MEASUREMENT OF THE CARDIAC OUTPUT 



577 



This goal has been approached in several ways. 

 Plasma albumin (labeled with I'^') and cells (labeled 

 with radioactive phosphorous, iron, or chromium) 

 may be quantitatively injected, and collected as timed 

 serial samples. The analysis of these separate samples 

 can be expedited by the use of automatic serial 

 counters which can be loaded with many samples and 

 will register the radioactivity of successive samples 

 without attention. If the amount of radioactivity in- 

 jected and the time and activity of the separate 

 samples are known, the indicator dilution curve may 

 be plotted and the blood flow calculated in accordance 

 with the principles described above. 



By far the most attractive approach is to induce a 

 change in some property of the flowing stream and 

 arrange for self-recording of that change. Continuous 

 automatic records of changes in conductivity were 

 first made by Gross & Mittermayer (54) in 1926 and 

 later by H. C. Wiggers (145) and H. L. White (143). 

 The technique is very simple but, by the very nature 

 of the determination, the indicator must be difTusible 

 and hence of limited usefulness in clinical conditions. 



The ease of measuring temperature with very small 

 probes (thermo junctions, thermistors) has led Fegler 

 (31) to take the very bold step of injecting saline at 

 room temperature in the hope that it would pass 

 through the circulation, gaining heat from the blood 

 only, so that the equililjrium temperature could be 

 used to calculate the dilution volume and hence the 

 flow. This method, known as the thermodilution 

 method, gives continuously recorded curves of tem- 

 perature change that look surprisingly like those of 

 dilution of a substantive indicator and are said by the 

 author to give correct flows, calculated on the basis 

 of the specific heat of saline, and of blood-saline mix- 

 tures — flows which are equal to simultaneous de- 

 terminations by the Fick method. The allegation that 

 heat is not gained from the tissues of the heart and 

 lungs is bolstered by the fact that a model with air 

 insulation is more effective than one with the air 

 spaces flooded with water. It is argued that blood 

 passing throue;h the lungs is insulated by air and gains 

 a small amount of heat. It also would not be expected 

 to gain inuch heat from the walls of the heart and 

 great vessels since it is in contact here with a relatively 

 small surface and for a very short time. 



The same principle has been modified by Fronek 

 & Ganz (41). They inject the cool saline or dextrose 

 solution from a catheter in an upstream jet which is 

 rapid enough to mix with a full cross section of the 

 blood vessel. A few millimeters downstream from the 

 jet a thermistor is affixed to the catheter to sense the 



temperature change and draw, by means of a galva- 

 nometer, a thermodilution curve. Excellent checks of 

 model experiments, regional blood flow, and cardiac 

 output are reported. This modification of the thermo- 

 dilution method is said to avoid heat exchange be- 

 tween point of injection and sensing. It also eliminates 

 the exploitation of such important side issues as the 

 circulation times and central volume. 



Continuous recording of radioactivity is an ap- 

 proach which has been exploited and bids fair to be- 

 come more and more useful as the apparatus becomes 

 more refined. Radiation hazard is a question that 

 deserves serious consideration, and radioactive sub- 

 stances with short biological half-life are to be chcsen 

 in clinical work. A physical half-life short enough to 

 avoid accumulation of radioactive waste and long 

 enough to use conveniently in the experiment is to be 

 desired. Dangers of repeated injection of radioactive 

 material are not always appreciated. 



Continuous recording of the radiation from I''*'- 

 treated serum albumin was successfully done by 

 Maclntyre and colleagues (93). They drew arterial 

 blood through a small tube coiled around a counting 

 rate meter. The calibration was done by assessing the 

 performance of the counter when the radioactivity 

 became constant (complete mixing with all the blood) 

 and standardizing the relation between radioactivity 

 in drawn blood and that of a dilution of the injected 

 indicator. 



The injection of radioactive material into a vein 

 with a counting rate meter placed over the heart re- 

 cording the concentration of radioactive material 

 passing through the heart gives the '"radiocardio- 

 gram." This shows an increase in radioactivity as the 

 injection enters the right heart and a second rise as 

 the blood enters the left heart (107). This could be 

 surveyed for circulation times and other landmarks 

 which could be empirically interpreted. Radiocardio- 

 grams have been calibrated from the deflection made 

 by the dilution of the indicator in the total blood vol- 

 ume (124) and treated quantitatively so as to calcu- 

 late the cardiac output though some reservation must 

 be admitted as to the accuracy of the calibration. 

 Curves were presented by Lammerant (86) and used 

 to calculate the cardiac output and the "pulmonary 

 blood volume." Actually, the calculation found the 

 pulmonary blood volume plus one-half that in the 

 heart. 



In order to measure the mean time of passage of 

 the indicator through a ventricle it is necessary to be 

 able to visualize the whole curve and make certain 

 calculations from it. Unfortunately, the first part of 



