Daniel Zinemanas

Effects of myocardial contraction on coronary blood flow: an integrated model.

The effects of myocardial contraction on the coronary flow are studied by means of an integrated structural model of left ventricular (LV) mechanics, coronary flow, and fluid and mass transport. This model relates global LV performance, and in particular coronary flow dynamics, to myocardial composition and structure and contractile sarcomere activity. Extravascular pressure is identified with hydrostatic tissue pressure,i. e., intramyocardial pressure (IMP), and is determined by the dynamics of myocardial contraction and fluid transport. Consistent with available experimental data, changes in myocardial function and contractile state are simulated by changing the sarcomere contractile properties or changing the LV loading conditions. The models predictions are successfully compared with a wide range of experimental studies; all but one were performed at a constant coronary perfusion pressure and maximal vasodilation. The results indicate a domiant effect of the myocardial contractile state on coronary flow and a dissocation between coronary compression and LV cavity pressure (LVP) when the pressure is controlled by load changes. However, when active sarcomere contraction is regionally impaired by lidocaine, LVP plays an important role in the coronary flow characteristics. The model adequately predicts observations on the effect of cardiac contraction on systolic and diastolic coronary flows, as well as the role of LVP at different loading and contractile conditions. The analysis supports the hypothesis that coronary compression, as mediated through IMP, is independent of LV loading conditions and depends on myocardial contractility and coronary perfusion pressure.

Relating mechanics, blood flow and mass transport in the cardiac muscle

Abstract Myocardial mechanics, perfusion and across-capillary mass transport are functionally related. The effects of these interacting phenomena on the performance of the left ventricle (LV) are investigated here. The effect of fluid balance on the diastolic and systolic intramyocardial pressures ( IMP ) and the interstitial and myocardial volumes as well as the global ventricular mechanics are of particular interest. The LV is approximated by a cylindrical geometry, containing blood vessels imbedded in the interstitial fluid and a fibrous matrix with active and passive elements. The coronary circulation is described by pressure dependent resistance-capacitance analog elements. Fluid and mass transport are calculated assuming an ideal semipermeable capillary wall and the lymphatic drainage depends linearly on the IMP . Changes in lymphatic flow are used to simulate edema formation, and its effects on myocardial mechanics and coronary flow. The empty beating and isovolumic contracting hearts are studied under constant coronary perfusion pressures. The model successfully predicts the corresponding changes of the coronary flow, the IMP , the LV pressure and the ventricular compliance. The simulated effects of a transient contractile dysfunction on the dynamics of fluid transport and coronary flow are in agreement with experimental data.

Surface viscoelastic effects in cell cleavage

The effect of passive surface traction on the cleavage of cells is incorporated in the cytokinesis hydrodynamic model of Zinemanas and Nir [Biomechanics of cell Division, pp. 281-305, Plenum Press, New York (1987)]. Different rheological behaviours were examined to model the surface tensions which arise due to the passive deformation of the cortex: a Mooney-Rivlin material, an STZC material and a viscoelastic material. The calculations show that passive surface tensions may play a significant role in determining the local surface deformations as well as in the modulation of the surface forces. Varying the rheological model has limited effect on the overall deformations. The latter appear to be affected mostly by contractile filament interactions.

Intramyocardial Fluid Transport Effects on Coronary Flow and Left Ventricular Mechanics

An integrated left ventricular (LV) model is used to solve, simultaneously and interactively, the LV mechanics, the coronary blood flow and capillary and interstitial fluid and mass transport, and to analyze the LV behavior under normal as well as pathological conditions. Accounting for the interstitial fluid mass balance in a LV flow-mechanical model allows to determine the LV wall volume in terms of the prevailing mechanical and flow conditions; it allows to uniquely define the flow-mechanical relationship and study pathologies, such as myocardial edema, which are directly related to changes in the myocardial fluid transport and content.

A fluid-mechanical model of deformation during embryo exogastrulation

Embryo deformation during gastrulation is simulated using a fluid-mechanical approach. The possibility that the deformation is governed by the forces originated at the embryo surface rather than by pulling by filopodia is considered. The slow viscous flow and the deformation during in vitro exogastrulation is solved using a boundary integral equation formulation. The results show a distinct anisotropic distribution of meridional and circumferential tensions, which are capable of producing the dynamics and shape of the archenteron development. This anisotropy suggests the existence of a specific biochemical activity at particular surface regions prior to and during the gastrulation process. The mechanical model can serve as a tool to help discriminate between the possible suggested mechanisms.

Fluid Mechanical Simulations of Cell Furrowing Due to Anisotropic Surface Forces

Cell division is a complex phenomenon ultimately leading to the formation of two new daughter cells. Accomplishment of this important cell function involves two different but related processes: mitosis and cytokinesis. The former is the process of nuclear division in which the continuity of the chromosomal set is maintained. Once the necessary cytoplasmic constituents and organelles are separated and redistributed, daughter cells are formed by physical division of the cytoplasmic matrix. This process of cell cleavage is known as cytokinesis.

Simulation of heat exchangers with change of phase

Abstract An algorithm for the simulation of vertical and horizontal 1-1 shell and tubes heat exchangers with phase change and one or more components is described. The algorithm calculates local values of the variables along the heat exchanger and accommodates changes of flow patterns. Comparison of the program with experimental data showed reasonable engineering accuracy for systems with one to three components.

Concepts and Controversies in Modelling the Coronary Circulation

Some major concepts and controversies associated with coronary flow dynamics and the interaction between left ventricular myocardial contraction and the coronary vasculature are highlighted. The controversies rise from the interpretations of the forces acting on the intramyocardial vessels and the mechanisms postulated to affect the coronary circulation. These include the waterfall concept and the pressure-dependent resistance and compliance models which are, in essence, refinements of the intramyocardial pump concept. These are reviewed in the light of recent models of the coronary circulation. Coronary ischemia, which is typically associated with the development of collateral flow, is also presented. In addition, the important role of the collagen mesh in the generation of intramyocardial pressure (IMP) at a wide range of loading conditions and the contribution of myocardial fluid transport to IMP distribution and the coronary vasculature are included in a comprehensive model relating the interactions of the above factors.

Integration of structure, function and mass transport in the myocardium

A left ventricular (LV) model that integrates muscle mechanics, coronary flow, and fluid transport, and accounts for the three-phase (fiber-blood-interstitium) myocardial structure and composition, is used to study the interactions between the mechanics, coronary flow and fluid and mass transport in the myocardium. Theoretical simulations elucidate the effects of ventricular load, coronary perfusion pressure, and fluid and mass transport on ventricular performance and coronary dynamics. The analysis yields a direct relation between cardiac function and structure to cardiac mechanics, coronary flow, and intramyocardial fluid (and mass) transport, and allows to study the interactions between coronary flow, ventricular and myocardial mechanics and intramyocardial fluid shifts.

Coronary flow: effects of myocardial contractile state, LV loading conditions, perfusion pressure and interstitial fluid transport

A comprehensive analytical study of the coronary flow dynamics covering a wide range of left ventricular (LV) operating conditions, including changes in the contractile state of the myocardium, LV loading, coronary perfusion and fluid transport conditions is presented. The integrated model, which also includes transcapillary-interstitial fluid (and mass) transport, is substantiated by available experimental data and predicts correctly the principal characteristics of the coronary flow under the imposed constraints and operating conditions, e.g., the significant dependence of the coronary flow on the contractile state and perfusion pressure and its rather small dependence on the LV pressure at the normal contractile state.<<ETX>>