Understanding murmurs and mitral valve disease

When your vet listens to your pet’s chest they are listening for any abnormalities, which includes murmurs. A murmur is a common finding in dogs with myxomatous mitral valve disease, but what exactly is a murmur? Let’s explore this further.

What are heart murmurs?

Heart murmurs are abnormal heart sounds which can be caused by two mechanisms –

  1. The most common cause is turbulent flow of blood. Instead of the blood flowing nicely and smoothly, it may be being forced under higher speed through a narrowed area into a wider one, the blood may be whooshing backwards as well as forwards or it could be because the thickness of the blood itself is changed.
  1. Less commonly a murmur could be caused by the vibration of a structure within the heart, such as a heart valve.

It is also worth noting that not all murmurs are caused by heart disease. Being anaemic can make the blood thinner (a decrease in blood viscosity) and make the heart pump faster, creating a murmur. A murmur is usually detected when listening with a stethoscope and can be very loud in some cases of MMVD. The vet will often assign a grade from 1 to 6 to your dog’s murmur as follows:

Hover over the grade to find out more!

Grade 1

The murmur is very soft and can only be heard in a quiet room with close concentration

Grade 1

The murmur is very soft and can only be heard in a quiet room with close concentration

Grade 1

The murmur is very soft and can only be heard in a quiet room with close concentration

Grade 2

The murmur is faint but can easily be heard

Grade 3

The murmur is moderately loud, about the same as the normal ‘lub dub’ heart sounds

Grade 4

The murmur is louder than the normal heart sounds, but no vibrations (‘precordial thrill’) are felt

Grade 5

the murmur is very loud and can also be felt vibrating on the chest wall (precordial thrill is present)

Grade 6

the murmur is very loud with vibrations on the chest wall (pre-cordial thrill is present) and can also be heard without the stethoscope having to be placed directly on the chest

It is important to note that the loudness of the murmur doesn’t always correlate with how severe the underlying heart issue is. Also, it doesn’t tell us what the cause of the murmur is either. This is where other tests will be required.

Your vet may also make a note of the timing of the murmur in relation to your dog’s heart sounds and its loudest point within the chest.

A quick note on innocent flow murmurs

Heart murmurs caused by mild blood turbulence can often be heard in young puppies, and most resolve without issue. These innocent murmurs are usually low intensity and resolve by 4-6 months of age, as the puppy grows.

This type of murmur doesn’t cause any issues to the puppy and doesn’t require treatment.

An innocent heart murmur is found on average in 28% of puppies under the age of 6 months, while in athletic dog breeds such as whippets, it has even higher percentages of up to 58% of puppies.

Murmurs that don’t resolve in puppies are usually due to congenital heart malformations, so if the murmur persists then investigation will be required.