698 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION 1 



been known for a long time that patients with pul- 

 monary emphysema in which situation there is more 

 air-breathing tissue, a poor sound conductor, between 

 the heart and the chest wall than in the normal, ha\'e 

 decreased intensity of heart sounds. The few cases of 

 emphysema that we have studied with the intra- 

 cardiac phonocardiogram would seem to bear out 

 this contention, but more examples are needed. 



Although the above information is interesting and 

 there are now techniques for comparing sound inten- 

 sity and frequency within the heart with that on the 

 chest wall, many questions remain unanswered. Exact 

 data on the energy decrement are lacking. The form 

 in which the vibrations are transmitted by the thoracic 

 tissues is not known, and the nature of the acoustic 

 coupling between the \arious tissues remains to be 

 discerned. 



PHYSIOLOGICAL BASIS OF ACOUSTICS 



Relationship of Heart Sounds to Events 

 of the Cardiac Cycle 



Since the acoustic events are determined by the 

 mechanical activity of the cardiac structures, any at- 

 tempt at understanding acoustics must be based on a 

 firm understanding of the events of the cardiac cycle. 



Consider first the relationship between the acoustic 

 events and the intravascular pressures as displayed on 

 one side of the heart. Figure i shows the heart sounds 

 and the intravascular pressures from the great vessel, 

 ventricle, and atrium. The data apply to either the 

 right heart or the left heart. The following features 

 of the intravascular pressures are important to our 

 discussion and should be noted. With the onset of me- 

 chanical activity of the ventricle, the ventricular 

 pressure rises rapidly to a peak (or more often a 

 plateau). As it rises it becomes first greater than the 

 atrial pressure and then a short, but nonetheless 

 appreciable time later becomes greater than the 

 pressure in the great vessel. The rise of ventricular pres- 

 sure above atrial pressure is associated with closure of 

 the atrioventricular valve (mitral or tricuspid), and 

 the rise of ventricular pressure above great vessel pres- 

 sure is associated with opening of the semilunar valve 

 (aortic or pulmonic). The motion of these two valves 

 is not synchronous. There is a definite time interval 

 between them. This interval, during which the ven- 

 tricular pressure is rising with no change in ventricular 

 volume, since both inlet nnd outlet valves are closed, 

 is called the period of isometric- (or more properlv, iso- 



VENT 



SOUNDS ■ 



-+4h 



-^+ 



4 I 



2 3 



^+- 



-fW- 



-L m^m 



V/////////A 



y///////////////f,\ 



miiiiiiih 



wiiini/x 



/A/ closed 



(_J open I 



FIG. I. Relationship of intravascular pressures to heart 

 sounds. Simultaneous pressures from great \essel, ventricle, 

 and atrium from one side of the heart. Normal heart sounds 

 also from one side of the heart: fourth (4), two components of 

 first sound (i), occurring with closure of the atrioventricular 

 valve and opening of the semilunar valve, second sound (2), 

 opening sound of the atrioventricular valve (o. snd) (put in 

 for the sake of completeness), and third sound (3). Action of 

 valves: atrioventricular valve (A-V) and semilunar vaK'e 

 (S-L). The hatched area indicates the interval during which 

 the valve is closed, and the clear area the interval during which 

 the valve is open. The valve motion is almost but not quite 

 reciprocal. There are two intervals, one at the onset and one at 

 the end of mechanical activity of the ventricle during which 

 both \al\es are closed. 



\'olumctric) contraction. With the cessation of \en- 

 tricular mechanical activity ventricular pressure drops 

 first below great vessel pressure and this is associated 

 with closure of the semilunar \al\e. Shortly thereafter 

 the ventricular pressure falls below atrial pressing, at 

 which time the atrioventricular valve opens. Again, 

 note that these two events do not occur simultaneously. 

 There is, therefore, a period of isovolumetric relaxa- 

 tion, during which \entricular pressure is falling and 

 both valves are closed. The significance of these two 

 isoNolumetric periods to the acoustic events \s\\\ be 

 made more apparent below. 



With the rise of ventricular pressure above great 

 vessel pressure note that the direction of the great 

 vessel pressure curve changes abruptly and becomes 

 virtualh' identical with that in the ventricle, and that 

 when ventricular pressure falls below great x'cssel pres- 

 sure the two go apart from one another. During the 

 time that the semilunar valve is open, therefore, these 

 two pressures are similar, and when the semilunar 



