Identifying heart murmurs and other heart sounds: should I investigate them all? - Veterinary Practice
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Identifying heart murmurs and other heart sounds: should I investigate them all?

Integrating history, signalment and auscultation findings is a valuable way to consider whether a dog requires further investigation of cardiovascular disease or not

Auscultation and heart murmurs in small animals: 2 of 3

This is the second article in this cardiology series on auscultation, with the previous part going back to the basics. In this article, we will focus particularly on heart murmurs in dogs. They can be graded similarly in cats, but tend to localise more sternally and the separation of apex and base is near-impossible because the heart is only the size of an egg.

This article contains audio files. For the best experience, we recommend using headphones.

What is a heart murmur?

A heart murmur is a low-frequency sound that is associated with the flow of turbulent blood. The turbulence of blood flow leads to the vibration of cardiovascular structures, and the murmur is timed with the inciting event.

What causes turbulence of blood flow?

The theoretical physics underlying laminar (organised) and turbulent (disorganised) flow is based on calculations looking at distilled water passing through glass tubes, so it is somewhat imperfect for the biological realities of pulsatile flows of blood. However, we can infer that several things increase turbulence and therefore are possible causes of heart murmurs. Generally, the more turbulent the flow, the greater the vibration of anatomic structures, and therefore the louder the murmur.

  1. High velocity flow: once blood flow velocity exceeds around 2m/s, flow murmurs will be generated. This is the main cause of heart murmurs in patients with stenosis, where the pressure gradient across a stenotic region is high enough to increase flow velocity above a particular level
  2. Wide vessel diameter: wider vessels carrying fast or accelerating flow cause more turbulence of the blood within them. This is why the aorta is a common site of flow murmurs (Figure 1): it is the widest vessel where blood accelerates rapidly to a high velocity
  3. Low viscosity: reduced blood viscosity means that turbulence occurs more easily within normal anatomic structures. This is the reason that animals with moderate to severe anaemia develop a “haemic murmur”. Reduced viscosity (relatively mild) may also be present in patients with pyrexia. The opposite is also true: animals with marked haemoconcentration or erythrocytosis are less prone to turbulence, so known heart murmurs may reduce in intensity or vanish
Phonocardiogram from a dog showing a systolic murmur
FIGURE (1) Phonocardiogram from a dog showing a systolic murmur, present between the normal S1 and S2 sounds. Echocardiography determined that this was caused by normal blood flow through the aorta – a “non-pathological”, “innocent” or “flow” murmur

Grading of heart murmurs

The intensity of heart murmurs is described according to a grading system which compares them to the heart sounds (Table 1). It is based on the point of maximum intensity (PMI) – almost all heart murmurs sound different as you move around the chest: some are focal and not audible anywhere else, but many can be heard more quietly elsewhere on the chest.

GradeComparison to S1 and S2Palpable thrill over PMIComments
IQuieter, focal, difficult to hearNoNot audible away from PMI
IIQuieter, easy to hearNoMay be focal
IIISame volume as heart soundsNoOften audible in more than one location
IVLouder than heart soundsNoAlways audible in more than one location
VLouder than heart soundsYesAlways audible in more than one location
VIMuch louder than heart soundsYesAudible with stethoscope 1cm off chest wall, or with ear only
TABLE (1) Guide to grading heart murmurs

Points of maximum intensity and differential diagnoses

The location and timing of the point of maximum intensity is an important factor to take into account when considering potential differential diagnoses (Table 2). Murmurs in systole at the left apex are caused by mitral regurgitation. The left base is the location of both the aortic and pulmonic valves, so systolic murmurs here may represent congenital aortic or pulmonic stenosis, or potentially could be innocent murmurs associated with aortic flow. The right apex is the location of the tricuspid valve.

