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Biomedical subjects

M Lewkowicz

Publications and source records attributed to M Lewkowicz.

5 recordsLinked to original sources
PubMed1990

Contraction and relaxation-induced oscillations of the left ventricle of the heart during the isovolumic phases.

A theoretical analysis is presented for the transient dynamical response of the left ventricle of the heart during the isovolumic contraction and relaxation phases of the cardiac cycle. Small oscillations of the left ventricular cavity pressure and wall motion are excited by the initial rates of filling and emptying of the ventricle as well as the rate of change in muscle fiber activation. The analysis applies to the genesis of the first and second heart sounds. The ventricle is modeled as a finite, thick-walled incompressible cylinder having a continuum of imbedded axial and circumferential active muscle fibers, which interacts with a fixed volume of an incompressible, ideal fluid. The solution is obtained using a two-timing asymptotic expansion procedure. The theoretical calculations of left ventricular pressure waveforms compare favorably with published recorded pressure waveforms. The amplitude spectra of computed waveforms contain information concerning the active elastic modulus of the fibers, which is a measure of cardiac contractility.

Animals↗
PubMed1989

Wall-thickness and midwall-radius variations in ventricular mechanics.

A fluid-fiber-collagen stress tensor is used to describe the rheology of the left ventricle of the heart. Linear theory is used to find the equilibrium solutions for the end-diastolic and end-systolic states of general axisymmetric shapes that are small perturbations of a thick-walled finite cylinder. The general problem can be studied by superposing the effects of variable midwall radius but constant wall thickness with those of variable wall thickness but constant midwall radius. A Fourier series representation is used to describe the midwall radius and thickness functions. Numerical calculations are performed to determine the deformed geometry and spatial distributions of tissue pressure, stresses, and fiber strains. The calculations proved to be highly accurate when compared to an analytical solution obtained for the special case of no fibers. The results show significant longitudinal differences when compared to results for the cylindrical geometry, with more sensitivity to variation in wall thickness than to variation in midwall radius.

Animals↗
PubMed1987

The first heart sound during the isovolumetric contraction.

We make the first attempt to construct a qualitative theory covering the whole process of the major part of the first heart sound from an electrical activation to the phonocardiographic observations at the thorax. We calculate the amplitudes and frequencies of the radiated pressures during the isovolumetric contraction period generated by the muscular wall of the left ventricle and by the valves considered as a spherical shell and two-dimensional membranes, respectively. The analysis shows that both the hemodynamic and the valvular theory are able to explain most of the characteristic features of the first heart sound (linear relation between the amplitudes of the radiated pressure and the slope of the left ventricular pressure-time curve; directional polarity of the amplitudes; equidistant frequency peaks with a decline in amplitudes). However, existing magnitudes of the set of physiological parameters involved seems to favour the hemodynamic theory of the first heart sound. The aortic valve can be neglected as a source of sound. The initial conditions (like valve closure velocity), according to our theory, cannot be important. The predicted time-plot and frequency spectrum of the radiated pressure show a general resemblance with the recorded ones. It is essential to have considerably more quantitative acoustic data both for normal and diseased hearts for subsequent theoretical development.

Heart Auscultation↗