Biomedical subjects

A Rohrbach

Publications and source records attributed to A Rohrbach.

6 recordsLinked to original sources

PubMed2001

Optical trapping of dielectric particles in arbitrary fields.

We present a new method to calculate trapping forces of dielectric particles with diameters D < or = lambda in arbitrary electromagnetic, time-invariant fields. The two components of the optical force, the gradient force and the scattering force, are determined separately. Both the arbitrary incident field and the scatterer are represented by plane-wave spectra. The scattering force is determined by means of the momentum transfer in either single- or double-scattering processes. Therefore the second-order Born series is evaluated and solved in the frequency domain by Ewald constructions. Numerical results of our two-force-component approach and an established calculation method are compared and show satisfying agreement. Our procedure is applied to investigate axial trapping by focused waves experiencing effects of aperture illumination and refractive-index mismatch.

Electromagnetic Phenomena↗

PubMed2001

[Effect of peridural analgesia on labor progress].

Epidural analgesia (EDA) is the most effective method of intrapartum pain relief. Its influence on the course of labor continues to be controversial. Although a cause-and-effect relationship has not been proven, this form of analgesia has been blamed for a host of adverse maternal/fetal events during labor, including prolonged first and second stage of labor, dystocia, malrotation of the fetal head and an increased risk of operative delivery (instrumental delivery, Caesarean section). Our own data from the Department of Obstetrics and Gynaecology at the University of Leipzig demonstrate that women with epidural analgesia had a longer duration of labor (the greater proportion taking more than 13 hours), although labor was often already protracted before the start of epidural analgesia. Early epidural analgesia with a cervical dilatation of less than 4 cm does not have any negative impact on the progress of labor. The duration of second-stage pushing and the rate of instrumental deliveries were not increased in our patients. Although the Caesarean section rate for women with an EDA was elevated, the total proportion of secondary Caesarean section remained unchanged despite increased use of EDA. Our findings suggest that women selected for intrapartal EDA already represent a population with an increased risk of an unfavourable course of labor, priming of the cervix, increased need of oxytocin and nulliparity. Pain relief in itself is sufficient indication for the use of intrapartal epidural analgesia.

Adolescent↗

PubMed2000

Observing secretory granules with a multiangle evanescent wave microscope.

In total internal reflection fluorescence microscopy (TIRFM), fluorophores near a surface can be excited with evanescent waves, which decay exponentially with distance from the interface. Penetration depths of evanescent waves from 60 nm to 300 nm were generated by varying the angle of incidence of a laser beam. With a novel telecentric multiangle evanescent wave microscope, we monitored and investigated both single secretory granules and pools of granules in bovine chromaffin cells. By measuring the fluorescence intensity as a function of penetration depth, it is possible through a Laplace transform to obtain the fluorophore distribution as a function of axial position. We discuss the extent to which it is possible to determine distances and diameters of granules with this microscopy technique by modeling the fluorescent volumes of spheres in evanescent fields. The anisotropic near-field detection of fluorophores and the influence of the detection point-spread function are considered. The diameters of isolated granules between 70 nm and 300 nm have been reconstructed, which is clearly beyond the resolution limit of a confocal microscope. Furthermore, the paper demonstrates how evanescent waves propagate along surfaces and scatter at objects with a higher refractive index. TIRFM will have a limited applicability for quantitative measurements when the parameters used to define evanescent waves are not optimally selected.

Animals↗

PubMed2000

Role of actin cortex in the subplasmalemmal transport of secretory granules in PC-12 cells.

In neuroendocrine PC-12 cells, evanescent-field fluorescence microscopy was used to track motions of green fluorescent protein (GFP)-labeled actin or GFP-labeled secretory granules in a thin layer of cytoplasm where cells adhered to glass. The layer contained abundant filamentous actin (F-actin) locally condensed into stress fibers. More than 90% of the granules imaged lay within the F-actin layer. One-third of the granules did not move detectably, while two-thirds moved randomly; the average diffusion coefficient was 23 x 10(-4) microm(2)/s. A small minority (<3%) moved rapidly and in a directed fashion over distances more than a micron. Staining of F-actin suggests that such movement occurred along actin bundles. The seemingly random movement of most other granules was not due to diffusion since it was diminished by the myosin inhibitor butanedione monoxime, and blocked by chelating intracellular Mg(2+) and replacing ATP with AMP-PNP. Mobility was blocked also when F-actin was stabilized with phalloidin, and was diminished when the actin cortex was degraded with latrunculin B. We conclude that the movement of granules requires metabolic energy, and that it is mediated as well as limited by the actin cortex. Opposing actions of the actin cortex on mobility may explain why its degradation has variable effects on secretion.

Actins↗

PubMed1998

[Sevoflurane in obstetric anesthesia].

In obstetrics, general anaesthesia is increasingly being replaced by peridural anaesthesia. The cases where general anaesthesia is still used are those involving increased risk. It is therefore not surprising that, for example, maternal mortality in cases of caesarian section under general anaesthesia has not decreased. Indeed, large-scale statistical analyses in the USA and the UK show an increase in relative risk. For this reason, continuing efforts have to be made to improve obstetric general anaesthesia. Inhalational anaesthetics have an important role to play, as they represent the only acceptable compromise between the dangers of maternal awareness and neonatal depression. We used literature data and our own experience of urgent caesarian sections to investigate whether the new inhalational anaesthetic sevoflurane is suitable for this purpose. In a comparison of sevoflurane and peridural anaesthesia, there was no difference in outcome for the child and the maternal results showed only the expected procedure-specific differences in circulatory parameters and early postoperative analgesia requirement. A second study involving continuous pEEG monitoring (SEF90) showed advantages of sevoflurane over isoflurane in the initial phase of surgery and in the recovery phase. However, the value of sevoflurane in obstetric anaesthesia will have to be confirmed in more extensive studies.

Anesthesia, Epidural↗

PubMed1998

[Halogenated anesthetic vaporizers: importance of maintenance regulations].

We report a serious dysfunction of 19 halothane vaporisers Vapor 19.3 (Dräger) which delivered a much higher concentration of agent than indicated on the dial. This inaccuracy was linked to a major corrosion of the inner layers of the vaporiser, with a deposit of zinc bromide and chloride in the bypass channel. The main cause for this dysfunction was the absence of an adequate maintenance of the vaporisers since their purchase 3 years before.

Anesthesia, Inhalation↗