Biomedical subjects

T E Wehrly

Publications and source records attributed to T E Wehrly.

7 recordsLinked to original sources

PubMed1998

A general approach to non-Markovian compartmental models.

Stochastic compartmental models are usually based on continuous-time Markov processes. Markovian models assume that the compartments have exponential retention times, which is known not to hold in certain applications such as calcium clearance from bone. A number of semi-Markov models with restricted families of retention times have been proposed recently. This paper presents a general, tractable procedure for determining from suitable data the estimated retention time distributions for assumed non-Markovian phenomenological models. The procedure is based on using phase-type distributions. These distributions can describe the long tails observed in calcium clearance data, and they frequently lead to complex eigenvalues that are often overlooked in practice. The procedure is illustrated on several proposed models for describing a particular data set.

Markov Chains↗

PubMed1996

Cluster analysis of MCMI scores of suicidal psychiatric patients: four personality profiles.

Millon Clinical Multiaxial Inventory (MCMI) scores from 299 suicidal psychiatric outpatients were cluster analyzed in hopes of identifying clinical subgroups and deriving specific treatment indications. The analysis revealed four personality profiles: Negativistic/Avoidant/Schizoid, Avoidant/Dependent/ Negativistic, Antisocial, and Histrionic/Narcissistic. This grouping was validated by examining demographics, diagnoses, and scores on several psychometric scales. Results showed few differences in demographics, diagnosis, or intelligence, but significant differences in depression severity, hopelessness, anxiety, problem-solving self-appraisal, and alcohol abuse. Implications for tailoring interventions for subtypes of suicidal patients are discussed.

Adolescent↗

PubMed1990

Generalized stochastic compartmental models with Erlang transit times.

This paper considers the use of semi-Markov process models with Erlang transit times for the description of compartmental systems. The semi-Markov models seem particularly useful for systems with nonhomogeneous "poorly-stirred" compartments. The paper reviews the Markov process models with exponential transit times, and illustrates the application of such models, describing the clearance of calcium in man. The semi-Markov model with Erlang transit times is then developed, and the solutions for its concentration-time curves and residence time moments are given. The use of semi-Markov models is illustrated with the same calcium data, and the results from the two models are compared. The example demonstrates that these semi-Markov models are physiologically more realistic than standard models and may be fitted to pharmacokinetic data using readily available software.

Mathematics↗

PubMed1989

Some generalized stochastic compartment models for digesta flow.

Digesta flow models have been based on linear compartment theory that assumes exponential retention times, and on a generalized theory that incorporates nonexponential (Erlang) retention times (Matis, 1987, Journal of Theoretical Biology 124, 371-376). This paper develops a new family of passage models for heterogeneous digesta by mixing the previous models with assumed parametric, usually gamma, mixing distributions. The utility of the resulting models is demonstrated with experimental data on two treatments, namely a chopped and a ground straw, given to each of four cows. Treatment differences are apparent in the preferred model form and in the means of the estimated mean residence times. The models are relatively easy to fit to data using standard estimation procedures, and they should have broad application to other compartment modeling problems with "heterogeneous particles."

Animals↗

PubMed1985

Use of residence time moments in compartmental analysis.

Residence time moments, particularly mean residence times, can be very useful in the modeling and kinetic analysis of physiological systems. They are defined and illustrated, and their practical utility is then explored by comparing the model formulation based on these moments with three alternative formulations: component half-lives, compartmental model rate coefficients, and multiexponential model coefficients and exponents. The residence time moment formulation is shown to have several advantages: a smaller number of parameters are usually needed to describe a biological system; they have a clear physical interpretation; their statistical "power" to detect certain treatment differences is greater; and they are computationally simpler in many situations.

Animals↗

PubMed1983

On some stochastic formulations and related statistical moments of pharmacokinetic models.

This paper presents the deterministic and stochastic model for a linear compartment system with constant coefficients, and it develops expressions for the mean residence times (MRT) and the variances of the residence times (VRT) for the stochastic model. The expressions are relatively simple computationally, involving primarily matrix inversion, and they are elegant mathematically, in avoiding eigenvalue analysis and the complex domain. The MRT and VRT provide a set of new meaningful response measures for pharmacokinetic analysis and they give added insight into the system kinetics. The new analysis is illustrated with an example involving the cholesterol turnover in rats.

Animals↗

PubMed1982

Effect of varying dietary ratio of sodium and chloride on the responses of lactating dairy cows in hot weather.

Twenty-four lactating Holstein and Jersey cows were fed concentrate mixtures formulated to supplement total diets with 1) no added sodium chloride, 2) 1.0% sodium chloride, 3) 1.14% calcium chloride, and 4) 1.45% sodium bicarbonate during a 9-wk continuous trial, Experiment 1. Responses measured were body temperature, respiration rate, milk yield, milk composition, and 15 blood variables to evaluate acid-base status and express metabolic profile. Treatments affected body temperature and pH, bicarbonate, total carbon dioxide, and base excess of blood. No treatment affected milk yield and composition. Differences in dietary electrolytes had little influence on responses. In a second experiment immediately following the first, a 1-km walking stress was imposed during midafternoon on 3 days; treatment did not influence change in response variables or recovery from stress although Jerseys recovered faster than Holsteins.

Animals↗