Mitral Valve Insufficiency

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Mitral valve insufficiency (MI) is inadequate closure of the valve that separates the left atrium from the left ventricle. This disorder can be due to backward movement of the valve (prolapsing), thickening of the valve, gradual (chronic) degeneration of the valve, bacterial infection of the valve, narrowing of the opening in the valve or dysfunction of the muscles or chords controlling the valve. Chronic degeneration of the valve is the most common cause of MI in dogs and cats.

This inability to close the mitral valve completely causes regurgitation (backward flow) of blood into the left atrium of the heart. This regurgitation increases the amount of blood that the left atrium has to pump, which may lead to volume overloading. As a means of compensation, the left atrium may dilate (expand). This is a condition known as left atrial enlargement. In later stages of disease, the left ventricle and veins of the lungs are generally enlarged as well.

Signs of MI include coughing, increased heart rate, increased respiratory rate and effort, abnormal heart sounds, decreased activity and reduced ability to exercise. MI is most commonly found in cats and older small breeds of dogs. Typical breeds of dogs affected by MI include Dachshunds, Toy Poodles, Mini Schnauzers, Chihuahuas, Cocker Spaniels, Boston Terriers and Bull Terriers. Large breed dogs such as German Sheperds and Great Danes have been known to have MI as well. Gradual degeneration of the mitral valve is an inherited condition in many old and young King Charles Cavalier Spaniels.


Diagnostic tools used in diagnosing MI include:

  • Thoracic Radiographs
  • Echocardiogram
  • Electrocardiogram

Common arrhythmias seen include bundle branch block (a block in the electrical conduction that controls contraction, causing skipped beats) and atrial fibrillation (irregular chaotic heart beat).


MI is usually treated medically. The goals of which are to:

  • Decrease the volume of blood the heart must pump
  • Remove fluid from the lungs (in later stages of disease)
  • Decrease the amount of regurgitation at the mitral valve
  • Increase the amount of blood the heart can pump out when it contracts (this would reduce the number of times the heart must contract)
  • Increase the animal’s overall comfort level

Drugs typically used in treated MI are:

  • Enalapril: helps to reduce the amount of water and salt that the body retains and reduces blood pressure. Feeding pet foods that are low in salt is also helpful.
  • Furosemide: helps to reduce fluid retained in the body and lungs. It also expands the vessels leading away from the heart and decreases the amount of blood the heart must pump.
  • Nitroglycerine: helps vessels going to the heart to expand. Expansion of these vessels helps reduce the heart’s workload.

It is also recommended that pets with severe MI are kept calm, cool and comfortable. They should not be allowed to overheat or get overly excited. Exercise should be reduced based on severity of disease.


Prognosis for MI depends on severity of disease and how early it is detected. Some pets with slight to mild MI need no treatment and require only occasional monitoring. Pets may live for many years without clinical signs. Once signs of heart failure are present, prognosis worsens. If MI is detected in early stages treatment with the above medications can be started and a pet can live a good quality of life with controlled disease.

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