Point of maximum intensityCause of murmurDifferential diagnosis
Left apex – systoleMitral regurgitationMitral valve disease
Dilated cardiomyopathy
Mitral dysplasia
Mitral endocarditis
Left apex – diastoleMitral inflowMitral stenosis
Left base – systoleAortic outflow
Pulmonic outflow
Aortic or subaortic stenosis
Aortic endocarditis
Innocent flow murmur
Pulmonic valve stenosis
Left base – continuousFlow between the aorta and pulmonary arteryPatent ductus arteriosus
Aorticopulmonary collateral vessels
Left base – diastoleAortic regurgitation
In very rare circumstances, pulmonic regurgitation can cause a murmur, but pulmonary hypertension must be present for this to occur
Aortic endocarditis
Aortic degenerative valve disease
Right apexTricuspid regurgitation
Diastolic murmurs on the right side are incredibly rare, and almost always caused by tricuspid valve stenosis
Tricuspid valve disease
Ventricular septal defect
Pulmonary hypertension
Right baseRadiation of a murmur from the left base through the aortaSame as for aortic outflow murmurs
TABLE (2) Different causes of heart murmurs cause PMIs at different locations of the heart and at different stages of the cardiac rhythm, so these are useful diagnostic tools when investigating murmurs

Additional heart sounds

Gallop sounds

Diastolic heart sounds are audible in large animals and may be a sign of pathology in dogs and cats. They would not be considered normal sounds in small animals, but do represent normal events in the cardiac cycle – the dual phases of diastolic ventricular filling. They are shorter and of lower frequency than the systolic heart sounds (S1 and S2).

  • S3: the third heart sound is caused by rapid deceleration of blood within the ventricular cavities as they fill after ventricular relaxation
  • S4: the second heart sound is caused by deceleration of blood against the closed aortic and pulmonic valves at the same time as the mass of ventricular myocardium relaxes rapidly in early diastole
FIGURE (2) This audio file and associated phonocardiogram is from a cat with a gallop sound – an additional sound (S3) caused by rapid deceleration of blood within the ventricular cavities as they fill after ventricular relaxation

In dogs and cats, it is not normal to hear the S3 and S4 heart sounds. Because of the normal heart size in these species, the diastolic events should not be audible. The only reason to hear an S3 or S4 is increased filling pressure – ie high atrial pressure – which causes rapid inflow into the left ventricle during diastole, generating a sound. This diastolic noise will cause a third heart sound (it is impossible to detect all four owing to the heart rate) known as a gallop sound (Figure 2), which is indicative of a high risk of heart failure. It is very difficult to differentiate a true gallop sound from other causes of additional sounds during the cardiac cycle, such as a split S2 (Figure 3) or a valve click (Figure 4).

It is worth noting that the often-used term “gallop rhythm” is incorrect – it is an additional sound; the rhythm may be normal or not and is not dependent on additional noise.

Split S2

As we know, in normal animals the S2 sound is made up of the simultaneous closure of the aortic and pulmonic valves and the associated deceleration of the blood column within the respective vessels, causing reverberation of tissues. Under particular circumstances, the closure of the pulmonic valve can be delayed, causing a dual sound at the end of systole – a so-called split S2 (Figure 3). Here, we refer to the individual sounds within this as the A2 (aortic) and P2 (pulmonic) sounds.

In humans, the S2 sound can split transiently during inspiration, but this is not the case in dogs and cats. A split S2 in dogs is always associated with severe pulmonary hypertension – it is not present in every dog with pulmonary hypertension, but it is a strong predictive finding if present.

FIGURE (3) In this audio recording and associated phonocardiogram from a dog with severe pulmonary hypertension, a loud and split S2 sound can be recognised, caused by a delayed closure of the pulmonic valve. The individual sounds are termed the A2 (aortic) and P2 (pulmonic) sounds

Mid-systolic click

Another cause of a third heart sound is the mid-systolic click. Like a split S2, the mid-systolic click is not easy to distinguish from a gallop sound on physical examination, but on a phonocardiogram the timing of the additional sound is clear (Figure 4).

