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[RC] Definition of Murmurs and Arrhythmias from the Merck Vet Manual. - Katrina Mosshammer

Heart murmurs are audible vibrations emanating from the heart or major blood vessels and generally are the result of turbulent blood flow or vibrations of cardiac structures such as part of a valve leaflet or chordal structure. Murmurs are typically defined relative to timing, intensity, and location, but can also be characterized by frequency (pitch), quality (eg, musical), and configuration (eg, crescendo-decrescendo). A systolic murmur occurs during systole and is typically either ejection (crescendo-decrescendo) or regurgitant (holosystolic, plateau). Ejection systolic murmurs demonstrate the greatest intensity during mid-systole and appear diamond-shaped on phonocardiography. They can be produced by stenotic lesions at the semilunar valves (eg, pulmonic stenosis or subaortic stenosis). Regurgitant systolic murmurs demonstrate a constant intensity throughout systole and can be caused by mitral or tricuspid regurgitation (eg, myxomatous degeneration of the mitral valve). Diastolic murmurs are typically decrescendo (decreasing in intensity through diastole) and a result of aortic or pulmonic insufficiency (such as that caused by aortic valve infective endocarditis). Continuous murmurs are most commonly a result of patent ductus arteriosus (a congenital cardiac defect) and occur throughout systole and diastole. Continuous murmurs vary in intensity over time, typically being most intense at the end of ventricular ejection and decreasing in intensity through diastole. A to-and-fro murmur occurs in patients that demonstrate both a systolic murmur and a diastolic murmur and can occur in patients with a ventricular septal defect and aortic valve insufficiency or in a patient with subaortic stenosis and aortic insufficiency.
In horses, early systolic and diastolic murmurs can be noted in the absence of heart disease or anemia. The point of maximum intensity is typically located over the left heart base. A short, high-pitched, squeaking, early diastolic cardiac murmur is sometimes seen in healthy young horses. Occasionally, systolic murmurs are noted in some cats secondary to an increase in right midventricular flow velocity without significant structural heart disease. Innocent cardiac murmurs are also commonly noted in immature cats and dogs (<6 mo of age) as a result of increased stroke volume.
Heart murmurs are classified as follows: Grade I?the lowest intensity murmur that can be heard, typically detected only while auscultation is performed in a quiet room; Grade II?a faint murmur, easily audible, and restricted to a localized area; Grade III?a murmur immediately audible when auscultation begins; Grade IV?a loud murmur immediately heard at the beginning of auscultation but not accompanied by a thrill; Grade V?a very loud murmur with a palpable thrill, the loudest murmur that is still inaudible when the stethoscope is just removed from the chest wall; or Grade VI?an extremely loud murmur that can be heard when the stethoscope is just removed from the chest wall.
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Arrhythmias are abnormalities of the rate, regularity, or site of cardiac impulse formation and are noted during auscultation. Other terms such as dysrhythmia and ectopic rhythm are also used to describe arrhythmias. The presence of a cardiac arrhythmia does not necessarily indicate the presence of heart disease; many cardiac arrhythmias are clinically insignificant and require no specific therapy. Some arrhythmias, however, may cause severe clinical signs such as syncope or lead to sudden death. Numerous systemic disorders may be associated with abnormal cardiac rhythms. (For discussion of specific arrhythmias, see Electrocardiography.)