8 5 8 



HANDBOOK OF PHYSIOl I » , -i 



CIRCULATION II 



AP j'ISOmmHg 



.124 



752 



ml/min 



-446 



4,059 



CONTROL 

 AP .-l26mmHg 



AF 

 AS 



iF A^/ID. 2mm A;2,87l i 

 / V / \ml/min 



«•»* 



m > 



fig. 27. Experimental graded coarctation of the descending 

 thoracic aorta. ID = internal diameter; AP = aortic pressure; 

 AF = aortic flow in the descending thoracic ; and AS = aortic 

 sounds. The sounds are taken by means of a barium titanate 

 phonocatheter downstream to the point of constriction. During 

 control, systolic pressure was 1 30 mm Hg, diastolic 86 mm Hg. 

 The peak systolic flow was 4,752 ml/min, while the mean flow 

 in the descending thoracic aorta was 446 ml/min. In spite of the 

 reduced peak flow, there was little reduction of mean during the 

 early stages of constriction because of change of contour of the 

 flow pulse. The pressure gradient, flow pulse contour and mur- 

 mur envelope follow the "contour rule." 



pulse. Presumably, this remarkable reduction in 

 cross-sectional area without reduction in flow is 

 attributable to the progressively increasing gradient 

 across the stenotic area. 



Beyond this degree of obstruction any further 

 change in the internal diameter becomes extremely 

 critical as far as blood flow is concerned. With an 

 internal diameter of approximately 2 to 3 mm, the 

 murmur consistently filled systole throughout, and 

 further reduction caused the murmur to increase in 

 duration beyond the second sound and extend into 

 the diastolic period. Internal diameters of 1 mm or less 

 frequently caused a continuous, high-pitched, blow- 



ing murmur distal to the site of coarctation in both 

 experimental animals as well as in congenital lesions. 



The Murmur Envelope and Contour Rule 



The "envelope" of a murmur is defined as the 

 amplitude of the full wave rectified murmur averaged 

 over several heart cycles. The term envelope is 

 similar to the "shape" of a murmur which itself 

 means amplitude of the unrectified murmur. The 

 envelope of a blowing murmur follows a "contour 

 rule," which means that it corresponds closely to the 

 contour of the flow pulse originating the murmur. 

 This is true because apparently once the critical 

 velocity is reached where turbulent flow begins 

 (turbulence is used in a general sense to indicate 

 nonlaminar flow), the amplitude of the resultant 

 turbulence or lateral velocities of the nonlaminar 

 flow is proportional to the mean axial velocity. 

 Further, the flow under these conditions is principally 

 viscous in nature and therefore the extant resistant 

 pressure gradient contour parallels the flow pulse 

 contour. The correspondence of the murmur envelope 

 and the pressure gradient to the flow pulse exists only 

 when the flow is highly viscous in nature (having no 

 significant reactance flow term), and may occur 

 without stenosis in a normal vessel under high- 

 velocity conditions producing nonlaminar flow or in 

 an aneurysm where nonlaminar flow may be achieved. 

 Some examples of the contour rule are given in 

 section VII. 



General Rules Relating Murmurs to Nonlaminar Flow 



From this and other studies in section VII, general 

 rules concerning the interpretation of frequency band 

 width and envelope (amplitude and duration) of 

 "blowing" murmurs may be made relative to the 

 functional anatomy of the source as follows: 



/) Blowing murmurs with high pitch and low 

 intensity are associated with small orifices through 

 which blood is flowing at high velocity, driven by 

 large pressure gradients. 



2) Loud, blowing murmurs of relatively low- 

 frequency spectrum, generally sounding coarse to 

 the ear are associated with relatively large orifices 

 through which large volumes of blood flow under 

 relatively high-pressure gradients. 



3) Very low-frequency (rumbling) murmurs of low 

 intensity are associated with turbulent flow beyond 

 large orifices through which blood flows under low- 

 pressure gradients. 



