My patient with aortic sclerosis has a loud systolic ejection murmur. What is the exact mechanism of this murmur?

We usually blame cardiac murmurs on the “turbulence” caused by blood flowing past an irregular valve surface but, believe it or not, how the murmur is created has been a matter of controversy. 1-4

For sure, murmurs are generated by disturbance of laminar blood flow (ie, turbulence) but over the years many have argued that turbulence per se fails to produce adequate acoustic force to be audible at the chest wall.2 Although challenged by some,1  the concept of “vortex shedding” borrowed from fluid dynamics is fascinating and has been proposed as a potential explanation.

Per this theory, just as a boulder causes a stream to separate and generate vortices, valves (particularly when abnormal) also create vortices. As the vortices are shed near the valve, they leave in their place relatively calm wakes which are then rapidly filled by flowing blood, creating the sound of a murmur.  

Two important variables in this theory are velocity and viscosity. When the velocity of blood flow increases substantially as in high cardiac output states (eg, fever, pregnancy), vortex shedding and the intensity of the murmur also increase. Similar phenomenon may be expected when the blood viscosity is lowered (eg, in anemia).

 

References

  1. Sabbah HN, Stein PD. Turbulent blood flow in humans: Its primary role in the production of ejection murmurs. Circ Res 1976;38: 513-24. https://www.ncbi.nlm.nih.gov/pubmed/1269101
  2. Alpert MA, Systolic murmurs. In Walker HK, Hall WD, Hurst JW. Clinical methods: The history, physical, and laboratory examinations. 3rd ed. Butterworths, Boston, 1990. https://www.ncbi.nlm.nih.gov/books/NBK345/
  3. Bruns D. A general theory of the causes of murmurs in the cardiovascular system. Am J Med 1959;27:360-74. http://www.amjmed.com/article/0002-9343(59)90002-6/fulltext
  4. Guntheroth WG. Innocent murmurs: A suspect diagnosis in non-pregnant adults. Am J Cardiol 2009;104:735-7. https://www.ncbi.nlm.nih.gov/pubmed/19699354
My patient with aortic sclerosis has a loud systolic ejection murmur. What is the exact mechanism of this murmur?

Can I assess the severity of aortic stenosis by physical exam alone?

Even in this age of high-tech medicine, physical exam is still a great starting point for assessing the severity of aortic stenosis (AS) even if you are not a skilled cardiologist like most.

Start out by listening over the right clavicle. If you don’t hear a systolic murmur, you can be pretty confident that your patient doesn’t have moderate to severe AS (>98% sensitivity, LR 0.10)1.

If you hear a systolic murmur look for combination of findings that may increase the likelihood of moderate to severe AS: slow carotid artery upstroke, reduced carotid artery volume, maximal murmur intensity at the second right intercostal space, and reduced intensity of the second heart sound.  The presence of 3 or 4 of these signs increases the likelihood of moderate to severe AS (LR 40), with less than 3 not helping much1.

When considered individually, many of the signs we often attribute to significant AS2 may not be as helpful in part because most of us are not skilled cardiologists and over the years the cause of AS has changed from primarily rheumatic heart disease-related to that advancing age and valve degeneration3.  

So it may not be surprising that murmur intensity (eg grade 3/6 or above) may have a poor sensitivity and is an unreliable predictor of the severity of AS when patients with left ventricular failure are also studied3.  Remember also that the absence of the 2nd sound may not distinguish between moderate and severe AS4

 

References

  1. Etchells E, Glenns V, Shadowitz S, et al. A bedside clinical prediction rule for detecting moderate or severe aortic stenosis. J Gen Intern Med 1998;13:699-704. https://link.springer.com/article/10.1046/j.1525-1497.1998.00207.x
  2. Etchells EE, Bell C. Robb KV. Does this patient have an abnormal systolic murmur? JAMA 1997;277:564-71. https://www.ncbi.nlm.nih.gov/pubmed/10376577
  3. Das P, Pocock C, Chambers J. The patient with a systolic murmur: severe aortic stenosis may be missed during cardiovascular examination. Q J Med 2000;93:685-8. https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/qjmed/93/10/10.1093_qjmed_93.10.685/1/930685.pdf?Expires=1500852139&Signature=TwyO6Z4fUfbPc1yiA~2xZC7jOjed0juH604DshdvRYy~VqeNQ57Sv1yE-LNsImthgQogkawMruBPdXn6PvVCVmdvXxE9QsMzQYhZ13JqXDTQhRiPBcsIBKDdROr~xbz0gp0nv-zEmjCp1M8-CXjrlVnjVtwJ6q2nIPTRW5h-CUOnDAmf8vCeJHRi2M9Dt3a4vGALDJQPaETvxKDfoADamBDtZHzzoCIH3OyXT3–jHRtv9AJI2uHlzN79Vzkh~oIrR-rI5mkHle3Yz0R3qIBY0l4P3PssMng~v-IXMNKS~Ghjav8YFTigHN23aEA5yUYllsC7hR25L6h9PA0SZP3QA__&Key-Pair-Id=APKAIUCZBIA4LVPAVW3Q
  4. Aronow WS, Kronzon I. Prevalence and severity of valvular aortic stenosis determined by Doppler echocardiography and its association with echocardiographic and electrocardiographic left  ventricular hypertrophy and physical signs of aortic stenosis in elderly patients. Am J Cardiol 1991;67:776-7. https://www.ncbi.nlm.nih.gov/pubmed/1826070
Can I assess the severity of aortic stenosis by physical exam alone?