Systolic modeling of the left ventricle as a mechatronic system: determination of myocardial fiber's sarcomere contractile characteristics and new performance indices.

Abstract

In this paper, the left ventricle (LV) is modeled as a cylinder with myocardial fibers located helically within its wall. A fiber is modeled into myocardial structural units (MSUs); the core entity of each MSU is the sarcomeric contractile element. The relationship between the sarcomere unit's contractile force and shortening velocity is expressed in terms of the LV model's wall stress and deformation, and hence in terms of the monitored LV pressure and volume. Then, the LV systolic performance is investigated in terms of a mechatronic (excitation-contraction) model of the sarcomere unit located within the LV cylindrical model wall. The governing equation of dynamics of the LV myocardial structural unit (MSU) is developed, involving the parameters of the series-elastic element (SE), the viscous element (VE) and the contractile element (CE). We then relate the MSU's force and displacement variables (in terms of SE, VE and CE parameters) to the LV pressure and volume, using the patient's catheterization-ventriculogram data. We thereby evaluate the MSU elements' parameters. We then determine the sarcomere (CE) 'force vs. shortening-velocity' characteristics as well as the power generated by the sarcomere (or CE) element. These are deemed to be important LV functional indices. When our computed sarcomeric peak-power is compared against the traditional LV contractility indices (by linear regression), a high degree of correlation is obtained. We have provided herein, a LV systolic-phase (cylindrical geometry) model whose wall contains the myocardial fibers having sarcomere units. We have expressed the LV myocardial sarcomere's CE (force vs. shortening-velocity) characteristics in terms of the LV pressure-volume data. These CE properties express the intrinsic performance capacity of the LV. Hence, indices containing these properties are deemed to reflect LV performance. In this regard, our new LV contractility index correlates very well with the traditional LV contractility index dP/dt(max).