Abstract:
To study the nonuniform mechanical function that occurs in normal and ischemic ventricular myocardium, a new method has been developed and validated. An array of 25 lead markers (approximately 4 x 4 cm) was sewn onto the epicardium of the anterior free wall of the left ventricle in an open-chest, anesthetized canine preparation. Biplane cineradiography was used to track marker positions throughout the cardiac cycle before and during episodes of acute ischemia induced by occlusion of the left anterior descending coronary artery. To estimate two-dimensional nonhomogeneous deformations in the region at risk and its border zone with normally perfused tissue, surfaces defined by bicubic Hermite isoparametric finite element interpolation were fitted by least squares to the three-dimensional marker coordinates in successive cine frames. Global smoothing functions prevented ill-conditioning in areas of low marker density. Continuous distributions of systolic finite strains referred to the end-diastolic state were obtained under normal and ischemic conditions without the conventional assumption of homogeneous strain analysis. Substantial regional variations in epicardial strains were observed in both the normal and ischemic heart. The method was validated in regions of small to moderate strain variations by comparing the continuous distributions of strain components with piecewise-constant measurements made using marker triplets and homogeneous strain theory. The influence of marker density was examined by recomputing strains from surfaces fitted to subsets of the original array. Further validation of moderate to large strain variations was obtained by simulating a nonuniform distribution of stretch across a planar sheet and computing strains both analytically and using the current method. The new method allows for more comprehensive measurements of distributed ventricular function, providing a tool with which to quantify better the nonhomogeneous function associated with regional ischemia.
