THE CEREBRAL CIRCULATION 



'753 



quired, however, some increase in precision is possible 

 by averaging values obtained from the two sides (74). 



Although the blood in one superior jugular bulb 

 represents a satisfactory mixture of the drainage from 

 the various cerebral tissues, it is not completely repre- 

 sentative of the drainage from both sides of the brain. 

 Dye injection studies indicate that about two thirds 

 of the blood in each jugular bulb is of ipsilateral ori- 

 gin, only one third having crossed the mid-line (101), 

 further substantiating the advisability of bilateral 

 sampling in studies on localized cerebral disease. 



Although there has been some indirect evidence 

 for the appearance of some blood of extracerebral 

 origin in the internal jugular vein (21, 88), the only 

 attempt to quantify this contamination found it to be 

 quite small in each of eight subjects (101 ). An average 

 of 2.7 per cent of the blood in the superior jugular 

 bulb was found to be derived from extracerebral 

 sources with a range from 0.0 to 6.6 per cent. 



METHODS OF STUDY 



A number of methods have been used for the stud) 

 of the cerebral circulation, both generally and Locally 

 in both animals and man. In measuring the total 

 cerebral circulation in animals, perfusion methods 

 have been widely used (9, 14, 29, 94, 97) and have, 

 in fact, yielded the most reliable results when atten- 

 tion was given to certain limiting features (14). It is 

 important, of course, that the perfusion flow which is 

 being measured be directed solely to the brain and 

 that all of the cerebral circulation be included in it. 

 In the rhesus monkey, this is possible because of the 

 rather definite isolation of cerebral blood flow from 

 that of the rest of the head, but in the cat and dog it 

 is necessary to demonstrate that such isolation has 

 indeed been secured. Another lesser problem is the 

 choice of a suitable flow meter, and bubble flow 

 meters, rotameters and Yenturi meters have been 

 used as has direct measurement of venous outflow. 



The flow of blood in the internal jugular of man 

 has been measured by means of a thermoelectric 

 technique (33), using a heated thermocouple enclosed 

 in a needle, the temperature at the thermocouple 

 being some function of the flow of blood about it. 

 Such a technique is difficult to make quantitative or 

 even reproducible (36) and has, in fact, been used 

 only to indicate roughly the relative changes in blood 

 flow in a single internal jugular. The Stewart principle 

 of dye dilution has been applied with varying degrees 

 of success to the measurement of cerebral blood flow 



in man (34, 78, 101). Since dye injected into one 

 internal carotid will be only partially mixed with the 

 venous return from the contralateral hemisphere 

 (101), it is necessary to secure samples of blood from 

 both internal jugular veins in order to make satisfac- 

 tory measurements. A more precise approach to the 

 problem entails the administration of the dye into 

 both internal carotids and the sampling from both 

 internal jugulars in order to obtain an approximation 

 to total blood flow through the brain (78). A modified 

 plethysmographic principle has been applied to the 

 brain of man (20), taking advantage of the rigidity of 

 the craniovertebral shell and the incompressibility of 

 its contents. This technique has yielded results which 

 are less than a third of the values obtained by other 

 methods, indicating that occlusion of the cerebral 

 venous outflow is only partially achieved by com- 

 pressing both internal jugular veins since appreciable 

 quantities of blood must Irak out by way of the spinal 

 \ enous plexus (4, 36). 



The Fick principle has been widely applied to 

 measurement of cerebral circulation, especially in 

 man. If cerebral oxygen consumption is assumed to 

 be constant, then hv this principle cerebral blood flow 

 would be inversely proportional to the arteriovenous 

 oxygen difference across the brain. Myerson and his 

 associates (75) made possible the sampling of cerebral 

 venous blood in man, and arterial venous oxygen 

 differences have been obtained under a variety of 

 conditions. The validity of relating these to cerebral 

 blood flow, of course, depends upon the validity of 

 the assumption of unaltered cerebral oxysjen con- 

 sumption which, without its independent measure, 

 can be made only on the basis of an astute guess. 

 Investigators who have used this technique have 

 made a large number of correct guesses (65), al- 

 though the record is by no means perfect. 



The nitrous oxide technique (47, 55) is also based 

 upon the Fick principle and, in addition, upon the 

 premise that the uptake by the brain of an inert gas 

 like nitrous oxide would, in contrast to oxygen, be 

 independent of the metabolism or functional activity 

 of the brain and would depend only upon gross com- 

 position. In this method, the integral of the arterio- 

 venous nitrous oxide difference 1 over a 10-min. period 



1 This was obtained in the original description of the nitrous 

 oxide technique by serial simultaneous sampling from the fem- 

 oral artery and internal jugular vein and by arithmetically 

 integrating the resulting curves (47). One modification of this 

 technique which may simplify the procedure consists in the 

 continuous sampling of the two blood sources, thus automati- 

 cally integrating them (92). Care must be taken to avoid timing 

 and dead-space errors in the latter procedure (106). 



