MEASUREMENT OF THE CARDIAC OUTPUT 



575 



140 



FIG. 17. Dye curves from perfusion of the heart and hings, and from the lungs only. Simultaneous 

 evaluation of the results of constant infusion of brilliant vital red and instantaneous injection of 

 Evans blue (T-1824). [From Howard et al. (78).] 



not give an exponential downslope. This may be due 

 to the fundamental nature of laminar flow in trans- 

 porting indicator (118) or to the fact that organs such 

 as the lung and kidney have rapid and slow circula- 

 tions with diff"erent flow volume relationships. When 

 glass models are set up in parallel with different flow 

 volume relationships the washout concentration curve 

 is not exponential and consequently cannot be extra- 

 polated (62). The descending curve from lungs per- 

 fused through the pulmonary artery and sampled 

 at the pulmonary vein is also not exponential (78) 

 (see fig. I 7). Each of these curves becomes exponential 

 when a mixing chamber is introduced in series. In the 

 model the mixing chamber was a glass bulb (62), and 

 in the lung perfusion experiment the perfusion was 

 made to include the heart; the injections were made 

 into the right atrium and the samples taken from the 

 aorta. As soon as the curve had rounded the peak it 

 became exponential and remained on the same course 

 down as far as the determinations could be made, i.e., 

 in the case of the lung experiment to i^-foo th^ peak 

 concentration. The mixing chamber had served to 



smooth out the irregularities of the curve as it left 

 the complex system and impress upon the downslope 

 exponential washout curve that would be expected 

 from dye passing out of a single reservoir. 



It is clear from contributions from Newman's lab- 

 oratory (103) that, unless special precautions are 

 taken to insure instantaneous mixing of dye into a 

 glass chamber model, the volume of the chamber is 

 not indicated by the rate of concentration change of 

 indicator in samples of the eflSuent stream. This was 

 recognized in 1932 (62) but does not alter the fact 

 that flow through a chamber, whether mixing is com- 

 plete or not, appears exponential and that the volume 

 of the chamber and the flow through the chamber can 

 be calculated as accurately as experimental methods 

 will allow, on the assumption that the washout curve 

 is exponential. 



Another indication that the downstroke of the in- 

 dicator dilution curve can be prolonged exponentially 

 is the success of those who have followed this pro- 

 cedure. This success consists of comparison of the 

 cardiac output, as measured by the dye injection 



