CHAPTER 17 



Measurement of the cardiac output 



WILLIAM F. HAMILTON 



Department of Physiology, Medical College of Georgia, Augusta, Georgi 



CHAPTER CONTENTS 



Kinetic Energy Flowmeters 



Electromagnetic Flowmeters 



Sonic Flowmeters 



Pulse Pressure Methods 



Pulse Contour Method 



Ballistocardiography 



Cardiometry 



X-Ray Cardiometry 



Dilution Methods 



Fick Method 



Respiratory Methods 



Injection and Infusion Methods 



THE FUNCTION OF THE HEART is tO pump blood IVoni 



the veins, through the lungs and out into the arterial 

 distributing system. If we think of the veins, the 

 right heart, and the lung vasculature as a low pressure 

 reservoir, and the systemic arteries as a high pressure 

 reservoir, we can say that the function of the heart is 

 to pump blood from the low to the high pressure reser- 

 voir and thus to display before the tissues a constant 

 supply of oxygen and nutrients as well as to transport 

 carbon dioxide and other wastes to the lungs, liver, 

 and kidney. 



In the early history of physiology quantitation was 

 subordinated to description in general terms. Thus to 

 prove that the blood circulated (flowed in a circle) 

 Harvey (72) had merely to show that a large quantity 

 of blood passed out of the heart — something between 

 10 and 41 pounds of blood in half an hour — and 

 that this quantity is more than can be drawn from 

 the body or can be made from food and drink. He 

 thus disposed of the Galenical tradition that blood 

 was made de novo in the liver and flowed and ebbed 



in the arteries. Similar experiments on the veins 

 showed that a large amount of blood was returned to 

 the heart by one-wa>- flow through these valved 

 structures. These arguments were not strictly quanti- 

 tative but sufficed to prove his thesis. 



Stephen Hales (35J made wax casts of the distended 

 ventricles of the horse and calculated the circulation 

 rate as the product of the volume of the left ventricle 

 and the heart rate. He came out with a very small 

 figure — 6 liters per min. One might expect the figure 

 to be large because he made no allowance for residual 

 blood. An explanation that comes to mind is the 

 possibility that the heart was in rigor mortis (con- 

 tracted) when the wax was poured in. 



These early gropings in the direction of quantita- 

 tion were not made in an environment that en- 

 couraged measurements of l:)iological function. What 

 microscopist would think of milliliters per minute 

 when he beheld the intricate hurrying of tlie micro- 

 circulation, and what follower of Darwin eager to 

 establish the truth of evolution would be interested 

 in quantitative figures about the circulation when 

 the gross anatomy of the heart and great vessels lent 

 themselves to his purpose much more cogently? 



Quantitation in terms of numbers was introduced 

 into biology by the early students of metabolism and 

 biometry. Galton used numerical measurements in 

 describing the characteristics of populations, and his 

 followers brought numbers into experimental biology 

 by the use of statistical computations of the prob- 

 ability that differences were "significant" or due to 

 "chance." The usefulness of this approach was 

 self-evident and caused biologists to supplement 

 their thinking with numerical ideas. 



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