Factors influencing left ventricular shortening in isolated canine heart.

To assess in the intact ventricle the steady-state influence of several mechanical variables on the extent of left ventricular midwall circumferential shortening, a pressure servo system was utilized in isolated canine hearts. The system permits continuous monitoring of ventricular volume and control of diastolic and systolic pressures. After determination of the diastolic volume at zero filling pressure (V0), a series of variably preloaded or afterloaded contractions were generated including the isovolumic state. Contractile state was manipulated in a positive (calcium, 12-18 mg/100 ml; norepinephrine, 0.4-1.4 mug/min) or negative (propranolol, 0.12-0.50 mg/min) direction. Force and length terms derived for a thick-walled sphere were expressed per cross-sectional area of muscle and length, respectively, calculated at V0. For any preload, an inverse linear (r greater than 0.96) force-shortening relation was obtained, and each line was identified by its slope and isovolumic load (sigma0). Both slope and sigma0 increased with positive inotropic agents (vis-a-vis propranolol) or increments in preload. Thus, in the intact ventricle an inverse linear relation characterizes the force-shortening relation with the amount of shortening determined by initial fiber length, afterload, and the contractile state of the myocardium.