The effect of intravenous fluid replacement on the response to mannitol in experimental cerebral edema: an analysis of intracranial pressure, serum osmolality, serum electrolytes, and brain water content.

Albino rabbits that had undergone a cryogenic insult over the left parieto-occipital cortex were analyzed for serum osmolality, serum electrolytes, brain water content, and intracranial pressure (ICP) following either a baseline infusion of intravenous (i.v.) fluid (45 mL total) for 3 hours or above-maintenance isotonic saline (73.5 +/- 12 mL or 90.5 +/- 1.5 mL) and mannitol therapy. The subgroups were compared amongst themselves and to sham-operated controls. Serum osmolality was elevated in the higher-dose mannitol subgroup compared with maintenance i.v. fluids subgroup (1 g/kg/h vs 1 g/kg/3 h; p < 0.05), accompanied by an insignificant reduction of serum sodium. A significant reduction in brain water in the injured left hemisphere was seen following high-dose mannitol in the subgroup that received less i.v. (maintenance) fluids than the group that received above-maintenance i.v. fluids (p < 0.025). No reduction in brain water was seen in the subgroup that received above-maintenance i.v. fluids (non-treated groups). Reduction of ICP was not found in the lower mannitol dose group. We conclude that the ability of mannitol to reduce cerebral edema is related to the total amount of i.v. fluid replacement. This implies that the amount of i.v. crystalloid fluid that is administered to patients with cerebral edema and raised ICP requiring mannitol for control needs to be carefully monitored.