Fluid balance

Cerebral edema is the final common pathway of most forms of brain injury and may reduce global or localized blood flow. Brain swelling due to increases in water content may occur because of cellular failure or breakdown of the blood-brain barrier. Unlike in the past, severe fluid restriction does not have a place as it leads to a reduced circulating blood volume, low blood pressure, and a rise in the viscosity of the blood. These may all contribute to inadequate flow to areas of the brain at risk.

Fluid restriction does not decrease the risk of brain swelling because the blood-brain barrier possesses unique permeability properties. It is impermeable to charged particles such as sodium ions, which can therefore form a greater hydrostatic gradient across the blood-brain barrier than oncotically active particles such as albumin. This contrasts with the action of these fluids in other organs. Therefore the influence of intravenous fluids on brain water content is related to how they affect plasma osmolality and not to their oncotic properties. It is reasonable to administer whatever intravenous fluids are necessary to maintain an adequate blood volume provided that plasma osmolality is not reduced in the process. In this way, cerebral blood flow is maintained without adverse effects on brain swelling.

Flow through the cerebral microcirculation and collaterals in ischemic brain is improved if blood viscosity is reduced. Many units aim for a hematocrit of 30 to 35 per cent after intracranial neurosurgery or brain injury and use 0.9 per cent saline or synthetic colloid solutions for volume replacement.

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