MEASUREMENT OF THE CARDIAC OUTPUT 



567 



measurement of cardiac output by X ray would seem 

 to be more definitive if it inv-olves the visualization 

 of the left ventricular cavity by means of infusions of 

 radiopaque fluids (119). Changes in the projection 

 of the area of the left ventricular cavity are certainly 

 more closely related to the stroke volume than are 

 changes in the projection of the whole heart or even of 

 the two ventricles. Evidence brought out by this in- 

 vestigator showed that the effective filling pressure 

 was not the predominant factor in determining the 

 diastolic area of the left ventricular projection, nor 

 was the diastolic size of this area predominant in de- 

 termining the systolic change in area (stroke). 



During the last 2 years this approach has been 

 carried much further by simultaneous and equally 

 meritorious researches in Sweden (52) and Texas 

 (13, 14). Both groups of investigators visualized the 

 left ventricle by injecting a relatively nontoxic radio- 

 paque material into the right heart and studying 

 cinefluorographic frames which showed the left 

 ventricular outline most clearly. Each group worked 

 out formulas for calculating the volume of the left 

 ventricular contents from its silhouette (see also 70). 

 Accuracy of the X-ray volume calculation was 

 checked against casts. 



Cineangiocardiographic exposures were made at 

 30 to 48 per sec projected life size and the formula ap- 

 plied to the silhouette of the radiopaque contents of 

 the left ventricle. Twenty, more or less, of these left 

 \entricular blood volume measurements were 

 laboriously made during each cycle, showing filling, 

 ejection, and isometric phases. 



The Texas workers showed that the increased stroke 

 volume of exercise was made by means of decreased 

 systolic volume rather than an increased diastolic 

 \olume (fig. 13, 14). The Swedish workers traced the 

 simultaneous cyclic changes in atrial and ventricular 

 volume and showed that the atrial \olume changed 

 reciprocally with that of the ventricle (confirming 

 what has been said above concerning ballistocardi- 

 ography, cf. 59, 60, 67). 



X-ray methods have led to clear insight into the 

 relation in the intact animal between the factors 

 which control the cardiac output such as diastolic 

 size, systolic size, stroke volume, and heart rate as the 

 animal changes from the resting to the active state. 



In general, it may be said that the pulse pressure 

 method, ballistocardiography, and the various forms 

 of X-ray cardiometry which do not involve measur- 

 ing the volume of left ventricular contents are methods 

 which depend for their validity on comparison with 

 an accepted method (see below). Comparison under 



one set of conditions may not validate the method 

 under other conditions. The chief advantage of these 

 methods is that they give data on beat-to-beat changes 

 and do not demand a steady state as do the classical 

 dilution methods. 



DILUTION METHODS 



In 1870, A. Pick wrote a much quoted but litde 

 read paragraph in the proceedings of a scientific 

 society in the ancient university town of Wurzburg 

 (32). This paragraph has been little read because it 

 carries a very simple message that is self-evident, once 

 it is grasped, and needs no careful rereading. It is 

 much quoted because it is a turning point in the de- 

 velopment of the quantitative measurement of blood 

 flow, and from its central idea or principle have come 

 many and various techniques that have given us the 

 soundest measurements of the output of the heart and 

 the flow of blood through organs. 



Adolph Pick had an unusually analytical viewpoint 

 in physiology. He is known at the present time in 

 physics for his law of diffusion in fluids, published 

 when he was 26 years old. This generalization of the 

 relation between rate of diffusion and concentration 

 gradient was developed mathematically in 1855, but 

 not proven experimentally for 25 years. 



In circulatory physiology he is known at the present 

 time as the originator of the dilution principle for 

 measuring blood flow, the so-called Fick principle 

 being a byword on the tongue of nearly all medical 

 students. This contribution again was not exploited 

 by Fick nor was it experimentally tested until ac- 

 curate methods of blood gas analysis had come into 

 general use in 1 886 (51) and again in 1 898 ( 1 50) . 



As a physiologist, Fick's chief interest was in the 

 physiology of muscular contraction, distinguishing 

 isometric and isotonic contractions and the eff'ect of 

 the length of fiber on strength of contraction. This 

 last development is the foundation of the Starling 

 principle, a most fundamental circulatory generaliza- 

 tion. 



The principle of dilution methods in general is 

 that if we know the amount of substance which enters 

 or leaves a stream, and the concentration difference 

 resulting from such entrance or removal, the size of 

 the stream can readily be calculated. The illustration 

 that Fick used, and most probably the only one he 

 had in mind, invohed gas transport by the blood and 

 can be shown by the simple example that if oxygen is 

 consumed at the rate of 240 ml per min, the arterial 



