- Timing: Murmurs are classified into three broad categories according to their timing within the cardiac cycle. Systolic murmurs
begin with or after S1 and end at or before S2, diastolic murmurs begin with or after S2 and end at or before S1, and continuous murmurs begin in systole and continue without interruption beyond S2 into all or part of diastole. Within this time frame, murmurs may be further characterized as early systolic, midsystolic,
late systolic, holosystolic, early diastolic, mid-diastolic, late diastolic, or holodiastolic.
- Intensity: Although the intensity of a heart murmur does not always correlate with disease severity, adhering to a grading
system permits meaningful serial examinations and clear discussion among clinicians. The intensity of heart murmurs may be
accentuated in patients with high cardiac output states or in thin or anemic patients, whereas obesity, muscular chest walls,
marked pulmonary disease, pericardial effusion, or decreased cardiac output may reduce their intensity. The system most commonly
used to describe the intensity of heart murmurs is based on a scale of I to VI (Table 1).
- Frequency: Audible frequencies vary from high to low and are best auscultated with the diaphragm or bell of the stethoscope,
- Configuration: Commonly auscultated murmurs are called plateau-shaped or band-shaped if they maintain similar intensity throughout their duration, crescendo-decrescendo or diamond-shaped if they build to a peak followed by a decline, or decrescendo if they taper from an initial peak (Figure 1).
- Location and direction of radiation: Murmurs are described by their point of maximal intensity over the cardiac valve area
where they are heard the loudest (Figure 3). Furthermore, the radiation characteristics associated with the direction of blood flow responsible for the murmurs may
help differentiate murmurs that display a similar point of maximal intensity, such as those due to pulmonic vs. subaortic
Left basilar systolic murmurs.
There are five types of left basilar systolic murmurs—innocent murmurs, physiologic murmurs, and murmurs caused by subvalvular
aortic stenosis, pulmonic stenosis, and atrial septal defects.
Figure 3. The areas of favored projection for murmurs on the left side of the chest include the left basilar region (1 & 2),
the left apical region (3), and the left parasternal region (4). Murmurs arising from the pulmonary valve in dogs are generally
loudest at the third intercostal space (1). Aortic valve lesions are usually most audible between the third and fourth intercostal
spaces just below the shoulder line (2), while mitral valve murmurs have a point of maximal intensity at the fifth intercostal
space near the costochondral junction (3). Another important area for auscultation in cats is the left parasternal region
(4). Heart murmurs associated with tricuspid valve disease in dogs are usually loudest at the right fourth intercostal space
near the costochondral junction (5). Similar to the left parasternal region, the right parasternal region (6) must be carefully
examined in cats to detect dynamic outflow tract murmurs.
- Innocent murmurs: Young animals without evidence of underlying cardiac disease may have these soft, grade I to III/VI left
basilar systolic murmurs. The murmurs are most commonly midsystolic and high-frequency and are devoid of substantial radiation.
Although their origin is uncertain, it is thought that innocent murmurs represent increased blood flow velocity and turbulence
through the right or left ventricular outflow tracts.3 These murmurs are usually nonprogressive and typically disappear in early adulthood.
- Physiologic murmurs: Similar to innocent murmurs, physiologic or functional murmurs tend to be soft, grade I to III/VI systolic
left basilar murmurs. They are recognized in conditions of high cardiac output (e.g. hyperthyroidism, pyrexia, pregnancy) or decreased blood viscosity (e.g. anemia) without substantial organic cardiac disease. These murmurs generally disappear after the underlying disease process
is resolved. While some cats with hyperthyroidism display soft physiologic murmurs because of increased cardiac output, they
may also display more intense and pathologic murmurs associated with systolic anterior motion of the mitral valve.
