Effect of mannitol on cerebral blood volume in patients with head injury

Mannitol has traditionally been the mainstay of medical therapy for intracranial hypertension in patients with head injury. The authors previously demonstrated that mannitol reduces brain volume in patients with cerebral edema, although whether this occurs because of a reduction in brain water, blood volume, or both remains poorly understood.
Objective of this study was to test the hypothesis that mannitol acts by lowering blood viscosity leading to reflex vasoconstriction and a fall in cerebral blood volume (CBV). The authors used 15O positron emission tomography to study 6 patients with traumatic brain injuries requiring treatment for intracranial hypertension. Cerebral blood flow (CBF), CBV, and cerebral metabolic rate for oxygen (CMRO2) were measured before and 1 hour after administration of 1.0 g/kg 20% mannitol. RESULTS: CBV rose from 4.1 6 0.4 to 4.2 6 0.2 mL/100 g (P = .3), while intracranial
pressure fell from 21.56 4.9 to 13.7 6 5.1 mm Hg (P , .003) after mannitol. Blood pressure, PaCO2, oxygen content, CBF, and CMRO2 did not change.
The study concludes that a single bolus of 1 g/kg of 20% mannitol does not acutely lower CBV. Another mechanism, such as a reduction in brain water, may better explain mannitol’s ability to lower intracranial pressure and reduce mass effect.