640 HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



ROLE OF RADIOISOTOPIC TRACERS IN DETERMINING 

 REGIONAL FLOW RATES 



When there are so many other good methods for 

 measuring regional flow rates, it is admittedly some- 

 what artificial to single out those contributions which 

 involve the use of isotopic tracer methods, and it would 

 be desirable to treat such contributions within the 

 context established by the other methods. This is not 

 the place for the broad review of the literature that 

 such comparisons would require, but it is expected 

 that other chapters of the Handbook will to a large 

 extent provide this context. 



The radioactive methods do have some unique 

 features, including susceptibility to the compartmental 

 analysis expounded above, and determination by 

 external detectors [Conn (17, 18), Anger & Upham 

 (i)], which justify their separate consideration. On 

 the other hand, in most kinetic studies which would 

 be good examples of uses of compartmental analysis 

 the chief interest is in other aspects of the behavior of 

 the labeled substance, with regional flow rates 

 being only an incidental problem, whereas in most 

 of the studies which are directed to the problem of 

 regional flow rates the methods of data analysis are 

 simply extensions of other indicator dilution methods. 

 The reader is again referred to Zierler's (74) chapter, 

 for the theory of the .Stewart-Hamilton method, but 

 some further examples will be mentioned here. A 

 number of articles (i, 2, 10, 13, 35, 37, 38, 40, 63) 

 which emphasize without being restricted to the 

 use of radioisotopic tracers in peripheral blood flow 

 measurement are collected in the volume edited by 

 Bruner (14). Some of the areas in which the contribu- 

 tions made through use of isotopic tracers are signifi- 

 cant are as follows. 



Cerebral Flow 



Lassen & Munck (41) have modified and adapted 

 the N2O method to the use of Kr'*^ in the measure- 

 ment of cerebral blood flow, and find the isotopic 

 method equally as satisfactory. In a later report 

 Munck & Lassen (45) recommend taking bilateral 

 internal jugular venous blood samples because the 

 two sides may give different results. Sokoloff (63) 

 cites a report by Lewis et al. discussing the further 

 advantages of Kr'^, which permits rapid and con- 

 tinuous measurement of total cereljral blood flow in 

 man. In this method, cerebral blood flow is calculated 

 minute bv minute for successive minutes bv the 



equation 



CBF 



Afti-i X / 



/: 



(C.4 - Cy) dt 



where C'.i and d- are the arterial and cerebral venous 

 concentrations as determined by counting blood 

 flowing through a glass coil in a well-type scintillation 

 counter, AQ, is the change in brain counts per minute 

 over the interval {t-i) to /, / is a proportionality 

 constant to convert brain and blood counting rates 

 to equivalent units and CBF is the total cerebral 

 blood flow in milliliters per minute. The mean total 

 cerebral blood flow during the middle 6 min of the 

 lo-min desaturation period studied was found to be 

 1 181 ± 132 ml per min in normal young men, which 

 is somewhat higher than corresponding N2O results, 

 probably reflecting some contribution from the blood 

 flow to other tissues in the head. The Kr method 

 had been proposed by Wechsler (73) for measurement 

 of cerebral blood flow in humans under acceleration 

 but he did not publish data. 



Kety (37, 38) has adapted the Fick principle to the 

 measurement of local cerebral blood flow in the cat 

 using CF.3P" and has obtained blood flow rates for 

 28 anatomical subdivisions of the central nervous 

 system. The values obtained range from a low of 

 0.14 ml per g per min for spinal cord white matter to 

 1 .80 ml per g per min for the inferior coUiculus. 



Myocardial Flow 



The clinical importance of knowledge concerning 

 the blood supply of the heart has stimulated intense 

 interest in finding new and better methods for meas- 

 uring the myocardial blood supply. For example, 

 Hansen et al. (31) have used Kr"^^ for studying 

 coronary i:)lood flow and obtained values in normal 

 controls comparable with values obtained using the 

 bubble flowmeter and N-.O methods. Conn & 

 Robertson (19) studied potassium exchange in dog 

 heart muscle by maintaining a constant arterial K''- 

 conccntration and interpreting the venous K^- con- 

 centration curve in terms of a two-compartment open 

 system. Assuming that the blood flow rate is the limit- 

 ing process in the plasma-extracellular fluid exchange, 

 this procedure offers another possible measvu'cment of 

 myocardial blood flow, but in common witli other 

 methods has the disadvantage of requiring venous 

 catheterization. Conn (17) discusses several papers 

 concerned with myocardial blood flow determinations 

 bv external countina; methods. Bing el al. ("i i) review 



