A Closer Look at Heart Murmurs in Cats

In this guide

Rewritten from source material originally published at: vcahospitals.com/know-your-pet/heart-murmurs-in-cats

A heart murmur is an abnormal heart sound, most often picked up during a routine stethoscope exam. It results from turbulent blood flow within the heart, and can be either 'innocent' (also called physiologic, meaning it has no real impact on health) or pathologic, meaning it is caused by an actual problem, either within the heart itself (cardiac disease) or outside it (an extracardiac cause).

Do all murmurs sound the same?

No. A murmur's loudness reflects how much turbulence is present in the blood flow, though loudness does not necessarily track with how serious the underlying cause is. Vets grade murmurs on a scale from I to VI: a Grade I murmur is very soft, may come and go, and is usually audible in only one spot on the chest, while a Grade VI murmur is loud, audible everywhere on the chest, and can even be felt as a vibration (called a 'thrill') when a hand is placed over the heart.

Murmurs are also described by when in the heartbeat cycle they occur, and by their duration and specific location. In cats, the large majority of murmurs happen during systole, the contraction phase when the heart is actively pumping blood out.

Innocent (physiologic) murmurs

An innocent, or physiologic, heart murmur has no bearing on a cat's health. One common form appears in young, rapidly growing kittens, often first noticeable around six to eight weeks of age, and is usually outgrown by four to five months; this type is entirely benign. Some adult cats also develop an intermittent murmur tied to a raised heart rate during stress, which disappears once the heart rate settles and has no health impact of its own. Physiologic murmurs are typically low grade, usually I to II, and are not accompanied by other symptoms.

Structural heart problems

The heart has four chambers, the left and right atria and ventricles, with blood flowing from the right side through the lungs to be oxygenated, then through the left side out to the body via the aorta. Valves between each chamber and major vessel prevent blood flowing backward as the heart pumps.

Structural heart disease involves some kind of physical abnormality disrupting that flow and creating turbulence, a leaky valve, a narrowed or thickened valve or vessel, or an abnormal opening between chambers. These problems can be congenital, present from birth, with a ventricular septal defect or atrial septal defect being the most common congenital issues in cats, alongside occasional defective valves, or acquired later in life, most often in the form of cardiomyopathy.

Extracardiac causes

Some murmurs, described as 'functional,' arise from problems entirely outside the heart: anaemia, low blood protein, fever or infection, pregnancy, obesity, or emaciation. In young kittens, anaemia or low protein levels are often linked to a heavy parasite burden, intestinal worms, blood parasites, fleas, or ticks. Other murmurs are 'secondary,' arising from changes elsewhere in the body; in cats, the most common cause here is hyperthyroidism, which can trigger a form of cardiomyopathy known as hypertrophic cardiomyopathy.

How is a murmur found, and how serious is it?

Most murmurs turn up during a routine stethoscope exam. From there, the vet will ask about your cat's health history and assess their general condition to judge whether further testing is warranted.

For a young kitten with a faint murmur, a vet may simply recommend rechecking in a few weeks to see whether it fades on its own, a good sign of an innocent murmur. Similarly, if an adult cat seems especially stressed during a routine exam, a follow-up visit when they are calmer can help clarify the picture. A murmur genuinely caused by structural or extracardiac disease will usually come with other symptoms too, though these can be subtle at first and only become obvious as the underlying condition progresses. The most common symptoms to watch for include poor appetite, weight loss or, in kittens, stunted growth, breathing difficulty, pale gums, lethargy, and weakness. An irregular heart rhythm or weak pulses detected alongside a murmur both raise the likelihood of an underlying cause, and in most such cases further diagnostic testing is recommended promptly, since early treatment tends to make the biggest difference.

What further tests might be needed?

Depending on the symptoms present, a vet may recommend x-rays, an ECG, or an ultrasound of the heart (an echocardiogram). If an extracardiac cause is suspected, bloodwork or other tests may be added. An echocardiogram that includes a Doppler study is generally the most useful tool for locating a murmur precisely, since it allows the vet to see the heart's size and movement directly and measure the speed and direction of blood flow through the valves and chambers, pinpointing exactly where the turbulence is coming from.

Treatment and outlook

Treatment always depends on the underlying cause. Physiologic murmurs need no treatment at all, though regular monitoring is still worthwhile to catch any new developments. Where an underlying problem is identified, treatment might involve a specialised diet, medication, or supportive care tailored to the specific diagnosis.

Prognosis varies enormously depending on the cause, ranging from excellent to genuinely guarded. A physiologic murmur carries a good to excellent outlook with no treatment required. Murmurs caused by a treatable extracardiac or functional issue often resolve once that underlying problem is addressed. For murmurs caused by actual cardiac disease, the long-term outlook depends heavily on the specific condition involved, and since every case differs, your vet is best placed to walk you through the available treatment options and what to expect for your particular cat.

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