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).

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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. 3^rd 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