Effect of the barbiturate methohexital on cerebral vessels and intracranial pressure

Reactions of pial arteries and veins as well as intracranial pressure (ICP) and mean arterial pressure during intravenous administration of methohexital were observed in two groups of cats totaling 24 animals in order to investigate hypothesized direct barbiturate effects on cerebral vascular volume. Group 1 had bolus injection of 1 mg/kg followed by infusion of 0.1 mg/kg/min over 60 minutes. This induced a 6 to 7% arterial dilatation at 50 to 60 minutes (not significant); a 20% reduction of pial vein calibers was observed (P less than 0.01). In Group 2, the same treatment protocol administered under conditions of ICP elevation to 20 mm Hg by the cisternal infusion of mock cerebrospinal fluid resulted again in consistent, although less extensive, venous caliber reduction (5 to 7%, P less than 0.01). In Group 2 controls, pial arteries dilated by 15 to 20%; this reaction was suppressed in the methohexital-treated animals. After stopping methohexital, however, arteries in the methohexital group dilated to almost the same extent as in controls. Mean arterial pressure remained stable in both groups. ICP was not reduced by methohexital. EEG-activity was slightly increased during treatment. We conclude that methohexital has a direct effect on vessel caliber and thus cerebral blood volume via absolute diminution of venous vessels and suppression of ICP-induced autoregulative arterial dilatation. This direct vascular effect, however, seems to be counterbalanced by unknown intracranial mass effects that prevent a reduction in ICP.