Intravenous fluid tonicity: effect on intracranial pressure, cerebral blood flow, and cerebral oxygen delivery in focal brain injury

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Abstract
An investigation into the role of intravenous fluid tonicity in determining intracranial pressure (ICP) after brain injury is described. The authors compare the results of infusion of a hypotonic fluid (Ringer's lactate, 270 mOsm/liter) to those of a hypertonic fluid (hypertonic sodium lactate, 500 mOsm/liter) in a porcine model of focal cryogenic brain injury. Hemodynamic parameters (ICP, regional cerebral blood flow (CBF), and oxygen delivery) and serum osmolarity were measured every 3 hours for 24 hours after injury. At sacrifice, the water content of the lesioned and nonlesioned cortex was determined by specific gravity. The cryogenic injury produced a significant increase in ICP and a significant decrease in CBF in all experimental groups. Maintenance infusion of hypertonic sodium lactate for 24 hours resulted in significantly lower ICP, higher CBF and oxygen delivery, and higher serum osmolarity than Ringer's lactate infusion. Cortical water content in the area of the lesion was similar in both groups, but in the uninjured hemisphere it was significantly lower in the hypertonic group. These data suggest that hypertonic maintenance fluid improves intracranial compliance by dehydrating uninjured cortex. Improved CBF in the hypertonic group may be due to dehydration of cerebrovascular endothelium and erythrocytes. By reducing ICP and improving CBF, hypertonic fluid administration may thus reduce secondary brain injury after head trauma.