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In systole, ventricular pressures exceed aortic and pulmonary artery pressures as well as atrial pressures. Outflow murmurs occur due to blood flow through a stenotic semilunar valve (aortic or pulmonary) or obstructed ventricular outflow tract. Regurgitant murmurs occur due to retrograde blood flow through an incompetent atrioventricular valve (mitral or tricuspid). Physiologic or flow murmurs occur due to increased blood flow through an otherwise normal valve. Ventricular septal defects result in left-to-right shunting of blood through the defect as left ventricular pressures usually exceed those on the right.
1. Causes: Tricuspid regurgitation without pulmonary hypertension, acute mitral regurgitation.
2. Auscultation: Early systolic murmurs begin together with the S1 and terminate in mid systole.
1. Mechanism: Blood flowing through a narrowed valve or increased cardiac output in a person with a normal valve area; rarely due to inflow of blood into a dilated vessel.
2. Causes: Aortic or pulmonary stenosis, hypertrophic obstructive cardiomyopathy, structural aortic or pulmonary artery abnormalities, flow murmurs due to hyperdynamic circulation (pregnancy, fever, thyrotoxicosis, anemia), atrial septal defect (flow murmur over the pulmonic valve), small ventricular septal defect. In young individuals with asthenia, a physiologic murmur may be audible.
3. Auscultation: Midsystolic murmurs begin after the S1 (after the end of the isovolumetric contraction phase) and terminate with or before the S2. The intensity of the murmur increases and decreases with pressure difference across the valve (crescendo-decrescendo). Decreasing preload results in a louder murmur if left ventricular outflow tract obstruction is the cause of the murmur.
1. Causes: Classically due to mitral valve prolapse but may be due to other mitral regurgitation caused by papillary muscle dysfunction.
2. Auscultation: Late systolic murmurs begin in late systole and terminate immediately before or with the S2. They are best audible over the apex. Murmurs accompanied by a midsystolic click are most frequently caused by mitral valve prolapse. Increased preload results in a later-onset click and murmur if mitral valve prolapse is present.
1. Causes: Mitral or tricuspid regurgitation (backflow of blood from the ventricle into the atrium), ventricular septal defect (blood flow between the left and right ventricle).
2. Auscultation: Pansystolic murmurs are audible during the entire systole (S1 cannot be separated from the murmur).