The benign murmur in puppies and kittens

April 8, 2026

An owner brings you a young patient – a new member of the family! In the early days of excitement, owners will be focused on planning a (normal) life with their pet. You start your clinical exam, place the stethoscope… and your stomach sinks – you hear a heart murmur.

Before panic sets in (for you or the owner!) it is worth remembering that not every murmur indicates structural heart disease. In young animals in particular, flow murmurs are a common and entirely benign finding. If you want to rule out congenital disease, referral is a great option.

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What is a flow murmur?

A flow murmur is a murmur generated by turbulent blood flow through a structurally normal heart. This may be more noticeable at higher heart rates. Rather than being caused by abnormal valves, septal defects, or outflow tract obstruction, these murmurs occur when blood moves rapidly through the cardiac chambers or great vessels.

These are sometimes referred to as physiological or ‘innocent’ murmurs.

Which patients are affected?

Flow murmurs are more commonly detected in puppies (and sometimes kittens), often during routine health checks or vaccination visits.

What do they sound like?

Typically, flow murmurs share several features:

Low grade, soft intensity (grade I–II/VI)
Systolic
‘Hollow’ or musical quality
Intermittent, or variable with heart rate

Importantly, the patient should otherwise appear completely normal: normal pulse quality, mucous membrane colour, and no signs of exercise intolerance, weakness, syncope, or respiratory compromise. Assessing how the patient is at home can be tricky if they haven’t been with new owners very long.

When should you be cautious?

The challenge in practice is determining when a murmur is likely to be innocent and when it warrants further investigation.

Features that should raise suspicion include:

Moderate (grade II-III/VI or louder) systolic murmurs
Diastolic or continuous murmurs
Palpable precordial thrill
Abnormal pulse quality (weak or bounding/hyperdynamic)
Poor growth, low body condition, or if the patient was the smallest in their litter
Association with other abnormal clinical signs or findings on examination
Murmurs found in older patients, particularly if they have not been noted before

What should you do in practice?

For a bright, asymptomatic young patient with a soft systolic murmur, it can be tempting to assume a benign flow murmur – particularly when everything else about the patient appears normal. However, it is not uncommon for patients with congenital disease to be clinically normal in the earlier days.

It is important to remember that flow murmurs are ultimately a diagnosis of exclusion. Some congenital abnormalities can initially present with relatively subtle murmurs, and distinguishing these from innocent murmurs based on auscultation alone can be challenging. You should not be expected to make this diagnosis definitively. Managing owner expectations is key – it is better to be cautious than regret not following up.

Practical steps include:

Listening to all cardiac regions (including high in the axillae) on both sides
Taking the patient to a quieter, calm environment and repeating auscultation
Careful characterisation of the murmur (timing, point of maximal intensity, radiation, and grade) – this will also help colleagues compare findings later
Repeating auscultation in 4-8 weeks, particularly if the murmur is detected during early vaccination visits
Seeking a second (or third) opinion from a colleague if you aren’t sure – ideally allowing them to listen before sharing your findings to minimise bias
Monitoring the patient’s growth, body condition, and clinical signs

Discuss echocardiography or referral early, and consider this if the murmur persists (particularly beyond six months of age), increases in intensity, or if any atypical clinical features are present.

The take-home message

Flow murmurs are common in puppies and kittens, and many will disappear as the patient matures. However, they should be approached with thoughtful clinical judgement and only diagnosed by exclusion.

A careful examination, appropriate follow-up, and a low threshold for further investigation will help ensure that structural abnormalities are not missed. At the same time, confirming an innocent murmur can provide enormous reassurance for owners who may have left the consultation worried about their new pet. It can also help distinguish a previously documented flow murmur from a new murmur later in life if the patient develops acquired cardiac disease – an important consideration for insurance purposes as well. Refer a case here, or get advice on a case you aren’t sure about.