Comparison of Directly Measured Left Ventricular Wall Stress and Stress Calculated from Geometric Reference Figures

Abstract

Mean left ventricular wall force was determined with a calibrated transmural auxotonic strain gauge in the left ventricle of six anesthetized, open-chest dogs with intact circulation. The gauge was oriented in the plane of the minor left ventricular equator, midway between the papillary muscles. Left ventricular internal volume was derived from the passive pressure-volume curve of the arrested heart and calculated mean wall stress was derived both from spherical and ellipsoidal reference figures for the left ventricle and compared with measured forces. Control left ventricular end-diastolic pressure averaged 3.0 ± 0.6 mm Hg (SE). At this level of end-diastolic pressure, measured peak wall stress averaged 97.2±14.4 g/cm², whereas calculated peak wall stress averaged 79.3±9.9 and 118.6±12.9 g/cm², for the spherical and ellipsoidal models, respectively. Measured end-diastolic wall force values averaged 9.4±4.5 and 29.2±8.1 g/cm² at an end-diastolic pressure of 3.0 and 12.3 mm Hg, respectively. In all cases, stress values calculated from spherical reference figures for the left ventricle were significantly lower than those measured directly. In four other experiments, using right heart bypass, the ventricular septum was exposed and active wall force was determined at two or more sites on the left ventricular minor equator. Wall stress at these sites differed by an average of 15.3%, indicating that stresses around the minor equator are relatively uniform. These studies lend validity to the application of geometric models in the calculation of mean wall stress and favor the application of an ellipsoid for the geometric reference figure.