THE PULSATORY VARIATIONS IN BLOOD PRESSURE 385 



A pronounced dicrotism of the pulse usually indicates a low blood 

 pressure, because a low tension permits the systolic-diastolic differences 

 and other fluctuations in pressure to become extreme. Conditions 

 of this kind frequently develop in the course of many wasting diseases, 

 and especially during fevers, such as typhoid, when a low peripheral 

 resistance is associated with an, as yet, efficient pumping force of the 

 heart. Any factor, therefore, which induces sudden and extreme 

 variations in pressure, or favors the elastic resiliency of the arterial 

 wall must tend to augment the dicrotism. For this reason, it is 

 usually very conspicuous in young people, but not in adults and 

 older persons, because their arteries have been rendered more rigid by 

 calcareous infiltration. 



Any discussion as to the cause of the dicrotic wave must first of all 

 take into Account that it may be a reflection traveling from the heart 

 outward, or that it may be a peripheral reflection passing inward. The 

 second possibility may be disposed of very quickly, because if it really 

 were a centripetal wave, it should be possible to obtain it apart from 

 the principal wave of the pulse. The latter has been proven to be of 

 central origin. Now, since the dicrotic elevation always keeps at a 

 definite distance from the apex of the primary wave, we are entirely 

 justified in concluding that it originates centrally and represents, 

 therefore, a centrifugal wave, traveling at the same velocity as the 

 principal one. 



Having established the direction of the dicrotic wavelet, it now 

 becomes a relatively simple matter to detect its cause. As may 

 readily be surmised, the latter must be sought in the closure of the 

 semilunar valves. A thorough distention of the aorta having been 

 attained, its walls recoil immediately upon the completion of the ventric- 

 ular systole and place the blood within under continued pressure. The 

 blood then seeks to escape in the direction of least resistance, namely, 

 toward the capillaries as well as toward the heart. The centripetal 

 movement of the column of blood is at first greatly facilitated by the 

 negativity resulting in the root of the aorta in consequence of the 

 ventricular discharge, but is suddenly cut short by the approximation 

 of the aortic semilunar valve-flaps. Being thus suddenly thrown 

 against the closed semilunar valve, a reflection results which is con- 

 veyed toward the periphery in the form of a wavelet superimposed upon 

 the principal wave. 



The dicrotic notch immediately preceding the dicrotic elevation, 

 seems to have its origin in the decrease in pressure resulting in the root 

 of the aorta at the beginning of ventricular diastole. As the aortic 

 walls recoil and force the blood against the closed semilunar valves, 

 a slight downward deviation of the latter results, because they are 

 no longer supported by the firmly contracted ventricular musculature. 

 This yielding of the '^semilunar floor," however, is very limited and 

 soon gives way to a rebound of the blood which in turn causes the 

 distention of the aorta described a moment ago as the dicrotic wave. 



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