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PubMed1997

Threshold parameters for a simple stochastic partnership model of sexually transmitted diseases formulated as a two-type CMJ process.

A simple stochastic model describing an epidemic of a sexually transmitted disease, accommodating the formation and dissolution of partnerships, was formulated within a two-type Crump-Mode-Jagers (CMJ) process in continuous time. A submodel, describing the formation and dissolution of partnerships, was formulated in terms of a semi-Markov process with a finite state space and death as an absorbing state. This model was then linked to a two-type CMJ process through offspring distributions. Numerical values of threshold parameters greater than one suggests that either the epidemic dies out with a probability significantly different form zero or it spreads explosively in the population.

Female↗
PubMedDate not supplied

An empirical method to refine personality disorder classification using stepwise logistic regression modeling to develop diagnostic criteria and thresholds.

This study of DSM-III-R personality disorder (PD) classification provides an empirical approach to determine (1) the discriminative power of each criterion and (2) the optimal number of criteria needed to diagnose the presence of each PD. A semistructured assessment of 110 outpatients was performed for the 11 PDs and their 104 diagnostic criteria. Sensitivity, specificity, and predictive powers were calculated for each criterion item. Logistic regression was performed to determine (1) univariate weightings of the individual criteria as applied to a given diagnosis, and (2) multivariate measures of the criteria that significantly improved the chi-square value in a stepwise fashion. The significant items were then equally weighted to calculate the optimal number needed to diagnose category membership. Of 104 PD criteria, 41 discriminated at a significance level of .05 or less, and each PD could be optimally diagnosed with fewer criteria than currently required. We can empirically reduce the number of criteria combinations comprising individual categories, decrease heterogeneity, and narrow diagnostic boundaries. This increases the likelihood of identifying etiological factors, predictors of clinical course, specific treatments, familial aggregation, and neurobiological correlates for the PD taxa.

Adolescent↗
PubMed2006

Rapid induction of cerebral hypothermia is enhanced with active compression-decompression plus inspiratory impedance threshold device cardiopulmonary resusitation in a porcine model of cardiac arrest.

OBJECTIVES: A rapid, ice-cold saline flush combined with active compression-decompression (ACD) plus an inspiratory impedance threshold device (ITD) cardiopulmonary resusitation (CPR) will cool brain tissue more effectively than with standard CPR (S-CPR) during cardiac arrest (CA). BACKGROUND: Early institution of hypothermia after CPR and return of spontaneous circulation improves survival and outcomes after CA in humans. METHODS: Ventricular fibrillation (VF) was induced for 8 min in anesthetized and tracheally intubated pigs. Pigs were randomized to receive either ACD + ITD CPR (n = 8) or S-CPR (n = 8). After 2 min of CPR, 30 ml/kg ice-cold saline (3 degrees C) was infused over the next 3 min of CPR via femoral vein followed by up to three defibrillation attempts (150 J, biphasic). If VF persisted, epinephrine (40 microg/kg) and vasopressin (0.3 U/kg) were administered followed by three additional defibrillation attempts. Hemodynamic variables and temperatures were continuously recorded. RESULTS: All ACD + ITD CPR pigs (8 of 8) survived (defined as 15 min of return of spontaneous circulation [ROSC]) versus 3 of 8 pigs with S-CPR (p < 0.05). In survivors, brain temperature (degrees C) measured at 2-cm depth in brain cortex 1 min after ROSC decreased from 37.6 +/- 0.2 to 35.8 +/- 0.3 in ACD + ITD CPR versus 37.8 +/- 0.2 to 37.3 +/- 0.3 in S-CPR (p < 0.005). Immediately before defibrillation: 1) right atrial systolic/diastolic pressures (mm Hg) were lower (85 +/- 19, 4 +/- 1) in ACD + ITD CPR than S-CPR pigs (141 +/- 12, 8 +/- 3, p < 0.01); and 2) coronary perfusion pressures (mm Hg) were higher in ACD + ITD CPR (28.3 +/- 2) than S-CPR pigs (17.4 +/- 3, p < 0.01). CONCLUSIONS: A rapid ice-cold saline infusion combined with ACD + ITD CPR during cardiac arrest induces cerebral hypothermia more rapidly immediately after ROSC than with S-CPR.

Animals↗
PubMed1989

The effect of a random initial value in neural first-passage-time models.

