MEASUREMENT OF THE CARDIAC OUTPUT 



579 



having the advantage that it leaves the circulation in 

 I iiour or so and can therefore be injected in large 

 amounts without staining the patient (132, 133). It is 

 thus possible to use a relatively insensitive pickup so 

 that changes in oxygen saturation do not interfere 

 badly. With this dye it is claimed that results with an 

 ear oximeter are as accurate and reliable as those with 

 a cuvette oximeter. 



Simultaneously with the development of the use of 

 the oximeter to record dye dilution curves, the photo- 

 multiplier tube was being adapted for the same pur- 

 pose in Bing's laboratory (39). The circuity was im- 

 proved independently in Newman's laboratory (103) 

 and in the Walter Reed laboratory (122). These in- 

 struments are not set up as differential photometers 

 and hence there is no compensation for changes in the 

 optical density of hemoglobin either as a result of 

 saturation changes, of quantitative differences or as 

 a result of changing red cell orientation resulting from 

 flow. The apparatus is quite bulky and is not con- 

 v'eniently used on the ear or on an unopened artery. 

 Its advantages are its linearity and stability. 



Calculation of the blood flow from the concentra- 

 tion of indicator downstream from a site of continuous 

 constant infusion was the original method of Stewart 

 (130). In fact, it is doubtful that Stewart appreciated 

 the possibilities of instantaneous (slug) injection (24) 

 before they were described bv Henriques in 1 9 1 3 



(75, 76). 



The calculation of the cardiac output from the 

 arterial concentration of a continuously infused indi- 

 cator demands the establishment of an equilil^rium 

 concentration plateau. At first, all the undyed blood 



must have left the heart and lungs, followed by blood, 

 all of which had been homogeneously mixed with dye. 

 If tliese conditions obtain before recirculation begins, 

 a usable plateau is evident. Since the continuous in- 

 fusion curve is the integral of the slug curve the in- 

 fusion curve can attain a plateau only while the slug 

 curve is maintained at zero concentration (63). This 

 would occur only if the slug is injected into the pulmo- 

 nary artery. If the infusion is made into the pulmo- 

 nary artery it is thus possible to find a short plateau. 

 If, on the other hand, an instantaneous injection is 

 made further upstream than the pulmonary artery, 

 dyed blood begins to recirculate before the first circu- 

 lation is over and the indicator concentration will 

 continue to mount in the arterial blood with no 

 plateau. 



There are further handicaps suffered by those who 

 work with a constant infusion. During the build-up of 

 the constant infusion curve there may be, as a result 

 of respiration or of biological instability, an increase 

 in blood flow. This would dilute the constantly infused 

 indicator and give rise to the appearance of a plateau. 

 Such spurious plateaus would not serve for calculating 

 the blood flow but might easily be taken for a useful 

 plateau (78). 



Although the height of the plateau and the duration 

 of the mean circulation time can be calculated from 

 the early part of the concentration cur\e resulting 

 from constant infusion (see Chapter 18), it is difficult 

 and the steps are not self-evident. 



It would be very convenient if an indicator could 

 be found which would not recirculate. It has been 

 shown in Cournand's laboratory (15, 117) that a solu- 



Constant infusion t V^ 



h[ n t 





Cfifl 



RA 



RV 



^Cmv 



PA 



W^ 



CARDIAC OUTfVT AT RCST AND EXEKISE MEASURED BT KRYPTON' 

 REST 1^ EXERCISE- 



-1— >H 



Cba 



y • tso 



Tissues 



FIG. 20. The constant infusion of a solution of radioactive krypton into tlic rigtit atrium gives 

 rise to a concentration of Kr*' in the pulmonary artery, which can be used to measure the instan- 

 taneous cardiac output. [From Rochester el al. (117)-] 



