PHONOCARDIOGRAPHY 



701 



and associated with a sound. Wliat does seem per- 

 tinent is that further study with an approach more 

 direct than correlation of acoustics with intravascular 

 pressure is needed to identify the precise physiological 

 events that occur at the time of the production of the 

 third heart sound. 



FOURTH HEART SOUND. As noted above, this sound oc- 

 curring in presystole comes as a result of atrial me- 

 chanical activity. Its relationship to atrial mechanical 

 activity is clear, for as the relationship between atrial 

 and ventricular mechanical activity changes, so does 

 the relationship between the fourth heart sound and 

 those produced by mechanical activity of the ventricle. 

 Changes in the length of the P-R interval are asso- 

 ciated with like changes in the interval between the 

 fourth and first heart sounds, and when the atrium no 

 longer contracts, as in atrial fibrillation, the fourth 

 sound disappears. For this sound, perhaps above all, 

 the precise physical correlate responsible for its 

 genesis remains to be established. It has only been 

 with studies of sounds from within the heart that the 

 true frequency of occurrence of this acoustic event lias 

 been appreciated (58). Whereas it is a rare event in 

 chest phonocardiograms, it is the rule rather than the 

 exception in intracardiac phonocardiograms. It may 

 well be that this sound, like the third sound, is related 

 to either inotion of the atrio\entricular valve (or some 

 other vahular motion) or primarilv to vibration of tlie 

 wall of the \'entricle. VVe have not seen, in our pres- 

 sure recording, any concrete evidence that at this time 

 ventricular pressure rises above atrial pressure. For 

 this event, therefore, one might well be justified in 

 saying that transient closure of the atrioventricular 

 valve does not occur. Furthermore, vibrations origi- 

 nating in the ventricle cannot always be responsible 

 either. We have made observations of the fourth sound 

 from within the right atrium in a patient with a com- 

 plete heart block. When atrial contraction occurred 

 during ventricular systole, the fourth sound was still 

 present, despite the fact that tliere could be no flow 

 into the ventricle at this time. It may well be that 

 this sound arises out of the vibrations set up by the 

 muscular contraction of the atrium, and, although 

 not independent of vibrations of blood, may be inde- 

 pendent of translation of blood with or without valve 

 motion. The problem is further complicated by the 

 suggestion that there may well be two sets of vibra- 

 tions rather than one. Some of our recordings have 

 suggested the presence of secondary vibrations of 

 greatest intensity in the ventricular inflow tract (we 

 have studied this only in the right ventricle) following 



the vibrations described above, and at a time when 

 one might e.xpect inflow of blood into the ventricle. 

 The fate of these in the presence of complete heart 

 block is not known to us. 



In summary then, when one considers the relation- 

 ship between the acoustic events and the intravascular 

 pressures as displayed on one side of the heart, there 

 are six separate events at which normal heart sounds 

 may occur: the two components of the first heart 

 sound complex associated with atrioventricular valve 

 closure followed closely by semilunar valve opening, 

 the second sound associated with semilunar valve 

 closure, which is closely followed by atrioventricular 

 valve opening, followed, in turn, by the third sound of 

 early rapid filling, and finally, by the fourth sound at 

 the time of late rapid filling. 



Consider now the actual circumstance, namely, that 

 there are two sides to every heart, and consequently 

 twelve separate acoustic events with each cycle. It is 

 necessary now to determine, from the order of the 

 mechanical events of the two circuits, the order of the 

 twelve acoustic events. For this purpose it may be more 

 meaningful to begin with the true beginning of the 

 cycle, the depolarization of the sinoatrial node. Elec- 

 trical activity spreads from here to the right atrium 

 and by direct continuity to the left atrium, and, as 

 expected, therefore, the mechanical activity begins 

 first in the right atrium. From this view of the cycle 

 the first acoustic event is the fourth heart sound from 

 the right atrium, followed by the fourth heart sound 

 from the left atrium. Figure 3, adapted for this presen- 



SOUNDS 



MC AO 



I 



AC 



FIG. 3. Relationship of order of mechanical activity to valve 

 motion and sounds. The order and duration of mechanical 

 activity is shown for each chamber. Right heart es'ents are 

 above the line, left heart events below. In addition the duration 

 of isometric contraction for each ventricle is shown. Valve 

 motion is depicted showing the order of events : mitral closure 

 (MC), tricuspid closure (TC), pulmonic opening (PO), aortic 

 opening (^40), aortic closure (AC), and pulmonic closure (PC). 

 .\t the bottom are shown the soimds : the two atrial soimds, the 

 four components of the first sound, and the two components of 

 the second sound. [.Adapted from the work of Braunwald el al. 

 (10).] 



