Simon Swift
May 2004

A heart murmur is caused by vibrations in the heart. or blood vessels. These vibrations are usually created by turbulence, and occasionally by vibrations of structures in the heart e.g, valves, Just imagine a. river flowing slowly and smoothly, that is laminar flow, Now picture a small fast-flowing stream with lots of currents and eddies, this is turbulent flow.

Turbulence is due to one of three things:
  1. High velocity blood flow
  2. Low blood viscosity - as seen in anaemia
  3. Flow from a narrow region into a large area
Each heart beat ejects a certain volume of blood, the stroke volume, Puppies have larger stroke volumes for their size than adult dogs. This means that for each heart beat, a relatively larger volume of blood is ejected out a smaller vessel when compared to an adult dog. When excited such as at a veterinary surgery, this can increase further and so the outflow velocity from the ventricles in the aorta or pulmonary artery can increase above a certain threshold and become turbulent creating a murmur.

Typically these innocent flow murmurs tend to be fairly quiet (up to grade 3), soft and blowing in character and occur at the beginning to middle of the heart heat, Also they are usually loudest over the aorta or pulmonary artery and can vary with body position. Unfortunately, it is impossible to differentiate between some mild congenital heart diseases such as sub-aortic stenosis (a narrowing under the valves at the base of the aorta) and a flow murmur. Most importantly, innocent flow murmurs disappear by 6 months of age.

If you have a puppy with a murmur that could be a flow murmur, you have two options:
  1. Have the puppy re-examined in 1 month. An innocent flow murmur should get quieter as the puppy grows. If the murmur is the same or louder, further investigations such as Doppler ultrasound examination would be indicated.
  2. Have a Doppler ultrasound examination performed if you need to know NOW. This will allow the cardiologist to assess the valves and the speed and direction of blood flow through the heart can be measured. This will differentiate between mild heart disease and flow murmurs.
Hence, if I pick up a quiet murmur in a young puppy at a show, I would recommend that the owner waits to see how it develops before making any drastic decisions. If it is an innocent flow murmur, it should gradually get quieter and disappear. Anything more significant will persist and require further investigation.

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