The effect of a random initial value is examined in several stochastic integrate-and-fire neural models with a constant threshold and a constant input. The three models considered are approximations of Stein's model, namely: (1) a leaky integrator with deterministic trajectories, (2) a Wiener process with drift, and (3) an Ornstein-Uhlenbeck process. For model 1, different distributions for the initial value lead to commonly observed interspike interval distributions. For model 2, a discrete and a uniform distribution for the initial value are examined along with some parameter estimation procedures. For model 3, with a truncated normal distribution for the initial value, the coefficient of variation is shown to be greater than 1, and as the threshold becomes large the first-passage-time distribution approaches an exponential distribution. The relationships among the models and between them and previous models are also discussed, along with the robustness of the model assumptions and methods of their verification. The effects of a random initial value are found to be most pronounced at high firing rates.

Action Potentials↗
PubMed1997

Epidemic thresholds and vaccination in a lattice model of disease spread.

We use a lattice-based epidemic model to study the spatial and temporal rates of disease spread in a spatially distributed host population. The prevalence of the disease in the population is studied as well as the spread of infection about a point source of infection. In particular, two distinct critical population densities are identified. The first relates to the minimum population density for a epidemic to occur, whilst the second is the minimum population density for long-term persistence to occur. Vaccination regimes are introduced that are used to measure the impact of spatially and nonspatially dependent intervention strategies. Specifically we show how a ring of vaccinated susceptibles, of sufficient thickness, can halt the spread of infection across space.

Animals↗
PubMed2000

Modelling the effects of electric fields on nerve fibres: influence of the myelin sheath.

The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibres, incorporating three different myelin representations, are compared. The three myelin representations are a perfectly insulating single cable (model A), a finite impedance single cable (model B) and a finite impedance double cable (model C). Extracellular stimulation of the three models is used to study their strength-duration and current-distance (I-X) relationships, conduction velocity (CV) and action potential shape. All three models have a chronaxie time that is within the experimental range. Models B and C have increased threshold currents compared with model A, but each model has slope to the I-X relationship that matches experimental results. Model B has a CV that matches experimental data, whereas the CV of models A and C are above and below the experimental range, respectively. Model C is able to produce a depolarising afterpotential (DAP), whereas models A and B exhibit hyperpolarising afterpotentials. Models A and B are determined to be the preferred models when low-frequency stimulation (< approximately 25 Hz) is used, owing to their efficiency and accurate excitation and conduction properties. For high frequency stimulation (approximately 25 Hz and greater), model C, with its ability to produce a DAP, is necessary accurately to simulate excitation behaviour.

Animals↗
PubMed1992

Some threshold and stability results for epidemic models with a density-dependent death rate.

Most classical models for infectious diseases assume that the birth and death rates of individuals and the meeting rates between susceptible and infected individuals do not depend on the total number of individuals in the population. While these assumptions are valid in some situations they are less valid in others. For example, for diseases in animal an insects populations competition for scarce resources might well mean that the death rate depends on the number of individuals. The present paper examines two epidemic models where the death rate is density dependent. For each model the possible equilibrium levels of disease incidence are determined and the stability of these equilibrium levels to small perturbations is discussed. The biological interpretation of these results is presented together with the results of some numerical simulations.

Birth Rate↗
PubMedDate not supplied

Biliary obstruction in a canine model: sequential study of the sonographic threshold.

Complete occlusion of the common bile duct was surgically created in eight dogs. Sequential gray-scale ultrasound studies were performed with serum bilirubin determinations. Dilatation of the intrahepatic biliary radicles was first recognized sonographically from five to eight days postoperatively (average 6.4 days). The findings indicate a temporal lag between the onset of obstruction and recognition by ultrasound. In the appropriate clinical setting, a delayed repeat examination may be advisable. No correlation between ductal dilatation and serum bilirubin levels was ascertained.

Animals↗
PubMed2003

Ergodic dynamics in a natural threshold system.

Numerical simulations suggest that certain driven, dissipative mean-field threshold systems, including earthquake models, can be characterized by statistical properties often associated with ergodic dynamics, in the same sense as stochastic Brownian motion. We applied a fluctuation metric proposed by Thirumalai and Mountain [Phys. Rev. E 47, 479 (1993)]] for statistically stationary systems and find that the natural earthquake fault system in California demonstrates similar ergodic dynamics.

Computer Simulation↗
PubMed1982

Central integration of pulmonary stretch receptor input in the control of expiration.

