The Contractile Properties of Human Heart Muscle: Studies on Myocardial Mechanics of Surgically Excised Papillary Muscles*

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Abstract
The contractile properties of human heart muscle were studied in left ventricular papillary muscles exised from 19 patients at the time of prosthetic replacement of the mitral valve. Analysis of the isometric length-tension relations revealed that the peak of the active tension curve was reached when the muscle was stretched to an average length that exceeded the initial length by 51%. Further increases in muscle length produced a precipitous rise in resting tension, while active tension declined. An inverse relation between afterload and initial velocity of shortening was observed in every muscle, extending to human heart muscle the concept of force-velocity relations. When initial muscle length was increased, isometric tension was augmented but the maximal velocity of shortening remained constants. The addition of strophanthidin or norepinephrine increased the maximal velocity of shortening as well as the isometric tension, whereas increasing the frequency of contraction augmented the maximal velocity of shortening only. It is suggested that the contractile state of the myocardium can be described by the force-velocity relation. These studies on the contractile properties of papillary muscles provide a framework for an analysis of the performance of the intact human myocardium in terms of fundamental muscle mechanics.

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