MEASUREMENT OF THE CARDIAC OUTPUT 



565 



erated in the venae cavae and atria should cancel 

 some of those generated in the ventricles and in the 

 great arteries, inasmuch as the average direction of 

 flow is opposite. Recent evidence (52) confirms the 

 idea given in an earlier report (67), that the atria 

 fill as the ventricles empty and vice versa (see fig. 14). 



The movements of the walls of the heart probably 

 also produce recoils that cancel and summate with 

 recoils related to the movement of blood. When the 

 inflow or the outflow was obstructed in dogs the 

 ballistic waves were either diminished with their 

 timing unaltered ( 1 34) or greatly changed in pattern 

 (121). Elimination of pumping certainly does not 

 eliminate ballistocardiographic forces; we must there- 

 fore conclude that the forces consequent on the move- 

 ment of blood are augmented or canceled by forces 

 originating in the movements of the cardiac walls. 



In resume it can be said that the ballistocardio- 

 graph records acceleration of blood but is silent as to 

 flow in a steady stream; that movements of venous 

 blood into the chest are reciprocal to the velocity 

 pulse of the aortic stream out of the chest and produce 

 opposite forces; that movement of the bloodless heart 

 makes large impacts as recorded by the ballisto- 

 cardiograph; and that the forces which can be shown 

 to be necessary for moving the stroke volume into the 

 arteries show up in the ballistocardiogram as having 

 been delayed, distorted, and greatly diminished. For 

 these reasons it seems best to look with skepticism at 

 the equations which purport to calculate the stroke 

 volume from the ballistic record (81, 105, 128) without 

 accounting for or even mentioning the inadequacies 

 and interferences mentioned above. 



This is not to belittle the usefulness of ballisto- 

 cardiography as an empirical diagnostic method and 

 as an intriguing and difhcult instrumental problem 

 (121), but rather to point out the many things that 

 must be incorporated into the theory of the ballisto- 

 cardiogram before it can measure the stroke volume. 



C.\RDIOMETRY 



In principle the cardiometer consists of a chamber 

 into which the heart, or better the beating ventricles, 

 are introduced. As the ventricles change volume 

 within the chamber there is a reciprocal change in 

 \olume of the space between the walls of the chamber 

 and the heart. Suitable instruments can then record 

 this change in volume from which the output of the 

 two ventricles can be calculated. One-half of this 



figure is the stroke volume which multiplied by the 

 heart gives the output per minute. 



The physiological investigation of the volume 

 changes due to the heart beat entered into its modern 

 phase with the paper of Henderson in 1906 (73). In 

 this paper earlier work was reviewed. It was pointed 

 out that it had been usual to include both atria and 

 ventricles within the cardiometer. Thus the earlier 

 records included a series of volume changes pertaining 

 to both atria and ventricles that might summate or 

 cancel the effect of one another on the volume curve, 

 making the latter very hard to interpret. 



In order to avoid these complications Henderson 

 designed his cardiometer to enclose only the ventricles. 

 The cardiometer (see fig. 12) consisted of a rubber 

 ball with a large opening at one end and a small one 

 at the other. Over the large hole a rubber dam was 

 applied which could be pierced with a hole large 

 enough to admit the ventricles and which would close 

 gently but airtight around the A-\' groove. The 

 smaller hole was for a tube connecting the cardiometer 

 with a loosely covered writing tambour. The covering 

 of the tambour was tied loosely so that pressure 

 changes within the cardiometer would be small and 

 have a minimal interference with cardiac action. In- 

 advertently this loose tying served another advantage 

 in that the record could be made with almost no com- 

 pression of the air within the system. If air is com- 

 pressed even in so small a measure that it would have 

 no appreciable hemodynamic effect, heat is generated. 

 If this heat is not dissipated, it will cause a pressure 

 increase that may be as much as 40 per cent greater 

 than it would be if the heat were immediately dissi- 

 pated and the pressure change were due to the com- 

 pression alone. 



Henderson's study of the shape of the ventricular 

 filling curve, the influence of heart rate, filling pres- 

 sure, the role of atrial contraction, and many other 

 influences makes rewarding reading even after the 

 lapse of more than half a century. Implicit in the 

 shape of the cardiac volume curve are such concepts 

 as optimal heart rate, the inadequacy of extreme 

 tachycardia, the uselessness of atrial contraction in 

 the presence of a normal venous pressure, and its 

 great importance when the ventricles are not ade- 

 quately filled. 



Henderson's cardiometer was used by Starling 

 (106) in documenting the "Law of the Heart" (see 

 Chapter 15), and by means of clear cardiometer 

 records relating diastolic size to stroke volume he 

 hastened the acceptance of this important generaliza- 

 tion. Wiggers & Katz (146) made technical improve- 