- Subaortic stenosis murmurs: One of the most common congenital cardiac conditions in dogs is a fibromuscular band just below
the aortic valve, called subaortic stenosis. This lesion obstructs left ventricular ejection, causing high-velocity, turbulent blood flow through the left ventricular
outflow tract and across the aortic valve. Subaortic stenosis produces variable-intensity, left basilar, systolic, crescendo-decrescendo
murmurs that may radiate extensively to the right cranial thorax. Mild subaortic stenosis often produces a soft, left basilar,
systolic murmur that is difficult to distinguish from an innocent or physiologic murmur. Moderate to severe subaortic stenosis
commonly produces a harsh, mixed-frequency, left basilar, systolic murmur that is more intense and longer in duration than
murmurs due to mild subaortic stenosis and than physiologic or innocent cardiac murmurs. These more severe murmurs may radiate
up the thoracic inlet along the carotid arteries and, on rare occasions, may be audible on top of a patient's head. Radiation
characteristics and evaluation of the femoral pulse quality often help distinguish severe subaortic stenosis (radiation up
the carotid arteries with weak femoral pulses) from severe pulmonic stenosis (rare radiation up the neck with normal femoral
- Pulmonic stenosis murmurs: The most common congenital anomaly of the pulmonic valve is valvular dysplasia wherein the leaflets
are fused or tethered. Their inability to open normally during right ventricular systole produces high-velocity, turbulent
blood flow across the pulmonic valve with radiation into the pulmonary arteries. So pulmonic stenosis produces a variable-intensity,
left basilar, systolic, crescendo-decrescendo murmur. Compared with subaortic stenosis murmurs, pulmonic stenosis murmurs
do not radiate as extensively to the right cranial thorax or up the carotid arteries, and the femoral pulse quality tends
to be normal. Pulmonic stenosis may be complicated by tricuspid insufficiency and signs of right-sided heart failure. It is
important to recognize that one component of the cyanotic congenital defect tetralogy of Fallot is pulmonic stenosis, but
this complex congenital heart defect is further complicated by a high ventricular septal defect, juxtaposition of the aorta,
and right ventricular hypertrophy. The therapies for uncomplicated pulmonic stenosis and tetralogy of Fallot are strikingly
different, so echocardiography is vital to distinguish between these conditions.
- Atrial septal defect murmurs: Although atrial septal defects are uncommon in dogs and cats, the defects are frequently accompanied
by a soft, left basilar, systolic, crescendo-decrescendo murmur. This murmur is produced by increased right ventricular stroke
volume (subsequent to the left-to-right shunting of blood) flowing across a normal pulmonary valve. An additional auscultatory
finding may include fixed splitting of S2.
Left apical systolic murmurs. The most common cardiac auscultatory abnormality in middle-aged to older small-breed dogs is a variable-intensity, left apical,
systolic, plateau-shaped murmur of mitral insufficiency. These murmurs vary in intensity from grade I to VI, depending on
the severity of insufficiency, the left ventricular to left atrial pressure gradient, the radiation characteristics of the
insufficient jet, and the presence or absence of comorbid conditions (e.g. pericardial or pleural effusion, obesity, abnormal chest conformation). Soft, grade I/VI mitral insufficiency murmurs are
usually early systolic to midsystolic and fail to obscure either S1 or S2. Louder, grade IV to V/VI mitral insufficiency murmurs are often harsh and mixed-frequency, radiate widely to the right thorax
(making documentation of tricuspid insufficiency difficult), and encompass all of systole.
Chronic degenerative valvular disease is the most common cause of mitral insufficiency, although diseases that produce eccentric
hypertrophy or dilatation of the mitral valve annulus (e.g. dilated cardiomyopathy, left-to-right patent ductus arteriosus, ventricular septal defect) or alterations of the mitral valve
apparatus (e.g. mitral valve dysplasia, valvular endocarditis, systolic anterior motion of the mitral valve) are also associated with mitral
insufficiency. Therefore, a left apical, systolic, plateau-shaped murmur is suggestive of mitral insufficiency although it
is not diagnostic of the underlying disease process.
Standardize your auscultation technique