DYNAMICS OF PULMONARY CIRCULATION 



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from the volume which prevails under more natural 

 conditions. In intact animal or man, indicator-dilu- 

 tion techniques have been commonly used to approxi- 

 mate the size of the pulmonary blood volume. 



stewart-hamilton: indicator dilution. This is an 

 indicator-dilution method (fig. 24) which entails the 

 introduction of a test substance into the venous side 

 of the circulation and the registration, from a systemic 

 artery, of its changing concentration with time (18, 

 hi, 184). This application was first proposed by- 

 Stewart (184), who held that the product of the flow 

 and the appearance time of the injected substance is 

 a measure of the capacity of the bed through which 

 the flow takes place; this idea was shared by Blumgart 

 and Weiss (184). Hamilton and collaborators (186) 

 showed that the mean circulation time rather than 

 the shortest circulation time should be used to calcu- 

 late the volume of blood in the vascular bed between 

 the point of injection and the point of sampling. In a 

 simple model, in which the entire stream passes the 

 points of injection and of sampling, the idea that the 

 product of the flow and the mean circulation time 

 measures the intervening volume is not only ac- 

 ceptable intuitively, but has also been checked in 

 models (186) and proved mathematically (444). 

 Since the mean circulation time is approximately the 

 time ordinate corresponding to the center of gravity 

 of the time-concentration curve (fig. 25A), the substi- 

 tution of the median for the mean circulation time 

 may introduce considerable error into the calcula- 

 tion (184). 



The injection into the venous circulation coupled 



with sampling from a peripheral artery defines only a 

 "central blood volume"; its limits are wide and vague: 

 it includes not merely the blood volume between the 

 needles, but also the volume of blood contained in the 

 other branches of the venous and arterial trees having 

 equivalent circulation times. It is a virtual volume 

 which corresponds to an anatomical volume only 

 under ideal conditions: if mixing of blood and tracer 

 is complete and uniform, if the system contains neither 

 stagnant nor sequestered blood and if there are no 

 preferential channels which are operating to short- 

 circuit the system. The use of mathematics to con- 

 struct a continuous infusion curve from the single in- 

 jection curve involves identical premises and does not 

 make the measurement of the pulmonary blood vol- 

 ume any more definitive. The continuous infusion of 

 a tracer substance into the central circulation does 

 provide an alternate approach for measuring the 

 pulmonary blood flow and central blood volume 

 (fig. 25B) (444); however, as in the case of the single 

 injection, the results promise to be less precise for 

 volume than for flow (444). 



When the test substance is injected into a peripheral 

 vein (instead of into the pulmonary artery), the cen- 

 tral blood volume includes the whole cardiac blood 

 volume. Many different radiological techniques have 

 been applied to the measurement of the cardiac 

 blood volume in dog and man (151). Despite theoreti- 

 cal reservations of various kinds — such as the diffi- 

 culty in separating the contribution of cardiac cavities 

 and walls to the radiographic picture of the heart — 

 the radiographic cardiac volume in dogs was found to 

 correspond, within 10 per cent, to the directly meas- 



Jt 



.6.4 mg/Lit 



< 

 tr 



t- 



o 



z 

 o 

 o 



20 



I - 



INJECT E D 

 TIME 

 Av CONC 

 6 



14.11 

 I 5 sec 



m g 



FLOW = 



CB V 



I 1.70 mg/Lit 

 14.11 



11.70 x 

 MCT 



I 5 

 1 1 sec 



= 4.82 

 Lit/min 



4.82 

 60 



x I I = 880 ml 



SECONDS 



fig. 24. Concentration-time curve inscribed by densitometer through which peripheral arterial 

 blood was drawn at a constant rate following injection of T-1824 into the pulmonary artery of a 

 normal human subject. At t = o, the indicator was injected. The calibration marks at the top of the 

 record indicate that a deflection of 1 cm is equal to a concentration of 6.4 mg of dye per liter of 

 blood. From such a record, the pulmonary blood flow, the mean circulation time (MCT), and the 

 central blood volume can be calculated as shown. 



