A murmur is likely to be found during careful auscultation. Auscultation is best performed with both the patient and the clinician in a comfortable position. Careful auscultation takes a reasonable period to perform and should involve listening on both sides of the thorax, palpation of the thoracic wall, and palpation of the arterial pulse.
In order to characterize a murmur adequately it is first necessary to think about the normal constituents of the cardiac cycle and how the murmur relates to those normal characteristics.
Audible heart sounds are numbered and referred to as S1, S2, S3, and S4.
S1 and S2 are the only sounds normally audible in the dog. These make up the Lub-Dup normally heard on auscultation. S1 corresponds to the sound generated in the heart and surrounding structures at the onset of systole on closure of the atrioventricular valves. This is usually the loudest heart sound and is heard best over the left apex. S2 corresponds to the sound generated in the heart and surrounding structures on closure of the pulmonic and aortic valves. This represents the end of systole and will be loudest at the left heart base.
Between S1 and S2 is ventricular systole. Between S2 and the following S1 is diastole. Diastole is much more variable in length. The pulse will rise during systole.
S3 corresponds to passive ventricular filling. The ventricle relaxes and blood passively flows in from the atria to the ventricle (approximately 75% of filling is passive).
S4 corresponds to active ventricular filling as the atria contract. Blood is forced into the ventricle.
S3 and S4 when audible in small animals, are always indicative of an abnormality. The presence of an audible diastolic sound implies that the ventricle is not filling normally i.e., there is poor relaxation. This is described as a gallop rhythm and sounds like Du-Lub-Dup. One can only characterise if it is an S3 or S4 gallop by recording a phonocardiogram. (Differentiating whether gallop is due to S3 or S4 is not likely to be of significance anyway.)
Splitting of S1 and/or S2 indicates asynchronous closure of the AV or outflow valves. A split S2 can occur if there is pulmonary hypertension for example. The pulmonic valve closes after the aortic valve leading to two audible sounds rather than one thus “splitting” the heart sound.
Heart Murmurs
A murmur indicates the presence of turbulent flow within an area of the heart due to disturbance to the normal laminar flow of blood within the heart and surrounding vessels. This is likely to occur when there is an increased velocity of blood flow, an increased volume of blood flow, a reduction in the blood viscosity or when there is regurgitation of blood across an insufficient (leaking) valve.
When Are You Likely to Hear Murmurs?
The vast majority of murmurs in small animals are systolic. This is because systole is the most active period of the cardiac cycle when ejection occurs and ventricular pressures are highest. Aortic regurgitation gives rise to a diastolic murmur.
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Systole
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Diastole
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AV valves closed M + T insufficiency
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AV valves open M + T stenosis (low pressure)
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Outflow valves open A + P stenosis
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Outflow valves closed A + P insufficiency
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Aortic pressure > PA pressure flow through PDA
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Aortic pressure > PA pressure flow through PDA
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LV pressure > RV pressure flow through VSD
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LV pressure = RV pressure No flow through VSD
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Abbreviations - M: Mitral, T: Tricuspid, A: Aortic, P: Pulmonic, PA: Pulmonary artery, PDA: Patent ductus arteriosus, LV: Left ventricle, RV: Right ventricle, VSD: Ventricular septal defect
Murmurs Described According To
- Timing/duration
- Intensity
- Location (point of maximal intensity)
Timing
The most important distinction as far as timing is concerned is systolic versus diastolic. Between S1 and S2 variable is systolic. Between S2 and the following S1 is diastolic. Some people try to define murmurs as early, mid, or late systolic or late diastolic. This can cause a lot of confusion and it is not as important as differentiating systole from diastole. It is especially difficult to accurately time murmurs at higher heart rates.
It is possible to have a murmur throughout systole and diastole. These murmurs are described as continuous. The classic example of this is the PDA (although other potential causes exist). If the heart rate is fast and it is difficult to distinguish systole from diastole then palpate the femoral pulse. Pulse pressure comes up (rises) soon after the onset of ventricular systole.
Location of Murmurs
The “heart base” area corresponds to the area of the outflow valves and is a fairly cranial position. In order to access this area you will have to move your stethoscope under the triceps muscle mass which may necessitate moving the animal’s leg forward on that side. The heart apex is the position on the left where the mitral valve will be most audible. On the right side the tricuspid valve is most clearly audible. VSD and PDA murmurs need not be at a valve position. PDA dorsal to the left heart base. VSD “diagonal” from the left heart apex to the right sternal border.
