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HANDBOOK OF PHYSIOLOGY "> CIRCULATION II 



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fig. 25. Schematic representations of concentration-time curves following injection of indicator 

 into central circulation. A: single injection curve. Recirculation of indicator occurs at arrow. The 

 dashed lines illustrate likely extremes of extrapolation of the downlimb to zero during the first cir- 

 culation of indicator. The shaded arrow represents the relative difference between the two esti- 

 mates of blood flow based on the two extrapolations. The vertical lines fi and £2 represent the two 

 estimates of mean transit time based on the two extrapolations. If recirculation occurs earlier, so that 

 the shape of the downlimb is uncertain, considerable errors may be introduced by the extrapolation. 

 B: constant injection curve. The times at which the indicator just appears and recirculates are identi- 

 cal with those in panel A. The dashed lines represent likely extrapolations to a plateau concentra- 

 tion. The shaded area between Pmax-i and Pmax-s represents the difference between estimates of 

 area above the extrapolated buildup concentration curves. The problem of recognizing the point of 

 recirculation is the same as for the single injection curve of panel A. [After Zierler (444).] 



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ured volumes (184); moreover, following epinephrine 

 overdosage, the radiographic cardiac volume was 

 found to constitute an unusually large fraction of the 

 central blood volume (151, 184). By substituting 

 cineradiography of the opacified intracardiac volumes 

 for conventional radiography, the precision of the 

 radiographic approach has been greatly enhanced 

 (73, 170, 367); this modification promises a reliable 

 measure of the volumes of the individual chambers in 

 normal man and dog. It remains to be seen if precise 

 measurements of this type can also be made in pa- 

 tients with pulmonary congestion and cardiomegaly. 

 Recently, the central blood volume has been ex- 

 perimentally narrowed to the pulmonary blood vol- 

 ume by the use of two catheters — one in the 

 pulmonary artery and the other in the left atrium. 

 Once placed, the catheters have been put to different 

 uses: a) for injecting a tracer substance into the pul- 

 monary artery and for sampling from the left atrium 

 (246, 293), and b) for injecting tracer substances into 

 both the pulmonary artery and left atrium, and 



sampling from the brachial artery, thereby determin- 

 ing the mean pulmonary arterial-left atrial transit 

 time (106, 278). Although the second of these ap- 

 proaches was designed to circumvent the theoretical 

 possibility of incomplete mixing in the left atrium, the 

 values for the pulmonary blood volume by both ap- 

 proaches have been not only similar, but also sur- 

 prisingly low. 



newman: exponential DOvvNSLOPE. The time-con- 

 centration curve of injected substance typically has a 

 descending exponential limb. According to Newman 

 (304), the slope of this line measures the volume of a 

 model through which water is perfused if there is 

 instantaneous and complete mixing of injected dye 

 and perfusate. If several chambers are perfused in 

 series, the slope indicates the volume of the largest. 

 Assuming that the lung volume is the largest of those 

 concerned in the circulation, Newman used the slope 

 to obtain a measure of the pulmonary blood volume. 

 In 1932, Hamilton et al. (186) had evolved an equa- 



