What is a heart murmur?

Heart murmurs are abnormal sounds created by a disturbance in the normal flow of blood through the heart. The murmur is audible as a whooshing or swishing sound that occurs during the normal cycle of the heartbeat.

Heart murmurs may be classified as congenital, or present at birth, or acquired, meaning that it has developed during the cat’s lifetime. Heart murmurs can be ‘innocent’ or ‘physiological’, where no apparent disease is causing the murmur.  However, a murmur may also be the first indication of heart disease.

What causes a heart murmur?

Heart murmurs can be caused by mechanical changes within the heart like narrowing of the blood vessels or chambers of the heart, blood leaking between the heart valves, or through defects within the heart muscle.  Systemic illnesses, like anemia, high blood pressure, or thyroid disease, may also cause murmurs. Occasionally, stress may cause a benign or physiologic murmur.

Do all murmurs sound the same?

No. The loudness of a murmur reflects the amount of turbulence that is present in the heart. However, the loudness of a heart murmur does not always correlate directly with the severity of disease.

How is a heart murmur detected?

In most cases, a heart murmur is detected when your veterinarian examines your cat’s heart with a stethoscope.

The murmur is then classified, or graded, to characterize its loudness. Low grade murmurs(Grade 1 and 2) are quieter than Grade 3-6 murmurs. The loudness of the murmur does not always correlate with the degree of heart disease.

Once a heart murmur is detected, your veterinarian may recommend further diagnostic tests in order to determine the cause, if treatment is needed, and the prognosis.  Tests may include:

  • Blood work and urinalysis
  • Blood pressure measurement
  • Chest radiographs (x-rays)
  • ECG (recording the electrical activity of the heart over a period of time using electrodes placed on the skin)
  • Echocardiogram (heart ultrasound)

After the diagnostic tests are performed, your veterinarian can then make a treatment plan that best suits the needs of your cat. Most diseases that cause heart murmurs, including primary heart disease, are treatable through medical management. Surgical intervention is very rarely recommended and is often contraindicated for many types of heart disease. Your cat’s prognosis will depend on the type and severity of disease that is present.  However, early detection and treatment will give your cat the best chance of survival.

If your pet is still a young kitten and the murmur is of low intensity, your veterinarian may recommend a re-examination in a few weeks time to track whether the murmur has changed in intensity or disappeared, indicating that it was likely an innocent murmur. Similarly, if your adult cat appears to be extremely stressed at the time of a routine health examination, your veterinarian may recommend a re-evaluation at a later time when the cat is calmer. 


The symptoms associated with murmurs depend on a variety of characteristics, including their grade, configuration, and location. If, however, the murmur is associated with structural heart disease, your cat may display signs of congestive heart failure such as coughing, weakness, or exercise intolerance.

Murmurs are graded by their intensity, usually on a scale of I-VI.

Grading Scale for Murmurs

  • Grade I—barely audible
  • Grade II—soft, but easily heard with a stethoscope
  • Grade III—intermediate loudness; most murmurs which are related to the mechanics of blood circulation are at least grade III
  • Grade IV—loud murmur that radiates widely, often including opposite side of chest
  • Grade V—very loud, audible with stethoscope barely touching the chest; the vibration is also strong enough to be felt through the animal’s chest wall
  • Grade VI—very loud, audible with stethoscope barely touching the chest; the vibration is also strong enough to be felt through the animal’s chest wall

Murmurs are also characterized by the time in which they occur during the heart cycle, and by whether they are long or short. Most murmurs are also characterized by their location, or where they are the loudest.

The vast majority of murmurs in the cat occur during systole, the phase of the heart cycle when the heart is contracting to pump blood out.

The specific characteristics of the murmur, along with any symptoms that your cat might be showing, will help your veterinarian to determine what is causing the murmur.


Unless heart failure is evident, your cat will be treated as an outpatient. The course of treatment will be determined based on the associated clinical signs. Kittens with low grade murmurs, for example, may require little or no treatment and the murmur may resolve itself within six months. Routine diagnostic imaging is recommended for cats with murmurs.

Treatment depends upon the underlying cause of the heart murmur or the turbulent blood flow. Physiologic heart murmurs do not require any treatment; however, regular monitoring of a cat that has evidence of a physiologic murmur is advised to ensure that no other problems develop. If the heart murmur is caused by an underlying problem, the treatment plan will be based on the diagnosis, and may include a combination of specialized diets, medications and supportive care.