Effect ofchanges incerebral blood flow on proportion ofhighandlowflowtissue inthebrain

SYNOPSIS Regional cerebral bloodflow was measured byinjection of133Xenon intotheinternal carotid artery in11 patients withcerebrovascular disease. Allpatients were studied undergeneral anaesthesia, first atnormocapnia andthenathypocapnia. The15minuteisotope clearance curves were analysed bycomputerbytwo-compartmental analysis andregional changes inflowandthe proportions offast andslowclearing tissue obtained attwolevels ofarterial CO2tension. Hypocapnia caused a fall inbloodflowwhich was consistently accompanied bya decrease intheproportionoffast clearing tissue. Regional changes were notsignificantly different fromthehemisphere mean changes. There was no correlation between changes inbloodflowthrough greymatterandthe proportion offast clearing tissue on ahemisphere mean basis, buton regional analysis thedatafrom 10outofthe11 patients showedthatinareaswherebloodflowthrough grey matterchanged most theproportion offast clearing tissue changed least andvice versa.A hypothesis hasbeenproposed to explain this phenomenon.