RESUMEN
The objective of this paper is the application of two-dimensional discrete Fourier transformation for solving the integral equation of the bioelectric forward problem. Therefore, the potential, the source term, and the integral equation kernel are assumed to be sampled at evenly spaced intervals. Thus the continuous functions of the problem domain can be expressed by their two-dimensional discrete Fourier transform in the spatial frequency domain. The method is applied to compute the surface potential generated by an eccentric dipole in a homogeneous spherical conducting medium. The integral equation for the potential is solved in the spatial frequency domain and the value of the potential at the sampling points is obtained from inverse Fourier transformation. The solution of the presented method is compared to both, an analytic solution and a solution gained from applying the boundary element method. Isoparametric quadrilateral boundary elements are used for modeling the spherical volume conductor in the boundary element solution, while in the two-dimensional Fourier transformation method the volume conductor is represented by a parametric boundary surface approximation.
Asunto(s)
Simulación por Computador , Impedancia Eléctrica , Análisis de Fourier , Campos Electromagnéticos , HumanosRESUMEN
The objective of this study is to analytically validate a boundary element (BE) formulation for the relationship between the transmembrane potential on the heart's surface and the potential on the body surface applying a concentric spherical test geometry. The relative difference (reldif) between the potential on the outer sphere of the test geometry computed analytically and numerically is determined by 3.59% for the coarse discretization (48 BEs) and by 0.46% in the case of the finer subdivision (192 BEs). In the inverse problem, the transmembrane potential on the inner sphere is estimated numerically from the electric potential on the outer sphere by using a minimum-norm least-square approach. The relative differences found are 20.2% when no measurement noise is added and 26.4% in the presence of 2% additional Gaussian noise. The BE formulation is also applied to real world data for solving the electrocardiographic inverse problem. A normal volunteer's inhomogeneous thorax (outer thorax surface, surfaces of the lungs, epicardial heart surface) is modelled by 424 BEs. The same inverse method is then applied in order to reconstruct the transmembrane potential on the epicardium from the measured body surface potential (BSP) data during normal ventricular depolarisation.
Asunto(s)
Electrocardiografía/estadística & datos numéricos , Corazón/fisiología , Humanos , Análisis de los Mínimos Cuadrados , Matemática , Potenciales de la Membrana , Modelos CardiovascularesRESUMEN
The estimation of pseudo primary current dipoles on a 2D-manifold in the atrial and ventricular myocardium and septum, and of the transmembrane potential on the endocardium and epicardium, from the magnetic heart field is investigated. The human thorax surrounding the heart is modelled by an inhomogeneous boundary element volume conductor model, including the outer thorax surface and the surfaces of the lungs. The influence of the blood mass is neglected. In the inverse problem Tikhonov's regularisation is applied. The regularisation parameter is determined by the L-curve method. An algorithm for iterative improvement is applied to estimate the pseudo primary current dipoles. Synthetic magnetic field and electric potential data are generated using a cellular automaton model of the entire human heart. Real world magnetic field data for a normal subject are analysed to demonstrate the practicability and effectiveness of the presented method.
Asunto(s)
Algoritmos , Simulación por Computador , Corazón/fisiología , Magnetismo , Modelos Cardiovasculares , HumanosRESUMEN
Transfer of pyrene-labeled phosphatidylserine catalyzed by the yeast phosphatidylserine transfer protein in vitro largely depends on the membrane lipid composition of artificial unilamellar acceptor vesicles. Negatively charged phospholipids markedly decrease the rate of protein-catalyzed phosphatidylserine transfer. Although biological membranes contain a significant proportion of negatively charged phospholipids they serve more effectively as acceptors than artificial membranes with a similar phospholipid composition, but without proteins. This result indicates that proteins present in biological membranes mask negative charges of phospholipids on the surface of acceptor membrane vesicles. When proteins of the membrane surface are removed by proteinase treatment this protective effect is partially lost. A correlation between the activity of the phosphatidylserine transfer protein in yeast cytosol and the extent of membrane biogenesis during growth could not be observed.
Asunto(s)
Proteínas Portadoras/metabolismo , Lípidos de la Membrana/metabolismo , Proteínas de la Membrana/metabolismo , Fosfatidilserinas/metabolismo , Citosol/metabolismo , Endopeptidasas , Fosfolípidos/metabolismo , Pirenos , Saccharomyces cerevisiae/metabolismoRESUMEN
A phospholipid transfer protein with a broad substrate specificity was isolated from yeast cytosol. The rate of transfer catalyzed by this protein in vitro is highest for phosphatidylserine; phosphatidylethanolamine, cardiolipin, phosphatidic acid and ergosterol are transported at a lower rate. In contrast to the yeast phosphatidylinositol transfer protein (Daum, G. and Paltauf, F. (1984) Biochim. Biophys. Acta 794, 385-391) the phosphatidylserine transfer protein does not catalyze the translocation of phosphatidylinositol or phosphatidylcholine. Using chromatographic methods the phosphatidylserine transfer protein was enriched approximately 3000-fold over yeast cytosol. The protein is inactivated by heat, detergents and proteinases. Divalent cations strongly inhibit the transfer of phosphatidylserine in vitro, and EDTA at low concentrations has a stimulatory effect.