Diastolic Geometry and Sarcomere Lengths in the Chronically Dilated Canine Left Ventricle

In 10 dogs left ventricular geometry and ultrastructure were examined 3 to 12 weeks after chronic ventricular dilatation was induced by means of a large arteriovenous shunt. Following cardiac catheterization, the hearts were arrested and fixed in diastole at the left ventricular end-diastolic pressures (LVEDP) which existed in the beating heart. Transmural LVEDP were increased in all animals (avg. 27 mm Hg). The diastolic pressure-volume relationship was shifted to the right, the average end-diastolic volume of the chronically dilated ventricles (avg. 103 ml) being larger than that of previously studied acutely dilated ventricles (avg. 72 ml, P <.01), but calculated diastolic wall stress values were not different. Sarcomere lengths in the chronically dilated hearts averaged 2.19±.02 µ ( SE ) (range 2.11 to 2.27), a value near the apex of the sarcomere length-tension curve, but not significantly different from sarcomere lengths in acutely dilated ventricles. Slippage between myofibrils, reflected by a loss of normal alignment of the Z lines, appeared to be one mechanism underlying this adaptation. The findings indicate that sarcomeres in the canine left ventricular wall are remarkably resistant to chronic as well as to acute overstretch. They further imply that little or no Frank-Starling reserve mechanism was available in these ventricles and raise the possibility that a descending limb of the heart's performance as a pump need not reflect a descending limb at the sarcomere level.