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Biomedical subjects

Robert Cowan

Publications and source records attributed to Robert Cowan.

9 recordsLinked to original sources
PubMed2006

Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode.

INTRODUCTION: Preservation of residual hearing in cochlear implant recipients has been demonstrated to be possible and provides the potential benefit of combined electric and acoustic auditory stimulation. A prototype 16-mm multichannel array has been designed to facilitate placement of 22 electrodes without damage to intracochlear structures. The electrode array is suitable for insertion via the round window membrane (RWM) or a small cochleostomy. AIM: To evaluate the insertion trajectory and the presence of trauma to intracochlear structures with the prototype electrode inserted by either the RWM or a scala tympani cochleostomy. MATERIALS AND METHODS: Eighteen fresh frozen human temporal bones were prepared for cochlear implantation using a standard transmastoid facial recess technique. Twelve electrodes were implanted at the University of Melbourne and 6 at the Medizinische Hochschule Hannover. In Melbourne fluoroscopy was used to monitor the insertions. Twelve prototype electrodes were inserted via the RWM. A further 6 electrodes were inserted via a small scala tympani cochleostomy. The cochleostomy was sited inferior to the RWM to avoid trauma to the basilar membrane and spiral ligament. Specimens were embedded and fixed with acrylic resin and the cochleae then examined histologically at 200-mum intervals using a grinding and polishing technique. RESULTS: Full insertion of the electrode was achieved without significant resistance in all RWM and cochleostomy specimens. In two RWM specimens fold-over of the electrode tip occurred, and in one specimen the electrode penetrated the spiral ligament to lie in an 'endosteal 'position. In one cochleostomy specimen the electrode was rotated within the cochlea to face laterally rather than towards the modiolus. The final electrode position differed for the two groups, with the electrodes inserted via the RWM lying in a more perimodiolar position along the first part of the basal turn. The average depth of insertion was 240 degrees for the RWM electrodes and 255 degrees for the cochleostomy electrodes. Histologic examination showed no damage in any specimen to the modiolus, osseous spiral lamina or basilar membrane. CONCLUSIONS: A prototype hearing preservation electrode array was inserted by either a RWM or a scala tympani cochleostomy without evidence of significant intracochlear trauma.

Basilar Membrane↗
PubMed2006

Characterization of Fe(III)-reducing enrichment cultures and isolation of Fe(III)-reducing bacteria from the Savannah River site, South Carolina.

The Savannah River site, South Carolina (SRS), has been subjected to heavy metal and radionuclide contamination. Dissimilatory Fe(III)-reducing bacteria, by reducing insoluble Fe(III) to soluble Fe(II), may enhance contaminant mobility through subsurface environments. In order to investigate populations of the indigenous iron-reducing microbes from the SRS, duplicate enrichment cultures were initiated using a 10% inoculum of 7 sediment/soil samples, and serial dilutions were made into Wolfe's minimal salts media amended with 50 mM Fe(III) floc, 10 mM acetate and 0.01% yeast extract. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to generate fingerprints of the cultures and track changes in the microbial communities through the dilutions. Cluster analysis determined the relatedness of individual fingerprints. Initial enrichment cultures exhibited complex fingerprints consisting of many individual T-RF peaks, and demonstrated low similarity between sites. After four serial dilutions the fingerprints were less complex and clustered at higher similarities. Several individual T-RF peaks became dominant in a majority of the fingerprints. Cloning and sequence analysis revealed the presence of microbes closely related to Clostridium and Bacillus species and to known iron reducers such as Geobacter species and Pantoea agglomerans. Several Fe(III)-reducing isolates related to Aeromonas, Bacillus and Clostridium species were obtained.

Aeromonas↗
PubMed2006

Tachydysrhythmias.

Tachydysrhythmias arise from different mechanisms that can be characterized as being caused by re-entrant circuits, enhanced or abnormal automaticity, or triggered after-depolarizations. The approach to the tachydysrhythmia should begin with distinguishing sinus from non-sinus rhythms, then assessing QRS complex width and regularity. This article review tachydysrhythmias.

