PHONOCARDIOGRAPH^■ 



707 



paradox. A more satisfactory term would be reversed 

 splitting of the second sound with a normal respiratory 

 response. The circumstances that produce this will be 

 discussed below. It should be obvious that expiration, 

 which increases intrathoracic pressure, induces a train 

 of events in each side of the heart opposite to that 

 described for inspiration. The effect of respiration on 

 the appearance time of the second sound in the normal 

 and in three other situations, discussed more fully 

 below, is depicted in figure 4. 



In addition to changing the appearance time of the 

 second sound, respiration also may alter intensity. 

 Inspiration by increasing the flow through the right 

 heart has the tendency to increase the intensity of 

 the sound. The .second sound shows this definitely, 

 the first not so well. It also increases the intensity of 

 the diastolic sounds originating in the right heart and 

 can be used as a means of identifying their site of 

 origin. Chest phonocardiograms show it for the fourth 

 sound, and for the presystolic gallop sound, sometimes 

 quite well but at other times less well. On the other 

 hand, intracardiac recordings of the.se two presystolic 

 sounds usually show quite marked respiratory vari- 

 ations. 



Effect of Alterations in Electrical 

 Activity on Heart Sounds 



Since the nature of the initiating electrical signal 

 determines in large part the succeeding inechanical 

 phase, it is not unexpected that alterations in the rate, 

 rhythm, and order of depolarization of the various 

 parts of the heart should manifest themselves in 

 alterations in cardiac acoustics. Analysis of the change 

 in acoustics induced by a change in the electrical 

 activity can be used to evaluate more fully cardiac 

 dynamics. It can also be used, as Levine & Harvey 

 (57J have very properly stressed, as an aid in the 

 diagnosis of the type of arrhythmia present. This 

 discussion will be limited to two types of altered 

 electrical response, heart block and bundle branch 

 block, since here some detailed information is availa- 

 ble concerning the physiological correlate between 

 electrical impulse and acoustic response. A catalogue 

 of the various acoustic manifestations of the different 

 types of supraventricular and \entricular arrhythmias 

 will not be given. Those interested in this phase of the 

 subject should refer to the excellent discussion by 

 Levine and Harvey. 



HEART BLOCK. This term is used to signify impairment 

 in the transmission of the electrical impulse from the 



atria to the ventricles. Traditionally, three degrees 

 are recognized: delay in transmission (first degree), 

 partial impairment with loss of the normal one-to-one 

 relationship between atrial stimulus and ventricular 

 response (second degree), and complete failure of 

 transmission in which a region below the block 

 assumes responsibility for the ventricles with the result 

 that two independent pacemakers exist (third degree). 

 Since the electrical abnormality involves an alteration 

 in the relationship of atrial activity to ventricular 

 activity, it is here that the characteristic alterations 

 in acoustics occur. There are changes in the late 

 rapid filling phase of the ventricle and, since this 

 affects the atrioventricular valves, it produces changes 

 in the first heart sound as well as in the fourth heart 

 sound. Little et al. (61, 62) and Boyer (8) have investi- 

 gated in detail the mechanism of alteration in acoustics 

 with changes in the time-course of atrial-ventricular 

 activity. Several clinical studies have also been re- 

 ported (4, 87, 93). The essential feature in the alter- 

 ations produced in the first heart sound is the position 

 in which the atrio\entricular valves are left at the 

 onset of ventricular mechanical activity. With atrial 

 contraction immediately preceding ventricular con- 

 traction, that is, in situations where the P-R interval 

 is shorter than normal, the atrioventricular valves are 

 left with their greatest aperture at the time of ven- 

 tricular contraction. In effect they have been dri\en 

 down into the ventricular cavity and have not had 

 time to float back toward a more closed position. 

 The consequently great distance that the valve leaflets 

 have to travel to close is associated with a loud first 

 heart sound. As the P-R interval lengthens to normal 

 values and beyond that to those seen with a minimal 

 degree of heart block, the atrioventricular valves have 

 time to float back toward a more closed position due, 

 most likely, to the pressure of blood behind the valve 

 leaflets. With a shorter distance for the leaflets to 

 travel there is a decrease in the intensity of the first 

 heart sound. Still further lengthening of the P-R 

 interval allows the leaflets to move again toward a 

 more open position and consecjuently increases the 

 intensity of the first heart sound. There is no doubt 

 that the variations in the range of movement of the 

 atrioventricular valve leaflets play an important part 

 in the obser\ed variations in first heart sound intensity 

 with changes in the P-R interval. Although this 

 feature alone can explam the phenomenon obser\ed, 

 the effect of the altered \entricular filling on the 

 time-course of ventricular contraction and its effect, 

 in turn, on the time-course of valve motion deserv'es 

 further studw C^linicallv, in cases of first dea;ree heart 



