386 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



The predicrotic wave or waves appear to be exaggerations of the 

 recoil produced by the ''fling" of the writing lever, but, contrary 

 to the inertia which gives rise to the pointed apex, or percussion-wave, 

 these secondary elevations are not dependent upon the initial upward 

 throw of the lever, but upon its rebound as it again endeavors to as- 

 sume the resting position. These oscillations, however, are destroyed 

 very shortly by the negative variation appearing in the form of the 

 dicrotic notch. The postdicrotic wavelets have also been regarded 

 as inertia movements of the instrument. It is more than probable 

 that the dicrotic elevation suffers an exaggeration in the same way as 

 the primary wave and hence, the writing lever and its connecting parts 

 can assume their position of rest only after they have passed through 

 several adj usting oscillations. Another view is that they represent after- 

 vibrations of the column of blood following in the wake of the dicrotic 

 wave. 



It should also be remembered that the tracings of the pulse taken 

 from different arteries, show certain differences regarding these minor 

 fluctuations. In explanation of this phenomenon it has been suggested 

 by Frank ^ that certain regions of the vascular system are so shaped that 

 they are capable of giving rise to special types of reflections which then 

 tend to modify the character of the principal pulse-wave. Thus, it 

 has been stated that the carotid pulse is influenced by waves reflected 

 from the circle of Willis, while the pulse in the descending aorta suffers 

 a slight modification in consequence of reflections from the bifurca- 

 tion of the iliac arteries. It is true, however, that many of these 

 secondary currents interfere with one another in such a way that they 

 become neutralized. 



Pulse Pressure. — When referring to blood pressure, we usually 

 have its average value in mind. It has been pointed out above that 

 this value may be determined most accurately by ascertaining the 

 arithmetic mean of the systolic and diastolic pressures, as registered 

 by the direct method. It may also be determined by the indirect 

 method, but only approximately, because this estimate must be based 

 upon the diastolic pressure. The mean pressure follows the diastoUc 

 minimum pressure more closely than the systolic maximum and hence, a 

 greater importance is frequently attached to the former than to the 

 latter. But as a definite numerical relationship between these factors 

 does not exist, the average blood pressure is usually determined in a 

 rough way by adding one-third of the systolic-diastolic difference to 

 the diastolic pressure. It has also been estimated at 75 per cent, of the 

 systolic pressure. 



The systolic-diastolic difference in blood pressure is generally desig- 

 nated as the pulse pressure. Thus, if a systolic value of 130 mm. 

 Hg is opposed by a diastolic value of 90 mm. Hg, the pulse pressure 

 equals 40 mm. Hg. Keeping this fact clearly in mind, the changes 

 which the pulse-pressure may undergo need not be considered in 



1 Tigerstedt, Ergebn. der Physiol., viii, 1909. 



