• 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, respectively.
• 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 stenosis.

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.

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.

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