Mid-systolic clicks are typically caused by mitral valve prolapse, and in the absence of a murmur they indicate very early mitral valve disease. As such, their only clinical significance is that they could be gallop sounds or a split S2.

FIGURE (4) Mid-systolic click sounds are not easy to distinguish on physical examination, but the timing of the additional sound is clear on associated phonocardiograms

Does every heart murmur require investigation?

Typical characteristics of a non-pathological (“flow”) murmur

FIGURE (5) Murmurs present only in early systole (A) could represent very mild stenosis or early mitral regurgitation, but those that last all the way through to S2 (known as holosystolic; B) are almost certain to represent pathology

Heart murmurs are much more common than heart disease in dogs, and in cats this is even more true. There are some crucial features of heart murmur auscultation which can help us to identify murmurs that will be considered low risk, compared to those considered to have a higher risk of representing heart disease.

A non-pathological heart murmur is caused by normal flow and has no clinical significance, so is often called an “innocent” or “flow” murmur. We can use a few factors to decide whether or not a murmur is likely to require investigation.

  1. Heart murmur intensity: louder heart murmurs are more likely to represent pathology. Grade I or II murmurs could feasibly be flow murmurs, but grade III murmurs are less likely to be. It is virtually impossible for a grade IV murmur to be anything but pathological
  2. Timing: fast flow will only occur during systole in normal dogs, so no diastolic murmurs are innocent
  3. Duration: murmurs that last all the way through systole are more likely to represent pathology. Flow murmurs are loudest or may only be present in the first half of systole (Figure 5)
  4. Localisation: flow murmurs almost all localise to the left base, but some are difficult to localise and may sound more like they are at the left apex. They may be easily audible at the right base (because aortic murmurs often radiate there) but not at the right apex
  5. Variability: flow murmurs often increase in intensity at higher heart rates and become quieter at lower heart rates. This is generally not the case for pathological murmurs in dogs, but can be for cats
  6. Association with other signs: innocent murmurs do not cause clinical signs and are not associated with other clinical findings that localise things towards heart disease, so if a dog presents with a heart murmur and a gallop sound, arrhythmia or a history of collapse or respiratory signs, then it should be investigated as a priority

Expert consensus on heart murmurs

In 2015, a consensus panel of veterinary cardiologists published a document making recommendations about how to approach incidentally detected heart murmurs in dogs and cats (Côté et al., 2015). Approaching heart murmurs in cats is more complicated than when approaching murmurs in dogs, but – broadly speaking – the six points above remain true. A heart murmur which is incidentally detected, grade II or quieter, focal, systolic and localised to the left base, and which varies in intensity with heart rate (eg Figure 1), is likely to be a flow murmur and therefore does not require any further investigation (Table 3).

Case exampleRecommendation
Three-month-old Bulldog with a grade III left basilar systolic murmur and reduced pulse quality. The murmur is easily audible at the left apex and right base in additionInvestigate
One-year-old Boxer with a focal, grade II left basilar systolic murmur which is loudest when the dog is excited and almost inaudible when he is calm. He is asymptomaticMonitor
12-year-old Chihuahua with an easily audible murmur at the left apex, which extends through to S2Investigate
Two-year-old Bull Terrier with a grade II early diastolic murmur at the left apex which is inaudible on the right sideInvestigate
18-month-old Bichon Frise with a grade II-III systolic murmur at the left base, which does not extend past mid-systole and is also quietly audible at the right baseMonitor
TABLE (3) Which murmurs should be investigated? Recommendations for examples of cases presenting with murmurs, based on signalment and auscultation findings


Despite data suggesting that many recent graduates’ skills plateau soon after graduation, listening to a wide range of heart murmurs should be helpful to increase people’s confidence with hearing new things.

The next article in this three-part series on auscultation will provide further case examples and guides on how to narrow down your differential diagnosis lists.

The content in this article is further discussed by Kieran Borgeat in Improve International’s Small Animal Cardiology Online Learning GPCert/PgC programme, of which Kieran is the course director. For more information, please visit the Improve International website.

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