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Murmurs are extra heart vibrations that are produced as a result of a disturbance in the blood flow -- enough, in fact, to produce audible noise. Often, the murmurs are classified according to a variety of characteristics, including their timing. Systolic murmurs, for example, occur when the heart muscle contracts; diastolic murmurs occur when the heart muscle relaxes between beats; and continuous and to-and-fro murmurs occur throughout all or most of the cardiac cycle.

Heart murmurs can occur in both dogs and cats. If you would like to learn more about how they affect cats, please visit this page in the PetMD health library.

Symptoms and Types

The symptoms associated with murmurs depend on a variety of characteristics, including their grade, configuration, and location. If, however, the murmur is associated with structural heart disease, your dog may display signs of congestive heart failure such as coughing, weakness, or exercise intolerance.

Grading Scale for Murmurs

• Grade I—barely audible
• Grade II—soft, but easily heard with a stethoscope
• Grade III—intermediate loudness; most murmurs which are related to the mechanics of blood circulation are at least grade III
• Grade IV—loud murmur that radiates widely, often including opposite side of chest
• Grade V—very loud, audible with stethoscope barely touching the chest; the vibration is also strong enough to be felt through the animal's chest wall
• Grade VI—very loud, audible with stethoscope barely touching the chest; the vibration is also strong enough to be felt through the animal's chest wall

Configuration

• Plateau murmurs have uniform loudness and are typical of blood regurgitation through an abnormal valvular orifice (regurgitant murmurs).
• Crescendo-decrescendo murmurs get louder and then softer and are typical of ejection murmurs due to turbulent forward flow.
• Decrescendo murmurs start loud and then get softer and are typical of diastolic murmurs.

Causes

Murmurs are caused by the following:

• Disturbed blood flow associated with high flow through normal or abnormal valves or with structures vibrating in the blood flow.
• Flow disturbances associated with outflow obstruction or forward flow through diseased valves or into a dilated great vessel.
• Flow disturbances associated with regurgitant flow due to an incompetent valve, patent ductus arteriosus, or a defect in the septum (the wall that separates the heart's left and right sides).

More specifically, the following are some conditions and diseases that may bring on murmurs:

Systolic Murmurs

• Anemia
• Hyperthyroidism
• Heartworm disease
• Mitral and tricuspid valve heart failure
• Cardiomyopathy and aortic valve insufficiency
• Mitral and tricuspid valve dysplasia
• Systolic anterior mitral motion (SAM)
• Dynamic right ventricular outflow obstruction
• Dynamic subaortic stenosis
• Aortic stenosis
• Pulmonic stenosis
• Atrial and ventricular septal defect
• Tetralogy of Fallot
• Mitral and tricuspid valve endocarditis (inflammation of the inner part of the heart)

Continuous or To-and-Fro Murmurs

• Patent ductus arteriosus
• Ventricular septal defect with aortic regurgitation
• Aortic stenosis with aortic regurgitation

Diastolic Murmurs

• Mitral and tricuspid valve stenosis
• Aortic and pulmonic valve endocarditis (inflammation of the inner layer of the heart)

Diagnosis

In order to determine exactly what is causing the symptoms, your veterinarian must differentiate between a wide range of abnormal heart sounds -- split sounds, ejection sounds, gallop rhythms, and clicks, for example. He or she also must differentiate between abnormal lung and heart sounds, and listen to see if timing of abnormal sound is correlated with respiration or heartbeat.

The location and radiation of the murmur, as well as the timing during cardiac cycle, is another way to determine the underlying cause. This can be accomplished by conducting a variety of tests, including chest X-rays, Doppler studies, and echocardiography. A complete blood count, meanwhile, is one of the preferred methods for confirming anemic murmurs.