MEASUREMENT OF THE CARDIAC OUTPUT 



557 



200 



180 



160 



140 



120- 



100- 



80- 



50 



40- 



20- 



20 50 



80 



110 



140 170 



FIG. 4. The relation of the \olume per m- of aortas fiom 

 human cadavers at pressures ranging from 20-170 mm Hg. 

 The different curves represent different age groups and aortas 

 from hypertensive patients. The change in aortic capacity 

 with change in pressure (slope) is quite constant from group 

 to group, whereas the capacity at any one pressure may vary 

 as much as 3-fold from group to group. [From Remington 

 etal. (112).] 



rarely, if ever, has the vahdity of methods of this type 

 been confirmed by independent workers. 



On the basis of the discu.ssion so far it would seem 

 that the pulse wave velocity is not very helpful in 

 assessing absolute arterial distensibility or uptake. No 

 other applicable method is available. It is necessary 

 then to test the assumption that uptake does not vary 

 in such a way as to stultify the use of pulse pressure 

 to calculate stroke volume. Uptake may vary a) with 

 pressure; h) with the individual under the influence of 

 biological variability disease and age; f) with physio- 

 logical condition; and d) with body size. To equalize 

 body size all uptake figures are given per square meter 

 of bodv surface. 



In figure 4 are shown curves calculated from a 

 table published by Remington et al. (112) in which 

 rings cut from 48 human aortas at different levels were 

 stretched and the aortic volume (per m^) pressure re- 

 lationship calculated from the tension and length of 

 the stretched rings. The relation of the lines indicates 

 that regardless of previous history, age, or aortic size, 

 the average increase in aortic volume per unit pressure 

 rise (slope) is very similar in all the categories illus- 

 trated. The aortic volume at any diastolic pressure 

 will vary widely, but the change in volume with 

 pressure is more constant. It is also seen that the 

 aortic volume changes almost proportionally with 

 pressure from qo to no mm Hg and that above this 

 figure the distensibility gradually falls off. This in- 

 dicates that at higher mean pressures the same pulse 

 pressure would be produced by a smaller stroke vol- 

 ume. 



The uptake of the aorta alone is much less than the 

 stroke volume. In the first place all the branches of the 

 aorta add their uptake. In addition, during systole the 

 blood continues to flow through the aorta and out the 

 arterioles. A table was constructed (112) in which the 

 aortic uptake curve (average of fig. 4) was increased 

 proportionately so as to give figures that would 

 average the same as those for the stroke index as cal- 

 culated by the Fick procedure. The figures are based 

 on the comparison of practically simultaneous intra- 

 arterial brachial pressure pulses and 83 Fick de- 

 terminations. 



The result of using the table on the pulse pressures 

 shown by the 83 cases in question is given in figure 5. 

 These results are empirical in the sense that the mean 

 of the Fick stroke index and of the pulse pressure pre- 

 diction are arbitrarily .set at the same value. The 

 scatter seems rather large on inspection but the corre- 

 lation coefficient is .79 and the average discrepancy 

 18.6 per cent. Other methods which have been pro- 

 posed gi\e the following discrepancies when tested on 

 these data: 34 per cent (5), 25 per cent (45), 24 per 

 cent (89). The principal cause of the greater dis- 

 crepancy between the calculation by the earlier 

 formulas and the measured result lies in the fact that 

 the older workers divided the pulse pressure, or some 

 derivative of it, by a constant. This approach does 

 not give the true relationship which at low normal 

 arterial pressures is a proportional (linear) and con- 

 stant relation between pulse pressure and stroke 

 index. Thus at pressures below 120 mm Hg each mm 

 Hg is produced by about i nil of stroke index. Above 



