MEASUREMENT OF THE CARDIAC OUTPUT 



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FiG. 14. Left ventricular (closed circles) and left atrial 

 volume changes (open circles) in the dog's heart, calculated 

 from cinefluorograms. Simultaneous ECG. [From Gribbe 

 el al. (52).] 



cardiac puncture in dosjs in the \ain search for a key 

 (such as hydrogen ion concentration, oxygen ten- 

 sion or CO-i tension) which might regulate the 

 circulation just as the respiration was thought at 

 that time to be regulated (cf. 57, 58, 71, 94). The 

 search for such a simple key to the control of the 

 circulation rate was naive because it seems clear at 

 the present time that the controlling role of the central 

 nervous system, the self-regulated demands for blood 

 on the part of active organs, and the reflex mechanisms 

 for the homeostatic control of the arterial pressure 

 all play a role in the complexly integrated mecha- 

 nisms by which the rate of the circulation is regulated. 



Even though CO2 tension, oxygen want, or other 

 simple key to the regulation of the circulation was 

 not found, much interesting information was un- 

 covered as to the effects of drugs, arteriovenous 

 fistulas, hemorrhage, trauma, and pneumonia. 



Cardiac puncture was not applied to man. The 

 human heart and even the human artery were held 

 inviolate until well into the present century. It was 

 not until 1930 that Bauinan & Grollman (4) punc- 

 tured the right ventricle of the human heart and used 

 the respiratory gas content of a sample of blood from 

 this cavity together with that of arterial samples and 

 the gas exchange to calculate the cardiac output. 

 When these investigators found that the cardiac 

 output by the direct puncture Fick method agreed 

 with a favored respiratory method (see below) they 

 carried their experiments no further, and no one 

 since has had the temerity to repeat them. 



At about this time the intrepid Forssmann (34) 

 catheterized his own heart several times, and Klein 

 (80) drew mixed venous blood using a catheter and 

 calculated the cardiac output. Cardiac catheterization 

 was used during the next decade for the visualization 



of radiopaque substances injected into the cardiac 

 cavities, and a significant advance was made in the 

 development of a nonwettable plastic catheter that 

 minimized the danger of intravascular clotting. 



The time was ripe for Cournand, Richards, and 

 their colleagues to begin their classical work on the 

 physiology of the circulation in health and disease 

 (18, 1 13). The new techniques available from cardiac 

 catheterization enabled these investigators to measure 

 the pressure pulses in the cardiac chambers and great 

 vessels as well as to take samples of mixed venous 

 and of arterial blood, so that many of the details of 

 hemodynamics of normal and abnormal circulations 

 could be worked out. This pioneering work demon- 

 strated the safety of the procedure so clearly that it 

 was taken up in many laboratories, notably those of 

 Bing (7), Dexter (22), McMichael (96), Stead (129), 

 and Wood (147). 



The accuracy of the cardiac output calculation 

 depends in the first place on getting a fair sample of 

 the mixed venous blood. As is well known from the 

 study of the fetal circulation, blood may pass through 

 the inferior vena cava and into the left atrium by way 

 of the foramen ovale without mixing much with the 

 .superior vena cava stream which crosses it and enters 

 the right ventricle. For this reason it is not surprising 

 that Shore ft al. ( 1 26) found that successive samples 

 taken from the dog's right atrium might not agree in 

 O; content. In man consecutive or simultaneous 

 samples show much better agreement than those 

 reported here. In general the effects of inadequate 

 mixing are reduced as the site of sampling is moved 

 downstream from the inflow tract of the right atrium 

 to the outflow tract. The samples agree better when 

 taken from the outflow tract of the ventricle and 

 better still when taken from the pulmonary artery 

 (20, 137). 



Aside from the possilsility that the \enous stream 

 is incompletelv mixed and essentially simultaneous 

 samples are variable in their oxygen content is the 

 possibility that the subject is in a changing state. 

 This is important because the fundamental assump- 

 tion on which the Fick procedure is based is that the 

 volume of oxygen taken up in the respiration equals 

 the volume of oxygen used by the tissues. Unfortu- 

 nately these quantities cannot be instantaneously 

 measured. The gas exchange and oxygen uptake can 

 be measured only at the mouth. The lungs are a large 

 reservoir which in their gas capacity and in the 

 quantity of blood which may be stagnant in the 

 alveolar capillaries are able to take up and give off 



