7o6 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



systole are all reduced. This would result in an earlier 

 appearance time of aortic \alve closure and the 

 aortic component of the second sound. 



From the acoustic standpoint the effect of inspi- 

 ration is to cause the aortic component of the second 

 30und to come earlier and the pulmonic component 

 to come later (g). Since normally aortic closure pre- 

 cedes pulmonic closure, inspiration has the effect of 

 increasing tiie gap between these two events, and can, 

 thereby, produce splitting of the second sound. Com- 

 ponents that could not be distinguished as separate 

 e\'ents may now be .so recognized. Components that 

 could be recognized may now be heard to be further 

 separated. The importance of this observation is 

 severalfold. First, by separating the components in 

 time two events may be observed and, as discussed 

 above, certify that there are two semilunar valves. 

 Second, the increase in splitting further certifies that 

 respiration is affecting semilunar valve closure, which 

 is the normal circumstance. Third, the increase in 

 splitting means that aortic closure precedes pulmonic 

 closure, which again is tiie normal phenomenon. All 

 three points provide valuable acoustic information, 

 since any deviation from this may well be associated 

 with some abnormality. Points one and two need no 

 further elaboration, but point three deserves some 

 clarification, as to why an increased degree of splitting 

 with inspiration identifies the order of the components. 

 Consider the various possible orders of events of semi- 



FIG. 4. Effect of respiration on mechanical activity and 

 second sound. The presentation is similar to that shown in fig. 

 3. Atrial events are not shown, nor is the duration of isometric 

 contraction. The vertical lines indicate the onset and end of 

 mechanical activity of each ventricle as it is for the normal at 

 the end of expiration. The onset of mechanical activity for left 

 bundle branch block (LBBB), right bundle branch block 

 (RBBB), and atrial septal defect (ASD), as depicted here, are 

 subject to revision as the matter is currently in dispute. How- 

 ever, there is general agreement as to the effect on the compo- 

 nents of the second sound, aortic closure (.42) and pulmonic 

 closure {P2). See text for detailed description. 



lunar valve closure. First, aortic closure precedes 

 pulmonic valve closure, even though they cannot be 

 heard as two separate events. Inspiration will increase 

 the splitting by the mechanism noted above. Second, 

 both events truly occur simultaneously. Here, again, 

 inspiration will increase the splitting and again, 

 although not separate before inspiration as they be- 

 coine separated with inspiration, aortic closure comes 

 first. Third, and finally, pulmonic closure may precede 

 aortic closure. In this case, as in the other cases, 

 inspiration can only cause pulmonic closure to be 

 delayed, if it affects its time of appearance. It cannot 

 make it come earlier. Similarly, inspiration can only 

 cause aortic closure to come earlier, if it affects its 

 appearance time. It cannot delay it. Therefore, with 

 inspiration the two components approach each other 

 and the splitting decreases or disappears if the events 

 become synchronous. In this circumstance therefore, 

 when pulmonic closure precedes aortic closure, inspi- 

 ration decreases rather than increases the degree of 

 splitting. There is the theoretical possibility, still un- 

 explored, that with pulmonic closure preceding aortic 

 closure, inspiration may not only cause the splitting 

 to decrease, then disappear, but then reappear again 

 as, in effect, the two components cross each other, and 

 perhaps go on to an even greater degree of splitting 

 than before inspiration. Such an observation would 

 be detrimental to the thesis that respiration can be 

 used to identify the components of the second sound. 

 The fact that this example just cited does not seem to 

 occur suggests that although respiration can alter the 

 duration of mechanical systole it cannot alter it to 

 such a inagnitude as to cause that great a shift in the 

 components of the second sound. Since the theoretical 

 possibility discussed does not occur, it means that 

 changes in the splitting of the second sound with 

 respiration can be used to identify the components 

 of the second sound. 



In summary, an increase iit the degree of splitting 

 with inspiration means that aortic closure precedes 

 pulmonic closure (at least during inspiration), a 

 normal finding. A decrease in the degree of splitting, 

 or disappearance with inspiration, means that pul- 

 monic closure precedes aortic closure, an abnormal 

 finding, and one that requires investigation. The 

 circumstance in which pulmonic closure precedes 

 aortic closure has been termed paradoxical splitting 

 of the second sound (36). This is an unfortunate term 

 in that it does not convey precisely what has occurred. 

 Furthermore, it may convey the erroneous impression 

 that there is also some difference in the respiratory 

 response, which is not the case. Indeed there is no 



