Effect of intravenous mannitol on cat pial arteries and veins during normal and elevated intracranial pressure.

In two series of cats, totaling 12 animals, the effect of 1 g of intravenous mannitol per kg over 15 minutes on pial vessels (cranial window technique, multichannel videoangiometry) and intracranial pressure (ICP) was investigated under circumstances of normal ICP and ICP elevated to 20 mm Hg by the cisternal infusion of mock cerebrospinal fluid. Under conditions of normal ICP, mannitol induced an 18% reduction of ICP at the end of the infusion and a further decrease to 38% below the initial level 10 minutes later. Pial veins and arteries remained more or less unreactive over 35 minutes, with changes never exceeding 2 to 3%. Administration of mannitol during elevated ICP (20 mm Hg) reduced ICP by 15% during the administration period; during the subsequent 15 minutes, ICP returned to pretreatment levels. Pial arteries remained unreactive during mannitol infusion; however, they started to dilate during the subsequent 15 minutes to +8%. Venous calibers showed no noticeable changes. It is concluded that mannitol in clinically relevant doses does not exert a significant constrictive effect on cerebral arteries and veins and that mannitol acts on ICP probably via an osmotic effect rather than by a direct effect on cerebral blood volume.