Myocardial fiber shortening in the circumferential direction produces left ventricular wall thickening during contraction.

When one bends the elbow by shortening of the biceps, a knot of muscle is observed in his or her upper arm, indicating that muscle shortening is converted to muscle standing in the perpendicular direction due to the incompressibility of skeletal muscle. A similar mechanism may work in the thickening process of the left ventricular (LV) wall. Although myocardial fibers of the left ventricle shorten by about 20% along the fiber direction when they contract, thickening of the LV wall during contraction often exceeds 50%. Thus, the aim of the present study was to clarify the mechanism by which myocardial fiber shortening produces such remarkable thickening of the LV wall. We hypothesized that myocardial fiber shortening in the circumferential direction causes myocardial transformation perpendicular to the fiber direction, thereby producing LV wall thickening. We evaluated this hypothesis using an incompressible model of the LV wall. In 15 healthy male volunteers (38±13 years), we calculated theoretical peak thickening values of the inner and outer LV wall layers and compared them with directly measured peak thickening values using Doppler strain imaging at the corresponding areas. The theoretical peak thickening and directly measured peak thickening were >60% in the LV inner layer. The theoretical peak thickening was correlated with the directly measured peak thickening in the inner (r=0.75, p<0.05) and outer (r=0.61, p<0.05) layers. We conclude that shortening of LV circumferential myocardial fiber and incompressibility of myocardium produce LV wall thickening during contraction.