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Mitral stenosis (MS) is the term used to describe a narrowing of the mitral valve, the valve between the heart's left atrium and left ventricle. Blood returning from the lungs moves through the leaves of the mitral valve on its way to the ventricle before being pumped to the body, but the importance of contraction of the left atrium in forcing enough blood through the narrowed mitral valve is not known. It has been suggested that atrial contraction could be an important cause of atrial fibrillation in MS patients when the chamber contracts but fails to deliver an adequate supply of blood to the ventricle. The contribution of atrial contraction to left ventricle filling was assessed using echocardiography in 30 patients with rheumatic MS. The volume of blood flowing through the mitral valve with each beat of the heart is the sum of two quantities: the amount that passively fills the left ventricle (even if there were no contribution from the atria) and the atrial contribution. The atrial contribution can be expressed as a percent of total mitral valve stroke volume (percent atrial contribution). It was calculated and correlated with other measures of mitral valve and atrial function. For the 30 patients as a whole, the atrial contribution varied between two percent and 34 percent. During bicycle exercise, the percent atrial contribution decreased by an average of three percent. In general, the atrial contribution to left ventricle filling varies according to the severity of MS, becoming less significant as the condition worsens. It is not a very important contribution during either rest or exercise and is therefore an unlikely cause of atrial fibrillation in people with MS. (Consumer Summary produced by Reliance Medical Information, Inc.)