In modeling the mechanics of the heart, a major problem is finding parameters on geometry, myofiber structure, tissue properties and hemodynamic loading. Measurement of these parameters is often cumbersome, and not realistic in clinical practice when trying to simulate cardiac mechanics patient-specifically.
In the real heart, geometry and structure is known to adapt to mechanical load, mainly directed to normalize mechanical load in the constituting tissue. Therefore, in stead of performing detailed geometric measurements, we reduce the problem of finding geometric parameters by using adaptation rules, based on physiological observations. Adaptation rules describe geometric or structural responses of tissue to changes in mechanical load.
An obvious rule is that cardiac mass increases with pump load. To distinguish concentric and eccentric hypertrophy, normalization of myofiber shortening during ejection is used additionally. A likely mechanism for that is the property of cardiac tissue to soften, when being subject to large deformation (strain softening). On cardiac structure, myofibers appear to orient so that both myofiber stress and myofiber strain are uniformly distributed within the heart. Furthermore, the sheet structure within the cardiac tissue appears to be directed along the planes of maximum shear deformation.
Using a set of appropriate adaptation rules, important features of the structure of the heart are generated by the model of heart mechanics itself. In investigating feasibility of patient-specific modeling of cardiac mechanics, in some examples it has been shown that the few measurements that could be made, were supplemented successfully by simulation of self-structuring of the heart.
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