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Biomedical subjects

W Wolf

Publications and source records attributed to W Wolf.

214 records · Page 12Linked to original sources
PubMedDate not supplied

Symbolic programs for structural identification of linear pharmacokinetic systems.

Most parameter estimation techniques implicitly assume that it is possible to determine the parameters of the system uniquely if there were no noise in the output. In practice, this is not always the case. Whether or not this assumption is true for a given input-output experiment is the problem of structural identification. Two programs utilizing symbolic matrix calculus are presented which aid in solving this problem for linear systems. Pharmacokinetic examples are given.

Computers↗
PubMedDate not supplied

Simulation of linear compartment models with application to nuclear medicine kinetic modeling.

Several techniques are evaluated for solving the linear ordinary differential equations arising from compartment models. The methods involve approximating the matrix exponential of the state matrix (i.e. the transition matrix). The computational efficiencies of these techniques, together with that of a general purpose differential equation solver, are compared for several models arising from radiopharmacokinetic studies. The matrix exponential calculations are performed using both Ward's Padé approximation method and an eigenvalue-eigenvector decomposition (QR factorization) of the matrix A. These two algorithms have been incorporated as simulation options into the programs of the ADAPT package. ADAPT consists of a set of high-level programs for simulation, parameter estimation and experiment design, developed primarily for basic and clinical research modeling and data analysis applications involving pharmacokinetic and pharmacodynamic processes. The advantages and disadvantages of these simulation strategies for solving linear kinetic models within a parameter estimation setting are illustrated and discussed.

Algorithms↗
PubMedDate not supplied

Objective motor response onset detection in surface myoelectric signals.

Precise detection of discrete motor events like the onsets of voluntary muscle contractions is a prerequisite for various psychophysiological approaches in sensorimotor system analysis. In biomedical research and clinical diagnosis, motor events frequently are determined from surface electromyographic (SEMG) signals by some computerized detection algorithm. However, little is known about the reliability and accuracy of these methods, which frequently rely on intuitive and heuristic criteria. Therefore, the systematic approach to computerized detection of discrete motor events from SEMG signals presented by this paper fills a basic gap in EMG signal processing. Based upon a dynamic process model for the SEMG signal, a formal detection scheme is established which incorporates the essential processing modules common to the majority of algorithms. In addition, using concepts of statistically optimal change detection in random processes, a new model-based algorithm is presented which serves as a reference for optimum performance. The validity of this concept is demonstrated for the specific example of accurate detection of muscle activation onsets in rapid voluntary contractions; the estimation error (i.e., the deviation between estimated and "true" onset time) was evaluated by statistical simulations for three representative methods. Results show a substantial decrease of performance of traditional methods in the case of highly variable dynamic muscle activation profiles and/or superimposed activation patterns (e.g., due to a secondary motor task simultaneously executed by the same muscle). The model-based approach provided significantly more accurate results, even when the exact model parameters were unknown but estimated from the SEMG signal actually measured. It is concluded that the detection algorithm has to be critically taken into consideration during interpretation of motor events resolved from SEMG signals. The process model together with the corresponding statistically optimal detector represents an efficient tool for selecting appropriate detection algorithms for a particular experimental condition, and it allows a quantitative assessment of their performance.

Algorithms↗
PubMedDate not supplied

Systemic treatment of venous leg ulcers with high doses of pentoxifylline: efficacy in a randomized, placebo-controlled trial.

Several small studies have indicated that the systemic administration of pentoxifylline may accelerate healing of venous leg ulcers. The goal of this study was to further evaluate these findings in a larger scale placebo controlled trial and to explore the effect of the dose of pentoxifylline on healing. The study used a prospective, randomized, double-blind, parallel group placebo controlled design in a multicenter outpatient setting. Patients with one or more venous ulcer were enrolled, with all patients receiving standardized compression bandaging for treatment for their ulcers. Patients were also randomized to receive either pentoxifylline 400 mg, pentoxifylline 800 mg (two 400 mg tablets), or placebo tablets three times a day for up to 24 weeks. The main outcome measure was time to complete healing of all leg ulcers, using life table analysis. The study was completed as planned in 131 patients. Patients receiving 800 mg three times a day of pentoxifylline healed faster than placebo (p = 0.043, Wilcoxon test). The median time to complete healing was 100, 83, and 71 days for placebo, pentoxifylline 400 mg, and pentoxifylline 800 mg three times a day, respectively. Over half of all patients were ulcer free at week 16 (placebo) and at week 12 in both pentoxifylline groups. Whereas the placebo group had only achieved complete healing in half of the cases by week 16, all of the subjects remaining in the group receiving the high dose of pentoxifylline had healed completely. Treatment with pentoxifylline was well tolerated with similar drop-out rates in all three treatment groups. Complete wound closure occurred at least 4 weeks earlier in the majority of patients treated with pentoxifylline by comparison to placebo. A higher dose of pentoxifylline (800 mg three times a day) was more effective than the lower dose. We conclude that pentoxifylline is effective in accelerating healing of leg ulcers.

Adult↗
PubMedDate not supplied

Inhibition of EMG activity in isometrically loaded agonist muscle preceding a rapid contraction.

It was recently suggested that a premotion silent period in isometrically loaded agonist muscle reflects transition process in switching motor program from an isometric to an isotonic condition. To test this hypothesis we investigated changes in EMG activity of a biceps muscle during motor preparatory phase in an entirely isometric paradigm. Five healthy volunteers produced rapid isometric elbow flexions superimposed on a slightly sustained contraction during self-paced and reaction time conditions. In addition to surface EMG, single motor units were recorded in agonist muscle prior to a rapid contraction. Silent period was found in responses of three out of five subjects, whereas inhibition of some but not all tonically active motor units was observed in all studied subjects prior to the phasic EMG burst. Inhibition of motor unit activity occurred more often in self-paced than reaction time contractions. Our results indicate that inhibition is not always powerful enough to produce a complete electrical silence, and weaker inhibitory effects were observed as a declining number of firing motor units. We conclude that the silent period or depression of the EMG activity is associated with changes in motor program from tonic to phasic muscle activation and is not merely a consequence of changes from an isometric to an isotonic condition.

Action Potentials↗