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Heart murmurs are abnormal, extra sounds of a relatively long duration. Heart murmurs represent auditory vibrations that occur as a result of turbulence within the heart or great vessels created by disruption of normal laminar blood flow.

Loud systolic murmur in a Beagle

Reynolds number is a dimensionless quantity that defines the variables that promote disturbed (non-laminar) flow in a vessel or chamber. When Reynolds number exceeds a critical value, flow becomes turbulent.

Reynolds Number = (Diameter)(Velocity)(Density) / Viscosity

Diameter = diameter of the chamber, orifice, or vessel
Velocity = velocity of blood flow (note that this is related to the diameter)
Density = density of blood
Viscosity = viscosity of blood (affected mainly by the red blood cell count and protein count)

Blood flow turbulence can be created by high-velocity flow, flow from a narrow region into a larger area, or low blood viscosity.

Causes of Heart Murmurs

Some common causes of blood flow turbulence producing heart murmurs include pathologic causes like:

  • Valvular insufficiency (regurgitation)
  • Valvular or chamber stenosis (narrowing)
  • Connections between the cardiac chambers (e.g. interatrial and interventricular defects)
  • Connections between the great vessels (e.g. patent ductus arteriosus)

Recognize importantly, though, with Reynolds number in mind, that turbulence may be produced in the absence of an organic heart lesion or heart disease! These non-pathologic causes include changes to blood viscosity or cardiac output, producing so called functional or physiologic heart murmurs. Examples include:

  • Murmurs associated with low blood viscosity produced by anemia or hypoproteinemia
  • Murmurs associated with high cardiac output states like pregnancy, hyperthyroidism, increased sympathetic tone, and chronic bradycardia
  • Murmurs in growing puppies and kittens associated with large relative blood volume and relative anemia. These typically resolve by 6 months of age.
  • Innocent murmurs in mature dogs and cats associated with mild turbulence in the left or right outflow tracts

These functional/physiologic murmurs tend to be soft to moderate in intensity, occur in early to mid-systole, and are most often heard over the left base.

It is often not possible to conclude that a murmur is functional on auscultation alone, despite the above characteristics. That is, the above characteristics above are not unique to functional murmurs – soft to moderate systolic murmurs over the left base could be pathologic in some cases. Conversely, it may be possible to conclude on auscultation that a murmur is pathologic (associated with an organic cardiovascular lesion or disease), related to its loudness, location, and/or timing.

Note that heart murmurs, particularly softer ones, may be masked by factors including obesity, pleural or pericardial effusions, loud respiratory sounds, or increased blood viscosity.