Background and Purpose Repeated use of mannitol in the setting of ischemic infarction is a controversial and poorly defined therapeutic intervention. The purpose of this study was to examine the effects of repeated mannitol infusions on brain water content and tissue pressure in a well-defined rat model of focal ischemic stroke.

Methods Mannitol infusions (0.5, 1.5, or 2.5 g/kg) were given by intravenous bolus 4 or 24 hours after 90-minute transient cortical ischemia in the territory of the right middle cerebral artery in rats and every 4 hours thereafter for a total of 24 hours. Fluid replacement was limited to 0.5 mL IV isotonic saline administered immediately after each mannitol dose. Control rats received 0.5 mL IV saline at the same intervals and were otherwise under ad libitum conditions. Water contents (percent H₂O) of whole hemispheres and of cortical biopsies were measured with the wet-dry method, and blood samples were analyzed for plasma osmolality and chemistries. In a subgroup of rats, tissue pressure was also measured within the hemisphere ipsilateral to the infarct.

Results Repeated mannitol infusions resulted in a dose-dependent increase in plasma osmolality and a dose-dependent decrease in the percent H₂O of the ischemic middle cerebral artery cortex and ipsilateral hemisphere. In contrast, percent H₂O of the contralateral cortex and hemisphere was significantly decreased only in the groups given the highest dose of mannitol (2.5 g/kg). Mannitol infusions at a dose of 1.5 g/kg begun 24 hours after reperfusion were also associated with a significant reduction of tissue pressure.

Conclusions In a rat model of ischemic cortical infarction, repeated mannitol infusions resulted primarily in a decrease in the percent H₂O of the infarct and ipsilateral hemisphere, as well as decreased tissue pressure.