MEASUREMENT OF THE CARDIAC OUTPUT 



559 



rings, the rings stretched, and the volume pressure 

 relationship plotted from the circumference-tension 

 relation (64). These curves are surprisingly uniform 

 from individual to individual, when corrected for 

 body size. Moreover, the slope of the plot of the uptake 

 of the dog's aorta against pressure is the same whether 

 the aorta is constricted on its removal from the body, 

 relaxed after several stretches, or constricted again 

 by epinephrine (i, 109; see also 1 12). 



This constancy of volume uptake of the aorta under 

 different conditions justifies considering the distending 

 effect of the pulse pressure in a very detailed manner 

 (64). Under different circulatory conditions the pres- 

 sure pulse contour may take diverse forms, with the 

 peak falling early or late in systole, and systole itself 

 lasting from 80 to 120 msec. At the instant of closure 

 of the aortic valves, each segment of the aorta and its 

 branches will have been distended as the pulse wave 

 passes down by very different pressure increments, 

 ranging from zero (diastolic) to peak systolic pressure. 

 The uptake of each segment will be conditioned by its 

 distensibility and by the pressure at time of valve 

 closure. The arterial uptake will, of course, be the 

 sum of the uptake of all the segments, and the stroke 

 volume the sum of the uptake and the arteriolar runoff 

 during systole. 



Evaluation of the stroke volume from the pulse con- 

 tour in this manner will, when its results are com- 

 pared with those of an independent method, prove the 

 soundness of measuring the cardiac output from beat 

 to beat from the pulse pressure contour, and will 

 establish the physiological conditions under which 

 the arterial uptake is a constant function of pressure. 

 In order to gather data for this inquiry it was necessary 

 to measure the uptake of aortic segments under con- 

 stant conditions, i.e., postmortem. The uptake of the 

 aorta as a whole and of the main arterial branches was 

 ascertained as follows. All the major branches from 

 the aorta were clainped and the smaller ones (seg- 

 mentals) tied off. A large cannula was placed in the 

 aorta via the ventricle and connected to a burette 

 which could discharge saline rapidly into the aorta 

 under air pressure. Optical records were taken of the 

 pressure changes during the discharge of known 

 amounts of saline and during '"diastolic" drainage. 

 These pressures were recorded o\'er a wide range and 

 the aortic uptake calculated from the injected volume 

 minus the systolic drainage. This latter was calcu- 

 lated, as indicated above, from the relation of the 

 systolic and diastolic areas of the optical pressure 

 record. These data gave the uptake of the aorta and 

 its branches over a wide range of pressures. Similar 



experiments were made by opening up in turn the leg 

 arteries, the arteries to the viscera, and those to the 

 head and forelegs. In this way the uptake of the several 

 arterial beds was assessed. For the purposes of this in- 

 vestigation it was necessary to separate the differences 

 in uptake of the various parts of the aorta itself This 

 was done by measuring the tension-circumference re- 

 lationship of rings cut from various parts of the aorta 

 and calculating the volume uptake from the.se. Since 

 the uptake of the whole aorta and all its branches was 

 known, a distribution of uptake for the arch, thoracic 

 aorta, and abdominal aorta, together with that of the 

 arterial branches coming from the several segments, 

 could be estimated. 



From these data, a compilation of the uptake of the 

 aorta and its branches could be made (64). A revised 

 form of this compilation is given in table 2 (108). All 

 measurements are given in milliliters per square meter 

 of body surface to account for differences in body size. 

 This is done for convenience, since most metabolic 

 and circulatory functions are customarily referred to 

 body surface rather than to body weight. In the prac- 

 tical application of this table and table i, it is best 

 to read values off from a plot of convenient .scales in 

 which the tabulated values are connected by a smooth 

 curve (fig. 6). 



Quite as important as the size of the uptake of the 

 several arterial segments is the time at which the 

 pressure in these segments is effective in distending 

 them to gi\'e rise to the uptake. This time is, as has 

 been indicated above, the instant of aortic valve 

 closure, and the pressure obtaining at this time in the 

 several arterial segments will depend upon the shape 

 of the systolic pressure pulse curve and the trans- 

 mission time out to these segments. This is illustrated 

 in figure 6. There are two pulses pictured; in one the 

 pressure is about normal. At the time the valves are 

 closed, pressure in the arch is about 39 mm Hg above 

 diastolic; that in the thoracic aorta and the branches 

 out to the head and forelegs (having similar trans- 

 mission times) is about 52 inm Hg, whereas that in the 

 abdominal aorta and visceral arteries is 47 and in the 

 leg arteries 23 mm Hg. These pressures are measured 

 from a calibrated central pressure pulse curve. The 

 arch pressure is measured at the incisura, and the 

 other pressures are measured at points determined 

 earlier in the pulse contour by the transmission times 

 as read from table 3 or from a graph plotted there- 

 from. Distention produced by these pressures is 7, 

 20, 5, and 3 ml per m- in the respective segments. The 

 uptake of the arterial tree as a whole is the sum of 

 these quantities or 35 ml per m- body surface. 



