Although global measures of ventricular pump function have been useful for assessing total cardiac performance, it is now well recognized that regional estimates of myocardial stress and strain distributions are needed to understand the underlying cardiac mechanics. There are many important reasons for studying myocardial mechanics: (1) Ventricular wall stress is a primary determinant of myocardial oxygen consumption and coronary blood flow; (2) Accurate distributions of ventricular stress and strain are required to interpret the localized contractile disorders caused by myocardial ischemia and infarct; (3) Growth and hypertrophy of intact cardiac muscle respond to changes in wall stress; (4) Understanding the ultrastructural basis of cardiac mechanics requires knowledge of the stress-strain relationships in intact myocardium. The general objective of these studies will be to gain a detailed understanding of the regional mechanics of the heart by determining the distributions of stress and strain in the intact ventricular myocardium. Since the direct measurement of wall stresses is not reliable, a newly developed computational method that realistically stimulates the complex three-dimensional geometry and fiber architecture, the large deformations, and the nonlinear, anisotropic material properties of the ventricular wall will be used to model the mechanics of the intact heart. The model will predict myocardial stresses and strains in the passive and active, normal and locally impaired heart under suitable loading conditions. It will also be used to analyze the mechanical effects of changes in coronary perfusion pressure and to investigate the significance of 'residual' stresses in the myocardium. To validate the theoretical models and assess the accuracy of their results, predicted ventricular wall strains (deformations) will be compared with experimentally measured strains. Three-dimensional regional deformations will be measured by biplane radiography of small metal beads implanted at sites in the left ventricular free wall of carefully controlled isolated arrested and isolated supported canine heart preparations. By altering the material parameters in the model to improve the agreement between the theoretical and experimental deformations, more reliable estimates of the material properties of intact cardiac muscle will be obtained. This information will provide a better insight into the relationship between structure and function in the intact myocardium, and the model itself will provide a quantitative means for interpreting the complex ventricular wall motions observed in the normal and abnormal heart.
