Aortic valve mechanics

Model and animal experiments emphasized the relevance of presence and flexibility of aortic sinuses in valve closure. In dogs the asymmetric deformation of aortic ring and the commissure displacements during the cardiac cycle were quantified. Finite element modelling made clear that the leaflet bundles are essential for a homogeneous stress distributio

A model approach to the adaptation of cardiac structure by mechanical feedback in the environment of the cell

The uniformity of the mechanical load of the cardiac fibers in the wall is maintained by continuous remodeling. In this proposed model the myocyte changes direction in optimizing systolic sarcomere shortening. Early systolic stretch and contractility increases the mass of contractile proteins. Cyclic strain of the myocardial tissue diminishes passive stiffness, resulting in the control of ventricular end-diastolic volume. Utilizing these rules of remodeling in our mathematical model yields that the natural helical pathways of the myocardial fibers in the wall are formed automaticall

Relation between left ventricular cavity pressure and volume and systolic fiber stress and strain in the wall

Pumping power as delivered by the heart is generated by the cells in the myocardial wall. In the present model study global left-ventricular pump function as expressed in terms of cavity pressure and volume is related to local wall tissue function as expressed in terms of myocardial fiber stress and strain. On the basis of earlier studies in our laboratory, it may be concluded that in the normal left ventricle muscle fiber stress and strain are homogeneously distributed. So, fiber stress and strain may be approximated by single values, being valid for the whole wall. When assuming rotational symmetry and homogeneity of mechanical load in the wall, the dimensionless ratio of muscle fiber stress (sigma f) to left-ventricular pressure (Plv) appears to depend mainly on the dimensionless ratio of cavity volume (Vlv) to wall volume (Vw) and is quite independent of other geometric parameters. A good (+/- 10%) and simple approximation of this relation is sigma f/Plv = 1 + 3 Vlv/Vw. Natural fiber strain is defined by ef = In (lf/lf,ref), where lf,ref indicates fiber length (lf) in a reference situation. Using the principle of conservation of energy for a change in ef, it holds delta ef = (1/3)delta In (1 + 3Vlv/Vw)

Adaptation of cardiac structure by mechanical feedback in the environment of the cell: a model study

In the cardiac left ventricle during systole mechanical load of the myocardial fibers is distributed uniformly. A mechanism is proposed by which control of mechanical load is distributed over many individual control units acting in the environment of the cell. The mechanics of the equatorial region of the left ventricle was modeled by a thick-walled cylinder composed of 6-1500 shells of myocardial fiber material. In each shell a separate control unit was simulated. The direction of the cells was varied so that systolic fiber shortening approached a given optimum of 15%. End-diastolic sarcomere length was maintained at 2.1 microns. Regional early-systolic stretch and global contractility stimulated growth of cellular mass. If systolic shortening was more than normal the passive extracellular matrix stretched. The design of the load-controlling mechanism was derived from biological experiments showing that cellular processes are sensitive to mechanical deformation. After simulating a few hundred adaptation cycles, the macroscopic anatomical arrangement of helical pathways of the myocardial fibers formed automatically. If pump load of the ventricle was changed, wall thickness and cavity volume adapted physiologically. We propose that the cardiac anatomy may be defined and maintained by a multitude of control units for mechanical load, each acting in the cellular environment. Interestingly, feedback through fiber stress is not a compelling condition for such control. [Journal Article; In English; United States

Three-dimensional blood flow in bifurcations : computational and experimental analyses and clinical applications

In this report, the issues discussed at a multidisciplinary symposium on blood flow in bifurcations are summarized. Topics adressed are (1) flow analysis in vitro models, using visualization and laser Doppler anemometer techniques, and numerical models; (2) the influence of (physiological) factors, such as vessel wall distensibility and vessel geometry, on the flow field; (3) the noninvasive assessment of arterial wall properties in humans, and (4) the noninvasive determination of flow patterns in humans, paying special attention to ultrasound techniques and magnetic resonance imaging. It was empasized that it is of utmost importance to obtain more detailed information, preferably three-dimensional, about flow field in bifurcations, not only from a diagnostic point of view but also to get more insight into the relation, if any, in between flow patterns and atheregenesis. It was agreed that plaque geometry and dynamics should be studied in more detail, especially in relation to plaque fissuring and rupturing. There is a need for the noninvasive assessment of wall shear rate and, hence, to be able to calculate wall shear stress, because these parameters have been shown to be important determinants of endothelial cell function