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.
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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.
- 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
- 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
- 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
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.
| Grade | Comparison to S1 and S2 | Palpable thrill over PMI | Comments |
|---|---|---|---|
| I | Quieter, focal, difficult to hear | No | Not audible away from PMI |
| II | Quieter, easy to hear | No | May be focal |
| III | Same volume as heart sounds | No | Often audible in more than one location |
| IV | Louder than heart sounds | No | Always audible in more than one location |
| V | Louder than heart sounds | Yes | Always audible in more than one location |
| VI | Much louder than heart sounds | Yes | Audible with stethoscope 1cm off chest wall, or with ear only |
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 intensity | Cause of murmur | Differential diagnosis |
|---|---|---|
| Left apex – systole | Mitral regurgitation | Mitral valve disease Dilated cardiomyopathy Mitral dysplasia Mitral endocarditis |
| Left apex – diastole | Mitral inflow | Mitral stenosis |
| Left base – systole | Aortic outflow Pulmonic outflow | Aortic or subaortic stenosis Aortic endocarditis Innocent flow murmur Pulmonic valve stenosis |
| Left base – continuous | Flow between the aorta and pulmonary artery | Patent ductus arteriosus Aorticopulmonary collateral vessels |
| Left base – diastole | Aortic 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 apex | Tricuspid 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 base | Radiation of a murmur from the left base through the aorta | Same as for aortic outflow 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
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.
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.
Does every heart murmur require investigation?
Typical characteristics of a non-pathological (“flow”) murmur
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.
- 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
- Timing: fast flow will only occur during systole in normal dogs, so no diastolic murmurs are innocent
- 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)
- 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
- 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
- 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 example | Recommendation |
|---|---|
| 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 addition | Investigate |
| 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 asymptomatic | Monitor |
| 12-year-old Chihuahua with an easily audible murmur at the left apex, which extends through to S2 | Investigate |
| Two-year-old Bull Terrier with a grade II early diastolic murmur at the left apex which is inaudible on the right side | Investigate |
| 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 base | Monitor |
Conclusion
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. |