Action Potentials↗
PubMed2004

An in vitro model for investigating impedance changes with cell growth and electrical stimulation: implications for cochlear implants.

The impedance of stimulating electrodes used in cochlear implants and other neural prostheses often increases post-implantation, and is thought to be due to fibrous tissue encapsulation of the electrode array. Increased impedance results in higher power requirements to stimulate target neurons at set charge densities. We developed an in vitro model to investigate the electrode-tissue interface in a highly controlled environment. This model was tested using three cell types, with and without charge-balanced biphasic electrical stimulation. Under standard tissue culture conditions, a monolayer of cells was grown over the electrode surface. Electrode impedance increased in proportion to the extent of cell coverage of the electrode. Cell type was a significant factor in the amount of impedance increase, with kidney epithelial cells (MDCK) creating the greatest impedance, followed by dissociated rat skin fibroblasts and then macrophages (J774). The application of electrical stimulation to cell-covered electrodes caused impedance fluctuations similar to that seen in vivo, with a lowering of impedance immediately following stimulation, and a recovery to pre-stimulation levels during inactive periods. Examination of these electrodes suggests that the stimulation-induced impedance changes were due to the amount of cell cover over the electrodes. This in vitro technique accurately models the changes in impedance observed with neural prostheses in vivo, and shows the close relationship between impedance and tissue coverage adjacent to the electrode surface. We believe that this in vitro approach holds great promise to further our knowledge of the mechanisms contributing to electrode impedance.

Animals↗
PubMed2003

Substrate inhibition kinetics for toluene and benzene degrading pure cultures and a method for collection and analysis of respirometric data for strongly inhibited cultures.

We present an evaluation of the qualitative and quantitative effects that high concentrations of benzene and toluene have on the growth rate of several pure cultures that use these compounds as their sole carbon and energy source. The cultures employed were five widely studied environmental isolates: Pseudomonas putida F1, P. putida mt2, P. mendocina KR, Ralstonia pickettii PKO1, and Burkholderia cepacia G4. Three cultures degraded toluene following a pattern consistent with the kinetic model of Wayman and Tseng (1976) while the other two followed a modification of this model introduced by Alagappan and Cowan (2001). The pattern followed for benzene degradation was different than that for toluene degradation for all four capable pure cultures and consistent with that described by the model of Luong (1987). Mechanisms of substrate inhibition and solvent toxicity are discussed, used to conceptually evaluate the reasons for the differences in inhibition behavior, and used to support a call for more widespread use of the empirical, terminal substrate concentration inhibition models employed here. We also present the methodology developed to overcome a limitation commonly encountered when attempting to collect oxygen uptake data for use in quantifying substrate inhibition kinetics. The experimental method was effective for use in the collection of high quality data and the substrate inhibition models most useful in representing the growth of bacteria on these solvents are those that show a complete loss of activity at high concentration rather than the more popular asymptotic inhibition models.

Benzene↗
PubMed2003

Assessment of Language Skills in Young Children with Profound Hearing Loss under Two Years of Age.

This article presents an initial evaluation of a technique known as the Diary of Early Language (Di-EL), designed to obtain data about early lexical development in young children with profound hearing loss using cochlear implants, hearing aids, or both. The validity of the Di-EL, a parent report technique, was examined through comparisons with other measures of language development. Lexical data reported by parents using the Di-EL was found to agree with that reported by the same parents for the same children using the MacArthur Communicative Development Inventories (CDI), although some differences in the lexical items were noted. Rate of lexical acquisition on the Di-EL was found to correlate highly with that measured by the CDI and with expressive language skills as measured by the Rossetti Infant Toddler Language Scale, suggesting that the Di-EL is a valid measure of early lexical progress. These results are discussed with reference to other diary studies, along with research and clinical applications of the Di-EL.

Journal Article↗
PubMed2002

Treatment of migraine with rizatriptan: when to take the medication.

