Right-sided systolic murmurs. Right-sided systolic murmurs are often due to tricuspid insufficiency or ventricular septal defects.
- Tricuspid insufficiency murmurs: The most audible region for tricuspid insufficiency murmurs is at about the third to fifth
intercostal spaces near the costochondral junction on the right hemithorax. Similar to mitral insufficiency, regurgitation
through the tricuspid valve produces a variable-intensity and variable-duration, systolic, plateau-shaped murmur, which is
often present in cases of chronic degenerative tricuspid valve disease, tricuspid valve dysplasia, dilated cardiomyopathy,
or pulmonary hypertension. Unfortunately, loud murmurs of mitral insufficiency frequently radiate to this location, making
differentiation of pure mitral valve involvement vs. mitral and tricuspid valve disease difficult based on the physical examination.
Murmurs of tricuspid insufficiency must also be differentiated from subaortic stenosis with marked right-sided radiation and
a left-to-right shunting ventricular septal defect.
- Ventricular septal defect murmurs: The murmurs that accompany ventricular septal defects vary depending on the location and
size of the defects. The severity of the lesion is often inversely proportional to the intensity of the murmur: larger defects
have softer murmurs, and smaller defects have louder murmurs. The most common murmur associated with a small, restrictive
left-to-right ventricular septal defect is a loud, high-frequency, systolic, plateau-shaped murmur with the point of maximal
intensity at the right cranial thorax. Larger, nonrestrictive, left-to-right ventricular septal defects produce softer, right-sided,
systolic murmurs, while patients with balanced ventricular septal defects due to marked pulmonary hypertension lack audible
murmurs and have a split S2.3
Parasternal murmurs. Cats with mitral valve dysplasia or left ventricular concentric hypertrophy (e.g. hypertrophic cardiomyopathy, hyperthyroidism, systemic hypertension) may have a left ventricular outflow tract obstruction
in which the anterior mitral valve leaflet is pushed, pulled, or sucked into the outflow tract. This phenomenon is often accompanied
by mitral insufficiency and high-velocity, turbulent blood flow through the left ventricular outflow tract, producing a harsh,
left apical or sternal, systolic murmur that intensifies with increasing heart rates and contractility. Heart murmurs associated
with dynamic right ventricular outflow tract obstruction display similar characteristics and can often only be differentiated
through Doppler echocardiography.
Diastolic murmurs. Diastolic murmurs are often due to aortic insufficiency or mitral stenosis.
- Aortic insufficiency murmurs: A left basilar, diastolic, decrescendo murmur of aortic insufficiency in an adult dog is an
important finding because it is often related to bacterial endocarditis. Bacterial colonization of the aortic valve followed
by erosion of the valve stroma promotes valvular insufficiency. Aortic insufficiency may also occur as an isolated congenital
defect, in conjunction with subaortic stenosis, or in combination with a ventricular septal defect subsequent to malalignment
of the aortic root. If the volume of aortic insufficiency is large, the soft, blowing decrescendo murmur is often accompanied
by a left basilar, systolic physiologic murmur because of the enhanced left ventricular stroke volume. Be sure to distinguish
this distinctive to-and-fro murmur from the continuous murmur of patent ductus arteriosus.
- Mitral stenosis murmurs: Mitral valve dysplasia may result in insufficiency of the valve, stenosis of the valve, systolic
anterior motion of the valve, or any combination of the three. Mitral stenosis, including valvular stenosis, supravalvular
stenosis, and cor triatriatum sinister, is uncommon in dogs and cats, and physical examination findings in these patients
are extremely variable. The classic murmur associated with mitral valve stenosis in people is a soft, low-pitched, rumbling,
left apical, diastolic murmur. Despite careful auscultation by clinicians in one study, only four of 15 dogs with mitral stenosis
displayed this classic left apical diastolic murmur, while eight of 15 displayed a left apical systolic murmur of mitral insufficiency.5
Continuous murmurs. Residual patency of the ductus arteriosus results in high-velocity, turbulent blood flow entering the pulmonary artery from
the descending aorta throughout both systole and diastole. So the characteristic murmur of a patent ductus arteriosus is a
variable-intensity, left basilar (axillary), continuous murmur that peaks in intensity near S2. The diastolic run-off associated with this abnormal communication will commonly produce bounding femoral pulses. Patent
ductus arteriosus murmurs may range in intensity from grade I to grade VI, depending on the size of the lesion, the aortic
to pulmonary arterial pressure gradient, and the radiation characteristics of the turbulent blood flow. In cases of a balanced
or a right-to-left patent ductus arteriosus, the murmur may be absent with a split S2.
Although uncommon, other potential causes of continuous heart murmurs include aorticopulmonary windows, pulmonary arteriovenous
fistulas, coronary arteriovenous fistulas, and abnormal communications between the coronary vasculature and right atrium.