1754 



lIVMIWIllK III I'HYSIill IK.Y - M.I kiil'Il'i Mill 0G1 111 



of equilibration is used as the denominator in the 

 Fick equation, while the quantity of the gas taken up 

 I iy unit weight of brain in the same time is the numera- 

 tor (47). The latter quantity cannot be determined 

 directly in man. If, however, the time of equilibration 

 with the inert gas is sufficiently long, the nitrous 

 oxide tension in the cerebral venous blood will 

 approximate its mean tension in the brain, and since 

 the brain-blood partition coefficient of the gas is 

 practically unity (51), its mean brain concentration 

 will be close to its concentration in mixed cerebral 

 venous blood. Direct analyses in a series of 16 dogs 

 (51) and indirect studies in man (66, 67) using the 

 7-radiation of K.r 7a have indicated that in the case of 

 the brain such equilibrium has, for practical purposes, 

 been achieved at the end of 10 min. This is in accord 

 with calculations based upon values recently deter- 

 mined for the blood flow in various portions of the 

 brain of the unanesthetized cat (63). It seems hardly 

 necessarv to point out that for those organs and condi- 

 tions where satisfactory equilibrium is not achieved 

 in the time interval under examination, the method 

 cannot be used with validity (85). 



The effect of the absorption of gas by cerebrospinal 

 fluid has been shown to introduce a small error in 

 the nitrous oxide technique under normal circum- 

 stances (1). This could be considerably increased in 

 conditions such as hydrocephalus or cerebral atrophy. 



One of the disadvantages of the nitrous oxide tech- 

 nique is the fact that it is incapable of measuring 

 rapid changes in cerebral blood flow. In order to 

 obtain such information in man, another modification 

 of the Fick principle has been made recently in 

 which a 7-emitting radioactive gas (Kr 79 ) is used 

 instead of nitrous oxide, permitting continuous esti- 

 mation of its uptake by the brain which, in conjunc- 

 tion with arteriovenous differences, makes possible 

 continuous calculation of cerebral blood flow did, 



67). 



Another limitation ol the nitrous oxide technique 

 is its applicability onlv to the brain as a whole, \et 

 the need lor information on regional circulation in 

 the brain is an obvious one. This problem has been 

 approached by direct visualization and measurement 

 ,,l superficial vessels of the brain (26, 27), or l>\ the 

 use of heated (76) or cooled (96 1 thermocouples 

 inserted into the substance 1 >! the brain. Neither of 

 lln-si- approaches has set been made quantitative, 



.mil techniques depending upon measurement of the 

 thermal conductivity of the living brain are subject 

 to erroi introduced bv metabolic heal and the man) 

 temperature gradients within the brain (68 



In recent years, measurements have been made on 

 regional circulation in the brains of animals in various 

 conditions by a technique which depends upon the 

 differential uptake of a radioactive inert gas by the 

 various tissues of the brain (53, 63). This can be 

 shown to depend upon the arterial concentration 

 curve, the solubility of the gas in the particular tissue, 

 blood flow and the diffusion process. In the case of 

 the brain, the diffusion can be shown not to be a 

 limiting factor so that, by measuring the various 

 parameters indicated, cerebral blood flow can be 

 calculated. 



NORMAL VALUES AND PHYSIOLOGICAL VARIATIONS 

 OF CEREBRAL CIRCULATION 



In table 1, mean normal values for cerebral blood 

 flow in man are presented as they have been reported 

 by investigators using a number of different tech- 

 niques. In table 2, mean values for regional blood flow 

 obtained in various areas of the cat brain by means of 

 the radioactive gas technique are summarized. 



In man the studies on the over-all cerebral circula- 

 tion have been quite extensive and the variety of con- 

 ditions sufficiently great that some generalizations 

 may be made for variation in cerebral blood flow 

 with different physiological states. 



A longitudinal study of the changes in cerebral 

 blood flow with age has not vet been undertaken, 

 nor has any one group of investigators systematically 

 studied the entire age span. The reliability of conclu- 

 sions based upon available data is considerably les- 

 sened by individual differences in the criteria ofselec- 



1 vi! 1 i'. 1 . \~alucs for Cerebral Blood Flow in Normal 

 Man Obtained by Different Groups Using larious 

 Mollifications of the Inert Gas Technique* 



* Each \alne tin' ieielii.il blood Mow represents the mean 

 of a series pei leu med on patients of approximately the same 

 age. 





