Si is diminished in first-degree heart block (delayed conduction from atria to ventricles). Here the mitral valve has had time after atrial contraction to float back into an almost closed position before ventricular contraction shuts it. It closes less loudly. Si is also diminished (1) when the mitral valve is calcified and relatively immobile, as in mitral regurgitation, and (2) when left ventricular contractility is markedly reduced, as in congestive heart failure or coronary heart disease.
Si varies in intensity (1) in complete heart block, when atria and ventricles are beating independently of each other, and (2) in any totally irregular rhythm (e.g., atrial fibrillation). In these situations, the mitral valve is in varying positions before being shut by ventricular contraction. Its closure sound, therefore, varies in loudness.
Si may be split normally along the lower left sternal border where the tricuspid component, often too faint to be heard, becomes audible. This split may sometimes be heard at the apex, but consider also an S4, an aortic ejection sound, and an early systolic click. Abnormal splitting of both heart sounds may be heard in right bundle branch block and in premature ventricular contractions.
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