heart murmurs are soft sounds, like those made during a forcible expiration with the mouth open. They are most often caused by any departure from the crisp sounds described here which reflect equally crisp function of the valves. Valves may become leaky, allowing backflow (incompetence), or they may become stiff and narrow (stenosis). Stenosis, for example, of the aortic valve leads to an abnormal turbulence during ejection of blood through it from the left ventricle, so it is a systolic murmur because it occurs during the phase of contraction (systole), preceding the second heart sound at the closure of the valve. Incompetence of this same valve allows some blood to flow back into the ventricle causing a diastolic murmur at the time of the second sound when it closes incompletely during ventricular relaxation (diastole). An incompetent mitral valve allows blood to regurgitate into the left atrium throughout ventricular contraction, causing a systolic murmur, whereas mitral stenosis causes a murmur as the blood flows from atrium to ventricle during diastole.
Other characteristic murmurs are caused by congenital defects, such as a hole in the ventricular septum, or a failure of closure of the connection between aorta and pulmonary artery which is present before birth (patent ductus arteriosus).
A murmur is classified with respect to its intensity (just audible, soft, moderate, or loud), the point on the chest wall where the murmur is heard best (which assists its localization to a particular valve or part of the heart), and the timing of the murmur in the heart beat (in relation to the first and second heart sounds).
Not all murmurs signify abnormality of the heart. Systolic murmurs, due to normal turbulence during ventricular contraction and ejection of blood, may be present particularly in young people, and not uncommonly in exercise and in pregnancy.
David J. Miller, and Niall G. MacFarlane
See also heart; sounds of the body.
"heart sounds." The Oxford Companion to the Body. . Encyclopedia.com. (December 13, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/heart-sounds
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