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Biomedical subjects

G Rau

Publications and source records attributed to G Rau.

At least 145 records · Page 8Linked to original sources
PubMed1990

Results with multichannel topographical bipolar recordings of the visual evoked potential in normals and amblyopes.

The method currently used to record topographical voltage distributions of visual evoked potentials is that of monopolar recording with respect to some arbitrary reference electrode. As an alternative we investigated the method of bipolar topographical recording of the checkerboard reversal evoked potential in four amblyopic patients and ten normal persons. The recordings were performed with a system of 14 bipolar channels, arranged in a rectangular grid with an interelectrode distance of 15% of the inion-nasion distance and centered 3 cm above the inion. The gradient distributions of the visual evoked potential obtained from normal persons showed at the time of component P2 a maximum about 3 cm above the inion. The healthy eyes of the amblyopic patients showed similar distributions and latencies. The amblyopic eyes showed normal or reduced amplitudes and normal or increased latencies with large checks and low amplitudes with smaller checks. The topographical distributions recorded with the bipolar recording system showed shifts of the maxima with the larger patterns in three of four amblyopic eyes. A technical comparison showed that these shifts can be recorded by the bipolar recording method with higher accuracy than with the monopolar method, promising a better understanding and more accurate diagnosis of amblyopia.

Adult↗
PubMed1990

Redefining cooling rate in terms of ice front velocity and thermal gradient: first evidence of relevance to freezing injury of lymphocytes.

A freezing process and the resulting injury or survival of biological cells is commonly characterized in terms of the cooling rate, B. Under certain circumstances, the cooling rate can be expressed as B = G.v, where G denotes the thermal gradient at the ice-liquid interface and v its velocity, respectively. To determine the influence of G and v on the morphology of the ice-liquid interface and on cell survival, a gradient freezing stage was designed. Flat capillaries could be pushed with constant velocity from a warm to a cold heat reservoir. With this setup both parameters, G and v, are independently adjustable and the resulting process of directional solidification can be observed dynamically in a light microscope. Human lymphocytes in phosphate-buffered saline with 10 vol% of dimethyl sulfoxide were used as biological test material. Viability was assessed by a membrane integrity test with fluorescein diacetate and ethidium bromide. All cells were cooled down to a final temperature of -196 degrees C and then rapidly thawed. The results obtained with this technique show that the viability determined after freezing and thawing with a certain cooling rate, B = G.v, may vary considerably depending on the imposed values of the thermal gradient, G, and the ice front velocity, v. In addition, the data seem to suggest that, first, the maximum viability which can be reached is governed by the cooling rate, and, second, this maximum for a given cooling rate could be achieved by establishing small temperature gradients and high interface velocities (about 30 degrees K/cm and 500 microns/sec, respectively, for the range of values of G and v tested).

Cell Membrane↗
PubMed1990

New methods for the development of pneumatic displacement pumps for cardiac assist.

The primary goal of the presented project was to develop a pump family with stroke volumes of 20, 50, 70 and 90 ml, which could be produced at low cost but with sufficient quality. The housing parts of the pump were thermoformed from technical semifinished materials. All blood contacting surfaces of the pump were coated with biomaterials in a controlled dipping process. During the design and fabrication process a professional CAD-system was used. This facilitated spatial presentations of pump components for first evaluations at the initial draft stages. The CAD-design data were then transformed to CNC-controlled lathes and mill's for the fabrication of pump tools. The stresses and strains of the moving blood pump components, such as membranes and valves, were precalculated by means of Finite-Element-Analysis (FEM). After completion of the pump, the internal flow fields were investigated by flow-visualization techniques using non-Newtonian test fluids, and the pump characteristics (function curves) were investigated in appropriate circulatory mock loops. The paper covers all above aspects from first draft to final fabrication and testing.

Biocompatible Materials↗
PubMed1990

[ECG in acute myocardial infarct. Continuous registration in thrombolytic therapy].

