Heart murmurs: what are they and what do they mean for my pet?

Hearing that your dog or cat has a heart murmur can be very worrying for any owner. We look to put your mind at rest with the lowdown on what murmurs are, what they mean and what to do for your pet’s best care.

Heart (cardiac) murmurs are common in our pets. While most aren’t serious and won’t require treatment, they can be an indication of an underlying problem and a useful early warning sign. They are always worth discussing with your veterinary surgeon and having investigated whenever that is appropriate.

What is a heart murmur?

A heart murmur is an abnormal sound that a vet hears when listening to a pet’s heart with a stethoscope. Like people, the heart should have two distinct heart sounds: lub-dub, lub-dub, lub-dub. When an additional “whooshing” sound is heard between or around those sounds, it’s called a murmur.

What causes a heart murmur in dogs and cats?

To understand this, it is useful to have an idea of the anatomy of the heart:

This animation shows how the blood flows through the heart from the top chambers, the atria, to the bottom chambers, the ventricles and then out of the major arteries to the body or the lungs.

Anything that changes the blood flow through the heart and creates abnormal turbulence can cause a murmur. A murmur may be congenital (that is, the pet was born with it) or can be acquired later in life.

Murmurs can reflect problems with the valves, the major blood vessels as they leave the heart, or the heart muscle. Here are some of the terms you may hear when discussing common causes of a murmur:

  • Mitral valve disease (MVD): valves between the left side, upper and lower chambers start to leak when the heart pumps
  • Endocarditis: the valves are distorted by infection or inflammation
  • Pulmonic or aortic stenosis: the valves in the major arteries are narrowed where they leave the heart
  • Patent ductus arteriosus (PDA): a blood vessel that was, correctly, there in the foetus in the womb and should have closed at birth, did not
  • “Hole in the heart”: a defect in the heart wall
  • Cardiomyopathy: weakening of the heart muscle in dogs (dilated cardiomyopathy) or thickening of the muscle in cats (hypertrophic cardiomyopathy)

Sometimes a murmur is caused by a medical problem outside of the heart, such as anaemia (a shortage of red blood cells); these heart murmurs are usually quiet, low-grade murmurs and often resolve after treating the underlying condition. 

How do you grade a heart murmur in cats and dogs?

Murmurs are graded on a scale of how loud they are. Since every vet hears them a little differently, the scale isn’t perfect but it very useful to describe murmurs and monitor them over time.

  • Grade 1: only heard with care, in a quiet room with a quiet animal
  • Grade 2: easily heard but quieter than the normal heart sounds
  • Grade 3: as loud as the heart sounds
  • Grade 4: louder than the heart sounds
  • Grade 5: loud with a thrill (vibration) that can be felt with the hand against the chest
  • Grade 6: very loud with a thrill; can be heard with the stethoscope not even touching the body

Does the grade of a murmur tell you how serious the heart disease is?

The answer to this is sometimes but not always. Quiet murmurs can sometimes reflect severe heart disease; loud murmurs don’t always mean an ill pet. It all depends on the precise problem, which is why investigation into the cause of the murmur is useful to recommend treatment and give an idea of the outlook.

What are the signs of heart disease in dogs and cats?

Signs can include all of the following:

  • Weakness or flagging on walks
  • The onset of a cough
  • Rapid, shallow or laboured breathing
  • A distended abdomen
  • Pale or blueish gums
  • Collapse or fainting, called syncope
  • Sudden hindleg weakness in cats, caused by a major blood clot

How are heart murmurs diagnosed in cats and dogs?

Tests will normally be recommended to diagnose the problem more precisely and could include:

  • Blood and urine tests to check for infection, anaemia, kidney, liver or thyroid disease
  • Blood pressure measurements, an ECG (electrocardiograph, meaning a heart trace) or chest x-rays.
  • An echocardiogram, that is, an ultrasound of the heart
  • A ProBNP blood test, which tests for indication that the heart muscle has been stretched by the heart being overworked from defect or damage.
  • Referral to a veterinary cardiologist.

Does a heart murmur mean my dog or cat has heart disease?

Not all animals with a heart murmur will suffer from cardiac (heart) disease. A heart murmur is a clinical finding, not a disease diagnosis. Some heart murmurs are benign or harmless and may go away on their own, particularly in puppies (more on this below). The only way to know the significance of your pet’s murmur is to work with your vet to find the cause.

My puppy has a heart murmur – should I be worried?

Some animals are born with a murmur already present – a congenital murmur, due to defects in the structure of the heart. Usually these are found during the first exam. if your vet suspects a congenital murmur then they will recommend further investigation, as some of these are serious.

However, ‘innocent’ puppy murmurs are common and found in as many as 1 in 5 young pups. They are usually heard at the first vet check at six to eight weeks of age and will disappear over the next few months. This type of heart murmur is usually low grade (quiet) and doesn’t affect the dog at all.

My pet is acting normally – does that mean the heart murmur is harmless?

Not necessarily. Dogs and cats may not have any symptoms, yet still have significant disease that warrants treatment. In some conditions, once the heart has begun to enlarge, treatment can slow the progression of the disease down and hopefully delay the onset of signs. Your vet will need to X-ray or ultrasound your dog’s heart to assess it, or refer you to a cardiologist.

Might my pet have a heart attack?

Signs of a heart attack in people include chest pain, shortness of breath and weakness. It’s scary to imagine our pets experiencing something so unpredictable and dangerous! Luckily, dogs and cats don’t have heart attacks in the same way that humans do, though in some cases dogs with heart disease will faint and collapse. Called syncope, these episodes can resemble a heart attack, but generally don’t cause permanent damage and the pet recovers quickly from them.

What are the treatment options for pets with heart murmurs?

Treatment options can include oral medication, specialised diets and supportive care, all of them designed to ease the symptoms related to the heart disease and where possible to slow the progression of the disease.

Cats may also be treated with drugs to reduce the incidence of blood clots occurring and reduce increased blood pressure.

Certain congenital heart defects like patent ductus arteriosus and pulmonic stenosis can be corrected surgically.

Your vet will tailor a treatment plan to improve your pet’s quality of life, as well as seeking to extend the time you have together. This is why investigating the cause of a murmur, as well as you are able to support, is so important.

Will a healthy diet and exercise help my pet?

The most common form of human heart disease is coronary artery disease. With this condition, plaques of cholesterol accumulates in the arteries, narrowing them so that the blood supply to the heart is disrupted. People are instructed to avoid fatty foods and exercise frequently to reduce this happening.

Luckily, your pet’s disease isn’t caused this way and, while a balanced diet is integral to your pet’s health, the standard “heart healthy” human diet won’t prevent your pet from developing heart disease.

As for vigorous aerobic exercise such as running, this can actually put more strain on your pet’s heart so your vet may recommend limiting your pet’s activity to support the heart and deter cardiac symptoms.

What can I do to help my pet at home?

There are two hugely useful things you can do at home:

Keep your pet slim, or commit to working – with veterinary help – slowly and carefully to slim an overweight pet. Being overweight puts a large strain on the heart.

Count your pet’s breathing rate over one minute, when they are asleep. Keep a record of this, as it gives your vet vital information about whether fluid is building up in the lungs, more accurately than any test in the clinic can do. Search for “pet respiratory rate app” to find helpful apps on how to do this.