A technique is described to localize the anterolateral papillary muscle and to assess its performance in vivo. Using this technique, we measured sequentially the pressure generated within the anterolateral papillary muscle and its changes in length during the cardiac cycle in eight open-chest anesthetized dogs. Pressure within the anterolateral papillary muscle was measured with a 1.6 mm diameter micromanometer probe. Its dimensional changes were measured with ultrasonic crystals. Pressure

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... n the anterolateral papillary muscle exceeded left ventricular pressure throughout the entire cardiac cycle. A difference of 200 + 23 mm Hg was present between systolic pressure in the anterolateral papillary muscle and left ventricular systolic pressure (348 + 25 vs 149 + 6 mm Hg) (p < .001). Shortening of the anterolateral papillary muscle began 25 2 msec after the upstroke of the aortic pressure, continued throughout isovolumic relaxation, and was maximal 68 5 msec after the apex of the aortic incisura. The extent and velocity of shortening of the anterolateral papillary muscle were maximal when pressure within the muscle was lowest. This temporal dissociation between pressure and dimensional changes of the anterolateral papillary muscle appeared to result from cyclic changes of loading imposed on the muscle.