A constitutive description of contracting papillary muscle and its implications to the dynamics of the intact heart

Fibre Orientation in Human Fetal Heart and Ventricular Mechanics : A Small Perturbation Analysis

The study of the topological organisation of myocardial cells is a basic requirement for understanding the mechanical design of the normal and pathological heart. Anatomical observations show that cardiac muscle tissue has a highly specialized architecture. We have made new quantitative measurements of fibre orientation through the heart wall by means of polarized light analysis on some thick sections of human fetal heart embedded in a resin and polymerized. A small perturbation method to find an equilibrium solution in a cylindrical left ventricular (LV) geometry with fibres running on toroidal shells of revolution is used to investigate the mechanical behaviour of three human fetal hearts (FH) of 14, 20 and 33 weeks of gestational age. The results of fibre strains and stresses presented for end-systolic state show significant differences when compared to results of the cylindrical geometry with regular helicoidal fibres running on cylindrical surfaces. Moreover, the toroidal shells of revolution explain shear stresses and strains in the transverse plane which also exist in the adult heart.

Controlled auxotonic twitch in papillary muscle: a new computer-based control approach

Based on new advancements in digital technology, we developed a PC- and DSP-based measurement and control system for isolated papillary muscle experiments. High flexibility was obtained through a three level control. Length or force was controlled real-time with a sample frequency of 5000 Hz. Muscle length and up to three segment lengths were measured simultaneously and each of these lengths could be chosen as feedback variable. Individual algorithms were implemented for different twitch types. Batches of twitches were organized in experiment protocols. The system included a new twitch type, namely a controlled auxotonic twitch. In this twitch, the muscle acted against a virtual ideal spring, giving a proportional change in developed force and shortening. The value of the virtual spring constant could be set on-line or defined in the experiment protocol. An increasing virtual spring constant represented a smooth transition from isotonic to isometric conditions.

Modelling the mechanical properties of cardiac muscle

A model of passive and active cardiac muscle mechanics is presented, suitable for use in continuum mechanics models of the whole heart. The model is based on an extensive review of experimental data from a variety of preparations (intact trabeculae, skinned fibres and myofibrils) and species (mainly rat and ferret) at temperatures from 20 to 27 degrees C. Experimental tests include isometric tension development, isotonic loading, quick-release/restretch, length step and sinusoidal perturbations. We show that all of these experiments can be interpreted with a four state variable model which includes (i) the passive elasticity of myocardial tissue, (ii) the rapid binding of Ca2+ to troponin C and its slower tension-dependent release, (iii) the kinetics of tropomyosin movement and availability of crossbridge binding sites and the length dependence of this process and (iv) the kinetics of crossbridge tension development under perturbations of myofilament length.

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 model's 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 dominant effect of the myocardial contractile state on coronary flow and a dissociation 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.

Contraction and relaxation of isolated cardiac myocytes of the frog under varying mechanical loads

The mechanics of cardiac systole and relaxation have been studied primarily at the level of the whole heart or intact muscle. End-systolic pressure-volume relations of frog hearts have been found to be load dependent, whereas those of the mammal are relatively load independent. On the other hand, myocardial relaxation as studied at the muscle level is load independent in the frog but markedly load dependent in the mammal. Interpretation of these studies is complicated because of the unknown contribution of extracellular connective tissue, neurohumoral factors, and, in the case of the heart, the complex chamber geometry. Therefore, it is valuable to study cardiac mechanics at the level of the basic unit of contractile activity--the isolated myocyte. The goal of this study was to subject isolated frog cardiomyocytes to mechanical loading paradigms that mimic those presented to the cells within the heart. In the first part of this study, the afterload and preload of contracting cells were varied to study their effects on the end-systolic force-length relation, which was consistently found to be load independent over the range of isotonic shortening tested (typically 5%). We also investigated the force-length-time response of the cells to test the concept of the heart behaving as a time-varying elastance. Our results suggest that in this regard the frog myocyte behaves like mammalian muscle, and they are consistent with the presence of a small viscosity within the cell. We conclude that the tissue structure of the frog heart may contribute to disparity in mechanical behavior at the different structural levels. In the second part of this study, we subjected isolated frog cardiomyocytes to four different loading paradigms to test the hypothesis that myocardial relaxation in the frog is independent of load. These sequences consisted of afterloaded contractions followed by conventional isotonic-isometric relaxation (ACCR) or afterloaded contractions followed by physiologically reversed isometric-isotonic relaxation (ACPR). Relaxation was measured under isometric conditions using a variable afterload with either the ACCR or ACPR paradigms. The decay of force was independent of the cell length at which it occurred or the amount of shortening prior to it within the contractile cycle. Relaxation also was measured as relengthening of the cell under isotonic late-load conditions, using the ACPR paradigm either with a variable afterload or variable late load. Relengthening had a time course that was unaffected by changes in afterload (i.e., extents of shortening) or late load (equivalent to the filling pressure for the heart).(ABSTRACT TRUNCATED AT 400 WORDS)

Finite extension and torsion of papillary muscles: a theoretical framework

We present a new analytical solution for the finite extension and torsion of a nonlinearly pseudoelastic, homogeneous, transversely isotropic, incompressible, solid circular cylinder. This solution can be used to guide the performance and interpretation of experiments, and to identify a specific functional form of a three-dimensional constitutive relation directly from data. We submit, therefore, that this solution can be used by experimentalists to quantify the multiaxial constitutive relations, including shear, of both passive and tonically activated papillary muscles for the first time.