Barbiturates in focal ischemia of primate cortex: effects on blood flow distribution, evoked potential and extracellular potassium.

Abstract

The effect of an ultra-short acting barbiturate, methohexital, on the distribution of blood flow in baboon [Papio cynocephalus] cerebral cortex was studied following occlusion of the middle cerebral artery under conditions of constant blood pressure. Further experiments assessed the effects of methohexital and pentobarbital on the threshold relationships between flow, cortical evoked potential amplitude and extracellular K activity. Regional flow was measured by the H clearance technique and the initial anesthetic was chloralose in all experiments. If flow after occlusion was > 25 ml/100 g per min, so that electrical activity was sustained, methohexital reduced flow in proportion to the flow (r [correlation coefficient] = -0.84, P < 0.001), but if flow was < 20 ml/100 g per min, and electrical activity was reduced or absent, a significant elevation in flow occurred averaging 3.4 ml/100 g per min (P < 0.01), an appreciable fraction of ambient flow. This result may be attributable to an inverse steal, blood being diverted into ischemic regions from vasoconstriction induced in relatively well-perfused areas. No statistically significant change was demonstrated either in the flow threshold for the abolition of the evoked potential or in that for the massive increase in K, although methohexital tended to decrease, and pentobarbital to increase, these thresholds. Methohexital significantly reduced the rate of decrease of the evoked potential for a given flow below the threshold. These effects may be among factors underlying any protective effect of barbiturate in focal cerebral ischemia on the neurological and neuropathological levels.