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In diastole, aortic pressure exceeds left ventricular pressure and pulmonary artery pressures exceed right ventricular pressures. Regurgitant murmurs occur when an incompetent semilunar valve allows backflow from the higher-pressure great vessels into the ventricles. Other murmurs occur as turbulent blood flow occurs from the atria to the ventricles over stenotic atrioventricular valves.
Diastolic murmurs are always pathologic.
1. Mechanism: Blood flowing back through a regurgitant aortic or pulmonary valve when ventricular pressure is lower than pressure in the aorta or main pulmonary artery.
2. Causes: Aortic regurgitation, pulmonary regurgitation.
3. Auscultation: A soft, “blowing,” high-frequency decrescendo-type murmur along the sternal border. The more severe the regurgitation, the louder the murmur. Pulmonic regurgitation increases with inspiration.
1. Mechanism: Blood flow from the atria to the ventricle through a stenotic atrioventricular valve.
2. Causes: Mitral stenosis, tricuspid stenosis, relative stenosis due to increased flow through the mitral or tricuspid valve (severe mitral or tricuspid regurgitation, left-to-right shunts), atrial myxoma. The Austin Flint murmur is functional mitral stenosis due to an aortic regurgitation jet directed at the mitral leaflets.
3. Auscultation: Middiastolic murmurs are usually low frequency, “rumbling,” and soft. They begin some time after the S2. An opening snap heard in diastole or presystolic accentuation (correlating to atrial kick) can be heard in mitral stenosis.
1. Mechanism: Increased flow through a stenotic valve in the late stages of ventricular filling (shortly before the S1) due to atrial contraction.
2. Causes: Tricuspid stenosis, mitral stenosis, Austin Flint murmur (functional mitral valve obstruction from the aortic regurgitation jet).
3. Auscultation: Usually a soft murmur of a crescendo-decrescendo or crescendo type. Murmurs of mitral and tricuspid stenosis are absent during atrial fibrillation.