A mechanical model of mitral insufficiency using canine papillary muscle preparation.

A canine papillary muscle is loaded to mimic the load of the myocardium in the wall of the left ventricle with atrio-ventricular valvular insufficiency. This mechanical model which simulates the atrio-ventricular valvular regurgitation is based on two simple assumptions. The assumptions arranged for the papillary muscle experiment are as follows: 1) the force that the myocardium encounters during muscle shortening is proportional to the muscle shortening velocity due to regurgitation through a narrow regurgitant orifice; and 2) the myocardium exerts a constant force while the aortic valve is open. The muscle shortening except the isotonic phase is ascribed solely to regurgitation since the aortic valve is closed during these phases. In the combined antegrade/retrograde ejection phase, which is characterized by a constant muscle force, the shortening velocity due to regurgitation is constant because of the assumed functional relation between the muscle force and shortening velocity. The amount of shortening assigned to regurgitation in this phase is given by the product of the velocity at the beginning point of the isotonic phase and the duration of this phase. The present in-vitro studies offer an alternative explanation for decrease in the regurgitant fraction as total load was reduced at a constant preload. The regurgitant fraction decreased as preload was increased at a constant total load in the present study. The regurgitant fraction also decreased by either isoproterenol or CaCl2 administration via the coronary artery.