Effect of the Twisting Motion on the Nonuniformities of Transmyocardial Fiber Mechanics and Energy Demand—A Theoretical Study

The contraction of the left ventricle (LV) is manifested by a distribution of strains and strain rates throughout the muscle thickness. Using a nested shell spheroidal model of the LV, which accounts for a fiber angle distribution from + 60° at the endocardium to — 60° at the epicardium, and the radial electrical activation pattern from the endocardium to the epicardium, it can be shown that endocardial layers undergo higher strains than the epicardial layers throughout the cardiac cycle, and higher length changes characterize the endocardial sarcomeres relative to the epicardial sarcomeres. However, the calculated nonuniformities in the sarcomeres’ shortening are significantly moderated when the physiological twisting motion of the LV around the longitudinal axis is accounted for. Thus, the twisting motion of the heart is a basic mechanism by which the sarcomere function is maintained within its physiological range. The model is extended to relate the local sarcomere length changes and stress to the local oxygen demand throughout the myocardium. The local oxygen demand is calculated from the local stress-sarcomere length loops, analogous to Suga’s pressure-volume area approach to evaluate the global LV oxygen demand. The oxygen demand of the inner-half LV wall is higher by 23 percent than the outer-half wall when twist is not accounted for. Consistent with physiological measurements, these results correspond to higher stresses as well as higher external work which characterize the endocardial sarcomeres. The twisting motion of the LV tends to equalize the mechanical parameters as well as the energy demand distribution across the myocardium.