For 35 patients with acute myocardial infarction undergoing thrombolytic therapy (31 patients with streptokinase and 4 with rt-PA), the treatment effect was assessed angiographically and correlated with ECG changes. These ECG changes were found by continuous registration of eight "strategically" located precordial leads in patients with anterior myocardial infarction, and five precordial leads plus three limb leads in those with posterior myocardial infarction. In 31 patients (group A), successful reperfusion could be verified by angiography, while in 4 patients the infarct-related vessel remained closed. In 3 further cases (group B), recanalization could be achieved only by additional mechanical intervention. The average duration of ECG surveillance was 87 min in group A and 85 min in group B. A significant change of R-amplitudes during this period of time could not be observed. ST-60-value of group A (total of ST-elevations and depressions 60 msec after S of all surveillance electrodes, divided by number of electrodes) was reduced under thrombolysis from 0.377 mV (SD +/- 0.270) to 0.133 mV (SD +/- 0.118) (p less than 0.004). This means a reduction of 64%, while the corresponding values in group B did not change (0.535 mV, respectively 0.546 mV) before and after therapy. When only the electrode with the highest ST-60-value was checked before therapy ("sensitive lead"), reduction in group A was as high as 71%. Except for two patients, all other 29 patients with successful reperfusion either precordially or in the limb leads showed a reduction of ST-segment abnormalities (ST 60) of more than 50% during surveillance time. This could not be observed in any of the unsuccessfully treated patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Evaluation of mechanical loading of a trileaflet polyurethane blood pump valve by finite element analysis.

The mechanical loading of the leaflets of a polyurethane blood pump valve at closing pressure (max 300 mmHg) and during the opening phase can be numerically calculated by the finite element method. Geometrical and physical non-linearities, supporting forces by neighbouring leaflets and manufacturing-related thickness distribution are taken into account. The valve housing is assumed to be rigid. Volume forces are neglected. The calculations showed that the critical strain limit of 4.5% (long-term failure limit) was exceeded at closing pressure and during valve opening. During valve operation the connection between leaflet and valve housing and the central leaflet region seem to be the critical areas.

Biocompatible Materials↗
PubMed1989

High-performance liquid chromatographic determination of ibuprofen, its metabolites and enantiomers in biological fluids.

An isocratic high-performance liquid chromatographic method to determine racemic ibuprofen (assay I) and its major metabolites (assay II) in biological fluids (plasma, urine, bile) using a conventional reversed-phase column is described. A third assay using beta-cyclodextrin as stationary phase (Cyclobond I) for the separation of the ibuprofen enantiomers is also described. A wavelength of 220 nm was used to monitor the substances. The sensitivity of the method was 0.1 microgram/ml for all three assays. The method was demonstrated to be suitable for stereoselective pharmacokinetic studies of ibuprofen in humans and animals.

Bile↗
PubMed1989

[Automated measurement of distance vision based on the DIN strategy].

A method for automated measurement of far vision is described which meets the test requirements laid down in the new DIN standards. The subject sits 5 m from a high-resolution monitor on which either Landolt rings or Snellen's types are generated by a computer. By moving a joystick the subject indicates to the computer whether he can see the critical detail (e.g., the direction of opening of the Landolt ring). Depending on the subject's input and the course of the test so far, the computer generates the next test symbol until the threshold criterion is reached. The sequence of presentation of the symbols and the threshold criterion are also in accordance with the DIN standard. Initial measurements of far vision using this automated system produced similar results to those obtained by conventional methods.

Distance Perception↗
PubMed1989

[Subjective noise perception and objective measurement of loudness following heart valve replacement with the St. Jude Medical and Duromedics Edwards bileaflet prostheses].

