Role of the papillary muscle in opening and closure of the mitral valve

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Abstract
Dimensional changes of the left ventricular anterolateral papillary muscle of 6 open-chest dogs were measured continuously throughout the cardiac cycle to evaluate the role of the papillary muscle in mitral valve opening and closing. Dimensional changes were measured with ultrasonic dimension gauges. Maximal shortening and maximal elongation of the papillary muscle followed maximal shortening and elongation of a segment of the free wall of the left ventricle by 65 .+-. 6 (SE) ms. Maximal elongation of the papillary muscle occurred 25 .+-. 2 ms after the onset of ejection. Maximal shortening of the papillary muscle occurred 68 .+-. 5 ms after the aortic incisura and 10 .+-. 2 ms after the crossover of left ventricular and left atrial pressure. The papillary muscle shortened 14 .+-. 4%. The percentage of papillary muscle shortening that occurred after the aortic incisura was 39 .+-. 7%, and the percentage of shortening that occurred after the crossover of left ventricular and left atrial pressure was 3 .+-. 1%. The observed shortening of the papillary muscle throughout left ventricular isovolumic relaxation suggests that the papillary muscle may have a role in opening the mitral valve. Elongation of the papillary muscle in the late portion of diastole appears necessary to permit proper closure of the mitral valve leaflets.

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