Understanding Heart Murmurs in Cats

In this guide

A heart murmur is an unusual heart sound, typically picked up by a vet listening with a stethoscope during a routine exam.

What Causes a Heart Murmur?

A murmur results from turbulent blood flow inside the heart. Some are labelled innocent or physiologic and cause no harm at all, while others are considered pathologic, meaning they are linked either to a structural problem within the heart itself, known as cardiac disease, or to a cause outside the heart, described as extracardiac.

Not All Murmurs Sound Alike

Loudness reflects the amount of turbulence present, though it does not always match the severity of the underlying condition. Vets grade murmurs on a scale from I to VI. A Grade I murmur is soft, sometimes heard only occasionally and in a single spot on the chest, while a Grade VI is loud enough to hear everywhere over the heart and can even be felt as a vibration, known in cardiac terms as a thrill, when a hand rests on the chest. Murmurs are also described by their timing within the heart cycle, their duration, and their location. In cats, most murmurs occur during systole, the phase when the heart contracts to pump blood out. Together with any symptoms present, these characteristics help a vet work out the likely cause.

Innocent or Physiologic Murmurs

These murmurs have no impact on a cat's health. One common type shows up in young, fast-growing kittens, often starting around six to eight weeks of age and usually disappearing by four to five months. Some adult cats also develop an intermittent murmur when their heart rate rises due to stress, which fades once the heart rate settles and carries no health consequences. Physiologic murmurs are generally low grade, around I to II, and produce no other symptoms.

Structural Causes of a Murmur

The heart is made up of four chambers: the left atrium, left ventricle, right atrium, and right ventricle. Blood travels from the right side of the heart through the pulmonary artery to the lungs to pick up oxygen, then passes through the left side and out through the aorta to the rest of the body. Valves between the chambers and major vessels stop blood flowing backward as the heart pumps. Structural disease disrupts this flow through issues such as a leaky valve, narrowing or thickening of a valve or vessel, or an abnormal opening between chambers. These problems can be congenital, meaning present from birth, such as a ventricular or atrial septal defect or a malformed valve, or acquired later in life, with cardiomyopathy standing out as the most common acquired heart condition in cats.

Extracardiac Causes of a Murmur

Some murmurs, described as functional, stem from causes outside the heart altogether: anaemia, low blood protein, fever or infection, pregnancy, obesity, or being underweight. In young kittens, a heavy parasite burden, from worms, blood parasites, fleas, or ticks, can trigger anaemia or low protein levels that lead to a murmur. Other murmurs are described as secondary, arising from changes caused by a separate disease. The most common cause of a secondary murmur in cats is hyperthyroidism, which can lead to a form of cardiomyopathy known as hypertrophic cardiomyopathy.

How a Murmur Is Diagnosed

A vet typically detects a murmur during a routine stethoscope exam, then asks about the cat's health history and assesses its general condition to judge whether further testing is needed. A young kitten with a soft murmur might simply be re-examined a few weeks later to see whether it fades on its own. A stressed adult cat might be re-checked later once calmer. Murmurs linked to genuine disease usually come with symptoms, though these can be subtle until the condition is fairly advanced: poor appetite, weight loss or stunted growth in a kitten, breathing difficulty, pale gums, lethargy, or weakness. An irregular heart rhythm or weak pulses raise the likelihood of an underlying problem, and further testing is generally recommended promptly once suspected, so any treatment can start as soon as possible.

Further Testing

Depending on the symptoms present, a vet may suggest X-rays, an electrocardiogram, or an echocardiogram, essentially an ultrasound of the heart. Blood tests or other investigations may follow if an extracardiac cause is suspected. An echocardiogram that includes a Doppler study is particularly useful, since it shows the heart's size and movement while measuring the speed and direction of blood flow through the valves and chambers, helping to pinpoint exactly where the turbulence causing the murmur is coming from.

Treatment and Outlook

Treatment depends entirely on the underlying cause of the turbulence. Physiologic murmurs need no treatment, though regular monitoring is still sensible to catch any new developments. Where an underlying problem exists, treatment may involve a specific diet, medication, and supportive care, depending on diagnosis. Outlook ranges from excellent to guarded depending on the cause. Physiologic murmurs carry a good to excellent prognosis, murmurs linked to a treatable extracardiac or functional issue may resolve entirely, and murmurs tied to structural heart disease vary considerably depending on the specific condition involved. A vet remains the best source of guidance on likely outlook and treatment options for an individual cat.

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