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WSV18-0194
TIPS FROM THE EXPERTS FOR THE APPROACH OF...
THE DOG WITH A HEART MURMUR
A. Boswood1
1Royal Veterinary College, Clinical Science and Services, London, United Kingdom
Approach to The dog with a heart murmur Adrian Boswood
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 thing 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.
Systole
Diastole
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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.
    AV valves closed M + T insufficiency
 AV valves open M + T stenosis (low pressure)
 Outflow valves open A + P stenosis
 Outflow valves closed A + P insufficiency
 Aortic pressure > PA pressure flow through PDA
 Aortic pressure > PA pressure flow through PDA
 LV pressure > RV pressure flow through VSD
 LV pressure = RV pressure No flow through VSD
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 S and S 12
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
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