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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



quality, and oftentimes this factor, regardless of time- 

 course or nature of the murmur enselope, points to 

 the site of origin. This feature is due apparently to 

 the nature of the principal \ibrating structure. One 

 highly characteristic murmur which is related to a 

 specific valvular deformity deser\es mention. When 

 one or more cusps of a semilunar valve become 

 everted (bent back toward the ventricle), the vi- 

 brations develop a predominance of a single fre- 

 quency, which lends a musical quality to the murmur 

 (5, 34). This happens most frequently to the right 

 anterior cusp of the aortic \alve. Tears in the free 

 margin of a cusp or perforations of a cusp seem to 

 present more often with sets of dominant vibrations, 

 and the murmur thus created is often harsh and not 

 so musical. 



Consider finally murmurs that arise from the atrio- 

 \entricular valve as a result of stenosis. In this circum- 

 stance there is an alteration in the relationship be- 

 tween the atrial and \cntricular pressure in diastole. 

 In many cases the contour of the \entricular pressure 

 is unchanged, and the main alteration is in the atrial 

 pressure curve. At the time of the opening of the 

 atrioventricular valve, at the end of the period of 

 isometric relaxation, the atrial pressure falls, but not 

 to the level of the ventricular pressure. The difference 

 between the two pressures for any given flow rate is 

 an index of the severity of the stenosis. There is a 

 continuous decline in atrial pressure until the onset 

 of mechanical activity of the atrium, at which time 

 there is a secondary rise in atrial pressure and a 

 consequent increase in pressure gradient. From the 

 standpoint of the acoustics the murmur can be ex- 

 pected to begin at the time of the atrioventricular 

 valve opening. This is in contradistinction to semi- 

 lunar valve diastolic miu'murs which begin with the 

 second sound component. As the gradient of pressure 

 falls there is a decline in the intensity of the murmur, 

 and with the increase in gradient clue to atrial ac- 

 tivity there is a secondary increase in murmur in- 

 tensity. Like semilunar valve stenosis the correlation 

 between pressure gradient and miumur is reasonable. 

 From this point several features may be noted. First, 

 in the mildest degrees of stenosis the atrial pressure 

 Cjuickly declines to ventricular pressure only to rise 

 above it again late in diastole. In this circumstance 

 the early diastolic component of the murmur, irre- 

 spective of intensity, is of short duration, and there 

 is a silent gap between it and the presystolic com- 

 ponent. As the severity of the stenosis increases, the 

 atrial pressure declines more slowly. This results in a 

 longer diastolic component with less of a gap between 



it and the presystolic phase. Finally, when the severity 

 of the lesion is sufficient to prevent atrial pressure 

 from reaching ventricular pressure before it rises due 

 to atrial activity, the gap between the two com- 

 ponents disappears (fig. 8). In this way one can see, 

 as Wood (106) has pointed out, that the duration of 

 the diastolic component of the murmur (without con- 

 sidering the presystolic component) is directly related 

 to the severity of the lesion. We have made this 

 correlation enough times now in our own cases to 

 attest to its validity and its importance. This same 

 phenomenon, since it does not depend on the atrial 

 component, applies and can be used when there is 

 atrial fibrillation. 



In atrial filjrillation without an atrial contraction 

 there is no late increase in the gradient of pressure. 

 It continues to decline to the end of diastole, and 

 consequently there is no secondary increase in minmur 

 intensity. It must be remembered that the change in 

 the murmur with atrial fibrillation is primarily the 

 loss of the presystolic accentuation. It need not be 

 the loss of presystolic murmur. The question as to 

 whether or not there is an audible murmur just before 

 systole depends on the severity of the stenosis, as 

 described above. Indeed the alteration in the murmur 

 with atrial fibrillation is only a particular example of 

 a more general rule that the nature of the presystolic 

 phase of the murmur depends on the nature of the 

 atrial actixity. When atrial actix'ity immediately pre- 

 cedes xentricular activity, as with a short P-R interval, 

 the atrial pressure rises just before xentricular systole 

 and is higli when mechanical activity of the \-entricle 

 begins. In this circumstance the presystolic phase of 

 the murmur is short and is increasing in intensity 

 when it is cut off by the atrioventricular vaKe closure 

 sound. .\s the P-R interval lengthens to normal 

 values and beyond, there is time for atrial pressure to 

 peak and begin to decline. In this circumstance, the 

 presystolic phase of the murmur is longer and the 

 peak intensity may occur enough before the onset of 

 the first sound to show a decline in murmur intensity 

 (fig. 9). In each case the murmur appears to be due 

 to the nature of atrial mechanical activity and its 

 effect on pressure gradient and, obviously, flow into 

 the \entricle. It is because atrial systole is most 

 commonly presystolic in time that this murmur is 

 most commonly presystolic. However, since the mur- 

 min- is dependent upon atrial systole and not upon its 

 relationship to \entricular systole, the term prc- 

 svstolic used for all of the variations seen may cause 

 confusion. The term atriosystolic, as used by W'ood 

 (107) and McKusick (68) conveys more precisely the 