The performance of heart valve prostheses is generally judged by the rate of valve-related complications and the hemodynamic performance. Patients may be severely bothered by the metallic click generated by the closure of the valve. In 74 patients after valve replacement with Duromedics Edwards (DE) (n = 38) and St. Jude Medical (SJM) (n = 36) prostheses the sound energy was recorded and correlated to the complaints of the patients. At a distance of 10 cm the DE valves produced a significantly higher sound pressure with 47 +/- 7 db(A) compared to the SJM valves with 39.8 +/- 5 db(A) (p = 0.001). The noise level was also different for the valves in aortic or mitral position. After splitting into frequency bands the highest sound pressure was observed in the high frequency ranges (8 to 16 kHz) which represents the metallic click. 65% of patients were aware of the noise generated by the valve, 16% had sleep disturbances and 22% would prefer a more silent valve type. 12 of 38 patients with DE valves and 4 of 36 patients with SJM valves wished to have a less noisy valve type (Chi square p = 0.003). In annoyed patients the valves produced a higher sound amplitude of 49 +/- 8 db(A) as compared to undisturbed patients with 42 +/- 6 db(A) (p = 0.002). The noise level of mechanical heart valves should be considered before selection of a prosthesis, because the metallic click bothers patients and the complaints correlate with measured sound energy.

Adult↗
PubMed1989

Velocity and shear stress distribution downstream of mechanical heart valves in pulsatile flow.

Ten mechanical valves (TAD 27 mm): Starr-Edwards Silastic Ball, Björk-Shiley Standard, Björk-Shiley Concave-Convex, Björk-Shiley Monostrut, Hall-Kaster (Medtronic-Hall), OmniCarbon, Bicer Val, Sorin, Saint-Jude Medical and Hemex (Duromedics) are investigated in a comparative in vitro study. The velocity and turbulent shear stress profiles of the valves were determined by Laser Doppler anemometry in two different downstream axes within a model aortic root. Depending on the individual valve design, velocity peaks up to 1.5 m/s and turbulent shear stress peaks up to 150 N/m2 were measured during the systolic phase. These shear stress peaks mainly occurred in areas of flow separation and intense momentum exchange. Directly downstream of the valves (measuring axis 0.55.dAorta) turbulent shear stress peaks occurred at peak systole and during the deceleration phase, while in the second measuring axis (1.5.dAorta) turbulence levels were lower. Shear stress levels were high at the borders of the fluid jets. The results are discussed from a fluid-dynamic point of view.

Blood Flow Velocity↗
PubMed1988

Model studies at mechanical aortic heart valve prostheses--Part I: Steady-state flow fields and pressure loss coefficients.

In a 3:1 scaled model of the human aorta models of the Omniscience, Björk-Shiley Convexo-Concave, Björk-Shiley Monostrut, Medtronic-Hall, Duromedics (Hemex) and the Saint Jude Medical heart valve prostheses are studied in steady flow representing the systolic peak flow phase. Detailed flow visualization experiments show flow separations at all inner ring surfaces as well as at most of the occluders. The resulting stagnation areas increase the risk of thrombus accumulation. Flow separations also stimulate vortex formation and turbulent mixing at the downstream jet boundaries and thus may intensify blood damage by turbulent shear stresses. The different influences of struts and occluder guides on the flow around the occluders are discussed. The effects of the individual valve components on the flow fields are analyzed and correlated with the resulting pressure losses.

Aortic Valve↗
PubMed1988

In vitro comparison of aortic heart valve prostheses. Part 1: Mechanical valves.

The hydrodynamic performance of a series of mechanical heart valve prostheses is measured in the aortic position. All experiments are performed in an electrohydraulic, computer-controlled pulse duplicator simulating the left side of the human circulatory system. Testing conditions are set according to a Food and Drug Administration interlaboratory comparison protocol with cardiac outputs of 3.0, 4.5, 6.5, and 8.0 L/min at a constant heart rate of 70 beats/min. Mean systolic pressure differences, volume losses, and energy losses, as well as dimensionless pressure losses and energy loss coefficients, are calculated from the recorded pressure, volume, and flow tracings. The results of 10 mechanical heart valve prostheses (eight tilting disc valves and two bileaflet valves) are presented and their clinical relevance is discussed.

Aortic Valve↗