of the intracranial vault (formed by the skull) and the intracranial contents. The latter is composed of three volume compartments: brain parenchyma, cerebrospinal fluid (CSF), and blood. By definition, intracranial hypertension exists when ICP is sustained above 15 mm Hg.

Brain

The brain parenchyma is composed of cellular elements and intracellular and interstitial water. The average adult brain weighs between 1350 and 1450 g and accounts for approximately 90% of the intracranial volume. This compartment may expand because of tumor growth or cytotoxic cerebral edema.

Cerebrospinal fluid

CSF occupies approximately 5% of the intracranial volume (i.e., 75 mL, of which approximately 25 mL is within the ventricular system). The rate of CSF production is about 0.35 to 0.40 mL/min in the normal adult. Expansion of this compartment occurs with communicating or obstructive hydrocephalus.

Blood

Intracranial blood accounts for the remaining 5% of the intracranial volume. The cerebral blood volume (CBV) is 3 to 7 mL/100 g brain weight. Elevation of the head decreases both CBV and ICP. Expansion of the blood compartment may result from cerebral hemorrhage or dilation of resistance or capacitance vessels (e.g., vasogenic cerebral edema). This compartment is most amenable to acute manipulation by the anesthesiologist (see following discussion). With few exceptions (e.g., cerebral vasospasm, profound hypotension), increases in cerebral blood flow (CBF) usually result in parallel increases in CBV and ICP.

Intracranial elastance

Historically, the intracranial pressure-volume relationship has been termed compliance in the medical literature. Compliance is defined as unit or units of volume (e.g., intracranial volume) change per unit or units of pressure (e.g., ICP) change (i.e., ΔV/ΔP). However, the pressure-volume curve presented in Figure 132-1 and most other textbooks actually depicts the reciprocal of compliance, or elastance.

Figure 132-1 Idealized intracranial pressure-volume curve. The horizontal segment depicts maintenance of intracranial pressure (ICP) via physiologic compensatory mechanisms that respond to expanding intracranial volume (e.g., tumor, hematoma). Once these compensatory mechanisms are exhausted, elastance is increased, and small changes in intracranial volume result in large changes in ICP. CPP, Cerebral perfusion pressure; CSF, cerebrospinal fluid. (From Drummond JC, Patel PM. Neurosurgical anesthesia. In: Miller RD, ed. Miller’s Anesthesia. Philadelphia: Churchill Livingstone Elsevier; 2009:2045-2087.)

Elastance is defined as ΔP/ΔV. Under normal physiologic conditions, small volume increases in any one of the three intracranial compartments results in little or no change in ICP. The compensatory mechanisms that initially protect against an elevation in ICP are (a) translocation of intracranial CSF through the foramen magnum to the subarachnoid space surrounding the spinal cord, (b) increased CSF absorption through the arachnoid granulations, and (c) translocation of blood out of the intracranial vault. Once these mechanisms are exhausted, abrupt increases in ICP occur in association with small increases in intracranial volume (see Figure 132-1). That is, intracranial compliance is decreased, or more correctly, intracranial elastance is increased.

Anesthetic considerations

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