MEASUREMENT OF THE CARDIAC OUTPUT 555 



I— I SEC H 



FIG. 2. {Lefl, above) : flow of blood, V, in an artery between the magnet poles, n. s., induces a 

 current in the electrodes, + and — . Right: diagram of Kolin's electromagnetic flowmeter. The 

 artery is placed in a sleeve, S, between two magnet poles. Pi, Pi. The induced current is led ofT 

 over LE from electrodes E\ (not shown) and Ei which are in contact with the artery. Left, lower: 

 pattern of aortic flow. [From Kolin (84).] 



by its mean velocity. During the transit time of the 

 sound wave a capacitor is charged at a constant rate 

 and then discharged when the sound burst arrives at 

 the receiving crystal. The maximum voltage of this 

 charge measures the transit time. The transit time is 

 slightly greater when the sound is transmitted up- 

 stream than when it is transmitted downstream. The 

 diflference in the voltages measuring the upstream and 

 downstream transit times is filtered, amplified, and 

 recorded so as to give a faithful measure of the instan- 

 taneous changes in the mean velocity of the blood 

 stream. Since the mount holds the \essel to a known 

 constant size, the flow can be derived from the velocity 

 of the stream. The flow through the aorta (plus the 

 estimated coronary ffow) or preferably the pulmonary 

 artery is, of course, the cardiac output. 



PULSE PRESSURE METHODS 



Methods for measuring the cardiac output, which 

 have been mentioned .so far, deal with some char- 



acteristic of the velocity of the stream in the aorta or 

 preferably in the pulmonary artery. The systolic dis- 

 charge, however, releases energy in another way 

 which can give a clue to the amount of blood ejected. 

 It stretches the aorta and thereby produces an increase 

 in pressure therein (pulse pressure). The pul.se pres- 

 sure, as a measure of the stroke volume, was first 

 suggested by Erlanger & Hooker (30). The aortic 

 distensibility or rather the distensibility of the whole 

 arterial tree raises the most important question in 

 assessing the relation between stroke \olume and 

 pulse pressure. If the arterial tree were highly dis- 

 tensible more blood could be forced into the arteries 

 during systole with a given change in pressure than 

 if the vessels were less distensible. Two assumptions 

 have been made, /) that arterial distensibility does 

 not change importantly and -') that important changes 

 in distensibility do occur but can be evaluated by 

 studies of the pulse wave velocity. This possibility 

 will be discussed first. 



The relation of pulse wave velocity to absolute dis- 

 tensibility (AT \P) or the amount of blood which, on 



