Three-dimensional finite element analysis of anterior-posterior variations in local sarcomere length and active fiber stress during left ventricular ejection

We used a finite element model for ventricular mechanics to interpret our experimental observations from the isolated supported canine heart that the amount of sarcomere shortening per ejected volume is similar at epicardial and midwall sites in the anterior and posterior left ventricular free walls, but any column during ejection sarcomeres are significantly shorter at the anterior epicardoum than at the other three sites. An elastance model for active fiber stress was incorporated in a three-dimensional model that included an accurate representation of the left ventricular free wall geometry and fiber angle distribution in the dog heart, and a nonlinear constitutive equation for the resting myocardium that was transversely isotropic with respect to the local fiber axis. This model was able to reproduce our experimental observations of anterior-posterior variations in sarcomere length during ejection remarkably well using a uniform stress-free sarcomere length distribution, the same transmural fiber angle distribution in all regions, and homogeneous material properties. Hence, our model suggests that geometric differences between the anterior and posterior free walls are responsible for variations in sarcomere lengths between these sites during ejection.