Papillary Muscle Shortening in the Intact Dog A CINERADIOGRAPHIC STUDY OF TRANQUILIZED DOGS IN THE UPRIGHT POSITION

Shortening of the anterior papillary muscle of the left ventricle was demon-strated in six intact, tranquilized dogs. Two small metal markers that had been surgically implanted 3-50 months earlier were cineradiographically photo-graphed during approximately ten sequential cardiac cycles in each of two orthogonal positions. Distances between markers were plotted for successive frames. The resulting curves were used to obtain maximum velocities of papillary muscle shortening and lengthening: 1.08 ± 0.29 muscle lengths/sec and 1.39 ± 0.48 muscle lengths/sec, respectively. From the two orthogonal planes, the average maximum spatial distance and the average minimum spatial distance between the markers were calculated. The mean percent shortening of 22.8 ± 6.5 % was surprisingly large: it approximated the distance from the foot to the peak of the ascending limb of the myocardial length-tension curve derived from isolated muscle studies. Mechanical studies on isolated papillary muscle have consistently shown reduced shortening wit