OBJECTIVE: To assess, in the setting of routine clinical practice, rizatriptan's effectiveness in treating acute migraine at its onset versus in a later phase of the attack. BACKGROUND: Although the efficacy and tolerability of rizatriptan have been well documented in clinical trials, no information is available about the impact of the timing of medication use on clinical outcome. METHODS: Migraineurs were enrolled in a prospective study and treated with rizatriptan 10 mg for two migraine attacks. Patients reported how they used the medication and treatment outcomes via an interactive voice response system. Outcome variables assessed included: time to onset of headache relief, headache severity 2 hours postdose and whether or not patients became largely symptoms-free (including associated symptoms), and when they were able to resume normal activities. Episodes in which patients took rizatriptan immediately after experiencing headache were designated "early use" and those in which patients delayed treatment until their headaches evolved to become moderate or severe were designated as "delayed use." Generalized estimating equations were used to assess the impact of early drug use and to control for age, gender, drug formulation, use of other migraine prescription medications before taking rizatriptan, attack sequence, migraine attack frequency, and recent migraine severity. RESULTS: A total of 1919 migraineurs with 3450 migraine attack episodes were evaluated, of which 1369 episodes were early use and 2081 delayed use. Compared to delayed-use episodes, episodes wherein patients took rizatriptan early were 1.33 times more likely to be characterized by onset of headache relief within 30 minutes posttreatment (95% confidence interval [CI], 1.11 to 1.60), 1.32 times more likely to result in a largely symptom-free state (95% CI, 1.12 to 1.57), and 1.34 times more likely to be associated with a return to normal activities (95% CI, 1.13 to 1.58) within 1 hour posttreatment. No significant difference between early-use and delayed-use episodes was observed with respect to the likelihood of there being no or mild headache 2 hours posttreatment (odds ratio, 1.05; 95% CI, 0.90 to 1.24). CONCLUSIONS: Whether taken at headache onset or later in the attack, rizatriptan was equally effective in relieving acute migraine headache by 2 hours postdose. Taking rizatriptan at the onset of headache was associated with more rapid relief of headache and reversal of functional disability.

Drug Administration Schedule↗
PubMed2002

Threshold, comfortable level and impedance changes as a function of electrode-modiolar distance.

OBJECTIVE: The study investigated the hypothesis that threshold and comfortable levels recorded from cochlear implant patients would reduce, and dynamic range increase, as distance of the electrode from the modiolar wall (radial distance) decreases. Two groups of cochlear implant patients participated; one group using the Nucleus' 24 Contour electrode array, and one group using the Nucleus standard straight (banded) array. The Nucleus 24 Contour array has been shown in temporal bone studies to lie closer to the modiolus than the banded array. The relationship of electrode impedance and radial distance is also investigated. DESIGN: The study, conducted at three centers, evaluated 21 patients using the Contour array, and 36 patients using the banded array. For each patient, threshold, comfortable levels and dynamic range were measured at four time points. Common ground electrode impedance was recorded clinically from each patient, at time intervals up to 12 wk. An estimate of the radial distance of the electrode from the modiolus was made by analysis of Cochlear view x-rays. RESULTS: Threshold and comfortable levels were significantly lower for the Nucleus 24 Contour array than for the banded array. However, dynamic range measurements did not show the predicted increase. In a majority of subjects, a significant correlation was found between the estimated radial distance of the electrode from the modiolus and the measured threshold and comfortable levels. This trend was not observed for dynamic range. The analysis indicates that other factors than radial distance are involved in the resultant psychophysical levels. Clinical impedance measures (common ground) were found to be significantly higher for the Contour array. However, the electrodes on the Contour array are half-rings, which are approximately only half the geometric size of the full rings as electrodes of the standard array. When the geometric electrode area in the two array designs are normalized, the trends in the electrode impedance behavior are similar. CONCLUSIONS: The results support the hypothesis that the relationship between the radial distance of the electrode and the psychophysical measures are influenced by patterns of fibrous tissue growth and individual patient differences, such as etiology and neural survival. Impedance measures for the Nucleus 24 Contour electrode array were higher than the banded electrode array, but this is primarily due to the reduction in electrode surface area. The different outcomes in impedance over time suggest differences in the relative contributions of the components of impedance with the two arrays.

Acoustic Impedance Tests↗