7o8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



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P-R INTERVAL 



FIG. 5. Relationship of intensity of first heart sound to P-R 

 interval. The effect of variations in the P-R interval on the 

 relative intensity of the first heart sound is shown. This in- 

 formation is from a dog with surgically produced complete 

 heart block. The curve has been drawn from the data of Boyer 

 (8). Similar curves obtained in children with complete heart 

 block have been published by Shearn et al. (93). 



block, with a constant and minimally or moderately 

 lengthened P-R interval, there is a selective decrease 

 in the intensity of the first heart sound. In cases of 

 third degree heart block or complete heart block the 

 full range of alterations in first heart sound intensity 

 is seen. Here the intensity for any given cycle is 

 dependent upon the coincidental relationship between 

 the atrial and ventricular acti\ity. Though no atrial 

 impulses reach the ventricle there is still for each 

 cvcle a P-R interval. Figure 5 taken from the work 

 of Boyer (8) shows the relationship between P-R 

 interval and intensity of the first heart sound. 



In situations in which the atrial sound reaches 

 intensities sufficient to be audible on the chest wall a 

 further feature of the acoustics in heart block is ob- 

 served. With P-R intervals of sufficient length to 

 allow atrial contraction to be over before ventricular 

 contraction begins, the atrial sound is heard as a 

 separate entity, as described above for the normal. 

 This applies also for a lengthened P-R interval and 

 for those cycles in complete heart block where atrial 

 activity is far removed from ventricular activity. For 

 those cycles with short P-R intervals the two sounds 

 may become fused. There is, then, this further factor 

 which lends itself to increasing the intensity of the 

 first heart .sound. 



BUNDLE BRANCH BLOCK. This term is used to signify 

 impairment in transmission of the electrical impulse 

 through the specialized conducting tissue of the 

 ventricle, in particular, the right or left bundle of 

 His. Other types of impaired ventricular conduction 

 including intraventricular conduction delay, arbori- 

 zation block, and peri-infarction block will not be 



considered here. In the common types of right and 

 left bundle branch block the featiu-e of importance 

 from the acoustic standpoint is the delay in the 

 impulse reaching the involved ventricle, with the 

 consequent delay in onset and cessation of mechanical 

 activity. This classical interpretation of the dynamics 

 of bundle branch block has recently been challenged 

 in part by Braunwald & Morrow (11). On the basis 

 of simultaneous right and left ventricular pressure 

 measurements in patients with bundle branch block 

 they have concluded that there is the expected delay 

 in some patients with right bundle branch block but 

 interestingly not in the cases with left bundle branch 

 block. Our own attempts to restudy this problem have 

 revealed to us the oftentimes great difficulty we have 

 in deciding on a pressure curve exactly where ven- 

 tricular contraction begins. It appears to us that the 

 problem is deserving of further investigation before 

 one can confidently cast out the traditional thesis. 

 Since, as mentioned, the change from the normal 

 involves a delay in the onset and the cessation of 

 \entricular activity (classical explanation") it follows 

 that one would expect changes in the time-course of 

 the acoustic events, which indeed do occur. Due 

 perhaps to the ctuTent inability to identify the com- 

 ponents of the first sound there is often question as to 

 the acoustic representation in the supposed delay in 

 the onset of \entricular acti\ity. Howe\er, the ability 

 to identify clearly the components of the second sound 

 make it much easier to note changes in the termination 

 of ventricular activity. In right bundle branch block 

 the greater than normal delay of right-sided e\'ents 

 causes the pulmonic component of the second sound 

 to occur even later after the aortic component than 

 in the normal (fig. 4). However, if the etiology of the 

 alteration is purely an electrical phenomenon and not 

 a mechanical one, then the response of the \entricles 

 to respiration should be normal. Therefore, with 

 inspiration there is still further delay in the pulmonic 

 component with a greater degree of splitting. In right 

 bundle branch block, then, the order of events of the 

 second sound is normal and the respiratory response 

 is normal, but the degree of splitting is greater than 

 normal throughout the respirators- cxcle. On the 

 other hand, in left bundle branch block, the classical 

 explanation holds that there is definite change in the 

 order of events. Here, the impulse activates the right 

 ventricle first and left ventricular actixity follows 

 rather than precedes right ventricular activity. In this 

 circumstance, the pulmonic component of the second 

 sound precedes the aortic component. Since the re- 

 sponse to respiration is presumably normal, inspiration 