Intensity of Murmur
Murmurs are graded out of six in terms of intensity
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I/VI
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Audible after a long time listening in perfect conditions. Quiet room, amenable patient, good stethoscope.
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II/VI
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Clearly audible as soon as the stethoscope is placed over the point of maximal intensity.
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III/VI
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Clearly audible and as loud as the normal heart sounds.
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IV/VI
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Audible louder than heart sounds but no thrill palpable, likely to radiate widely over the thorax.
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V/VI
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Thrill palpable over the point of maximal intensity at the skin surface
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VI/VI
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Audible with the stethoscope lifted off the chest.
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Murmurs can also be described according to their radiation, pitch, and shape. These characteristics are less important than the three already referred to.
Radiation
When a murmur is audible at a site other than the point of maximal intensity it is said to radiate. It will radiate more loudly in certain directions due to the direction of the turbulent jet giving rise to the murmur, or along structures adjacent to the site of origin of the murmur. Aortic stenosis murmurs tend to radiate up the carotid arteries and are sometimes audible over the head. Mitral murmurs radiate dorsally within the thorax.
Pitch
High pitch murmurs may be more likely to be ejection murmurs and low pitch murmurs may suggest regurgitant flow. Fairly loose terms and probably more useful when trying to establish if a murmur has changed or if more than one murmur is present in the same animal.
Shape
Description from phonocardiogram diamond shape, crescendo, crescendo – decrescendo, etc. Decrescendo murmurs get less intense over time the classic example of this being aortic regurgitation.
What Else Are We Listening For?
Intensity of heart sounds. May be muffled with pleural or pericardial fluid. May be marked if there is gross cardiomegaly.
Listen for heart rhythm. Compare to pulse rate and rhythm. Always take the pulse at the same time as listening to the heart.
Finally listen to the lungs. Respiratory disease is often mistaken for heart disease due to similarity of the signs - breathlessness, coughing, etc. A lot of older small breed dogs with lung disease will have incidental heart murmurs. Listen for wheezes and crackles over lung fields. Crackles may be evident if there is pulmonary oedema present.
A Murmur Has Been Discovered - What Now?
Following the discovery of a murmur there are a number of important questions that should be considered which will help narrow down the likely nature of the underlying disease process.
How Old Is the Dog?
Animals younger than 3–4 years of age are more likely to have congenital disease rather than acquired disease.
What Breed and Size Is the Dog?
Many congenital diseases have strong breed associations. With acquired heart disease, degenerative mitral valve disease is more likely to occur in small breed dogs whereas larger dogs tend to develop dilated cardiomyopathy.
How Audible Is the Murmur?
Louder murmurs often, but not always, signify the presence of more serious disease.
Is the Animal Showing Any Clinical Signs Consistent with Heart Failure?
Signs such as increased respiratory rate and effort, exercise intolerance, lethargy and collapse episodes may occur as a consequence of heart disease leading to inadequate function of the cardiovascular system. If such signs are present it is more likely that an animal’s murmur indicates the presence of significant disease.
Are there other signs present suggestive of compromised function of the cardiovascular system these could include pallor, cyanosis, venous congestion, ascites, cold extremities, an audible arrhythmia, etc.
Further Investigation
Many factors may determine whether or not a dog will undergo further investigation.
Factors to consider include the following.
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Factor
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More likely to investigate…
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Less likely to investigate if…
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Age of patient
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Younger patient More likely to have definitive treatment and lengthy period of benefit
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Older patient
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Audibility of murmur
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Loud murmur ≥ III/VI
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Quiet murmur
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Timing of murmur
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Diastolic, continuous
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Systolic
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Presence or absence of clinical signs
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Clinical signs are present
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Clinical signs are absent
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Intended use of dog
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Dog used for breeding or as athlete
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Sedentary neutered dog
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Level of owner anxiety
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Owner anxious to know cause of murmur
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Owner happy to wait and see
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Anticipated cardiovascular stress
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Patient likely to be anaesthetised or receive fluids in near future
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Patient not expected to undergo any CV stress
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What Is the Best Method of Investigation?
Usually the initial preferred diagnostic test for investigation of the presence of a heart murmur is echocardiography including Doppler echocardiography. This is often sufficient for a diagnosis to be reached although other tests may be required to determine the severity and impact of the patient’s disease.