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P Wach

Publications and source records attributed to P Wach.

At least 55 records · Page 3Linked to original sources
PubMed1995

An iterative approach on magnetic source imaging within the human cortex--a simulation study.

A simulation study on magnetic source imaging within the human brain from a synthetic evoked magnetic field is presented. An inhomogeneous boundary element (BE) head model (cortex cerebrospinal fluid) was built up from real magnetic resonance imaging (MRI) cross-sections. In the forward problem, one or two rotating primary current dipoles (PCDs) are located at arbitrary sites within the auditory cortices. The PCDs should represent focal and distributed neural activities, respectively. The reconstruction space (predefined by a priori morphological information) is defined as a surface within the three-dimensional cortical volume, with an averaged distance of 0.005 m to the outer cortex surface. The reconstructed pseudo primary current dipoles (PPCDs) are not restricted to any particular direction. The observation space consists of two concave surfaces closely above the scalp. Each observation surface contains 37 observation points. An iterative Wiener filter estimation (WFE) is applied in order to reconstruct PPCD distribution from simulated magnetic field data. This iterative WFE approach enables the simultaneous localization of focal and distributed activities. Aspects on the correlation between neural activities are not investigated within this paper.

Algorithms↗
PubMed1995

A simulation study on magnetic source imaging with a realistic model of the entire human heart.

A computer model study on magnetic source imaging from magnetocardiographic data is presented using a cellular automaton model of the entire human heart in the so-called forward problem. A homogeneous boundary element (BE) torso is built up from real magnetic resonance imaging (MRI) cross-sections. The heart model, which has a realistic anatomical shape, is positioned inside the BE torso. In the forward problem the spread of excitation is simulated by applying a modified Huygen's propagation principle. The magnetocardiogram (MCG) and electrocardiogram (ECG) can then be computed following the bidomain theory. From the simulated MCG data, pseudo primary current dipole (PPCD) estimation within the electrically active tissue is performed. The reconstruction space is defined as a surface in the middle of the atrial and ventricular myocardium and septum. The observation space consists of two mutually perpendicular planes closely above the torso surface on the frontal and the left lateral side, respectively. An iterative minimum-norm approach is applied in order to reconstruct PPCD distributions. The errors in PPCD estimation arising from noisy data and regularization algorithms are investigated in more detail.

Computer Simulation↗
PubMed1994

[Simulation of a one-dimensional impulse transmission along a heart muscle fiber with varied intracellular specific resistance--unidirectional block].

The subject of this study is the modeling of the spread of excitation along a ventricular myocardial fibre based on the one-dimensional "cable" (transmission line) theory. In particular the influence of a varying intracellular c resistivity, R, and of the extracellular potassium concentration [K]o on the propagation is investigated. The membrane model used is the Luo-Rudy model, with which such cellular phenomena as supernormal excitability and Wenckebach pattern can be simulated. The specific resistivity, R, comprises the resistivity of the myoplasm and that of the gap junction. The phenomen of the unidirectional block (UDB) can be simulated in the ischaemic/non-ischaemic transitional zone. Here, the resistivity peak within the ischaemic border zone--caused by collagenous tissue--in combination with a gradient of the R along the fibre-plays a basic role. This simulation study makes it evident that variations in the R-profile have a considerably greater effect on the propagation of a stimulus than variations in the [K]o-profile.

Collagen↗
PubMed1993

Pharmacokinetic model for the absorption of subcutaneously injected soluble insulin and monomeric insulin analogues.

A subcutaneous insulin absorption model is presented for parameter estimation from the time course of plasma insulin. Modifications of a published model were made for the absorption of soluble insulin and monomeric insulin analogues in the range of therapeutic concentrations and volumes. The modified diffusion-dissociation model with distributed parameters was approximated by a multiple-compartment model. Subcutaneous absorption of soluble insulin and monomeric insulin analogues with various volumes, concentrations, and injection depths was simulated. The model for soluble insulin exhibits volume, concentration, and injection depth dependent absorption, as experimentally observed. It was found that binding of soluble insulin in the subcutaneous tissue is negligible for U-40 and U-100 strengths. The absorption of identical doses (10 U) of soluble U-40 insulin was markedly faster (T-50% = 159.4 min) than the absorption of U-100 (T-50% = 196.2 min). According to the simulation results, the absorption rate of monomeric analogues is not dependent on concentration. No significant chances of the absorption rate could also be observed by varying volume and injection depth of the monomeric analogues.

Computer Simulation↗
PubMed1991

[Further advances in the salt sensitivity hypothesis in man].

The hypothesis, proposed by us since 1981, that genetically determined salt sensitivity exists in the normotensive population has been confirmed by other groups. We propose that in salt sensitive subjects an augmented upregulation of alpha-2 combined with simultaneous downregulation of beta-2 adrenoceptors by a high salt diet (resulting in an increase in the "operative" adrenoceptor ratio) is responsible for the rise of blood pressure. In salt resistant subjects the "operative alpha-2/beta-2 adrenoceptor ratio" does not increase on a high salt intake. The adrenoceptor changes in salt sensitive subjects probably lead to an increased central sympathetic outflow (through receptor changes in certain brain areas) and to simultaneous enhanced end-organ response in resistance vessels and in the kidney, causing enhanced vasoconstriction and enhanced sodium reabsorption. Long term follow up of salt sensitive normotensives will show, whether they develop "essential hypertension" in the future.

Absorption↗
PubMed1990

Light scattering by blood components after supplying glucose.

Investigations were performed in order to find out whether the glucose concentration in liquids can be determined by means of light scattering. Both static light scattering and photon correlation spectroscopy (PCS) were used. Neither of them revealed a possibility of determination of glucose concentration in a pure glucose solution. But for glucose-protein-solutions a clear correlation between intensity of light scattering and glucose concentration was detected due to glycosylation of proteins. In blood serum it is solely possible to measure non-enzymatic reaction products between glucose and proteins and to determine the influence of Amadori products on protein structure. Therefore not even indirect conclusions on the present glucose concentration are possible.

Blood Glucose↗
PubMed1989

[Glucose determination by means of NMR spectrometry].

NMR spectroscopy could be a method for non-invasive determination of glucose concentration in the future. In this connection, 13C-NMR spectroscopy is of special interest. In vitro investigation of blood serum samples in 13C-NMR revealed significant changes in signal intensities reflecting the different glucose concentrations in these samples. The further development of noninvasive in vivo glucose determination by means of NMR spectroscopy is closely associated with technological progress.

Blood Glucose↗