The dynamics of the central processing of the discharge pattern from vagal pulmonary afferents that mediate the expiratory facilitatory reflex have been investigated. These studies involved the development of mathematical models based on analogs of neurophysiological principles such as temporal summation and threshold crossing. These models, which are capable of predicting the expiratory duration for arbitrary discharge patterns, were verified through comparison of their prediction with experimentally obtained relationships between expiratory duration (TE) and waveform parameters of various input patterns. These relationships were obtained by electrical activation of the largest vagal afferent fibers in bilaterally vagotomized, pentobarbital-anesthetized dogs. A parallel two-component model with long time constants (ca. 0.8 and 18 s) was best able to describe the experimental responses. This model suggests that 1) central integration of pulmonary stretch receptor (PSR) input is similar to long time-constant temporal summation; 2) central inspiratory inhibition (no vagal input) may share a common mechanism with vagal processing; 3) PSR-induced inhibition is a linear function of discharge frequency; and 4) TE depends on both the trajectory of lung deflation and the tonic activity at functional residual capacity. These characteristics embody information regarding specific neural arrangements and properties within the respiratory centers.

Action Potentials↗
PubMed1994

Temporary change of compound action potential amplitude after intense sound exposure.

Intense sound induces a temporary or permanent threshold shift. The aim of this study is to make an experimental model for temporary threshold shift (TTS) using guinea pigs. In this study, two kinds of sound exposure were used for inducing a threshold shift in the guinea pig; one of 110 dB SPL at 10 kHz for 10 min and the other of 120 dB SPL at 10 kHz for 5 min. The former condition produced different levels of reduction in compound action potential (CAP) immediately after sound exposure and different steady levels of CAP amplitude after recovery (within 120 min) in 10 guinea pigs. The latter condition produced a total loss of CAP amplitude immediately after sound exposure in 8 of 9 guinea pigs, and this condition continued for 120 min in 7 of the 9 guinea pigs. Additionally, in 2 of 3 guinea pigs which were exposed the sound of 120 dB SPL at 10 kHz for 5 min, the CAP amplitudes were almost the same as those of control animals 1 day after the exposure. One the basis of these results, we consider the sound of 120 dB SPL at 10 kHz for 5 min to be the optimal intensity and frequency of sound and duration of exposure for producing a TTS in the guinea pig.

Acoustic Stimulation↗
PubMed2004

Modelling anaerobic biomass growth kinetics with a substrate threshold concentration.

Many bacteria have been observed to stop growing below a certain substrate threshold concentration. In this study, a modification of the Monod kinetics expression has been proposed to take into account this substrate threshold concentration observed in bacterial growth. Besides the threshold concentration no additional parameters have been added to the kinetic expression and so, only the substrate threshold concentration and the half-saturation constant have to be estimated for model calibration purposes. Furthermore, for parameter estimation purposes, practical identifiability of this new function has been studied and the results have been satisfactory. The new model has been applied, as an example, to a simple anaerobic model to simulate the competition for hydrogen between sulphate reducers and methanogens in a thermophilic methanol-fed bioreactor. Oscillatory behaviour and mathematical instabilities have been avoided by using the proposed model. Parameter sensitivities have also been calculated along the simulation period in order to investigate the importance of hydrogen threshold concentration parameters.

Bacteria↗
PubMed2000

Using thresholds based on risk of cardiovascular disease to target treatment for hypertension: modelling events averted and number treated.

OBJECTIVE: To estimate the impact of using thresholds based on absolute risk of cardiovascular disease to target drug treatment to lower blood pressure in the community. DESIGN: Modelling of three thresholds of treatment for hypertension based on the absolute risk of cardiovascular disease. 5 year risk of disease was estimated for each participant using an equation to predict risk. Net predicted impact of the thresholds on the number of people treated and the number of disease events averted over 5 years was calculated assuming a relative treatment benefit of one quarter. SETTING: Auckland, New Zealand. PARTICIPANTS: 2158 men and women aged 35-79 years randomly sampled from the general electoral rolls. MAIN OUTCOME MEASURES: Predicted 5 year risk of cardiovascular disease event, estimated number of people for whom treatment would be recommended, and disease events averted over 5 years at different treatment thresholds. RESULTS: 46 374 (12%) Auckland residents aged 35-79 receive drug treatment to lower their blood pressure, averting an estimated 1689 disease events over 5 years. Restricting treatment to individuals with blood pressure >/=170/100 mm Hg and those with blood pressure between 150/90-169/99 mm Hg who have a predicted 5 year risk of disease >/=10% would increase the net number for whom treatment would be recommended by 19 401. This 42% relative increase is predicted to avert 1139/1689 (68%) additional disease events overall over 5 years compared with current treatment. If the threshold for 5 year risk of disease is set at 15% the number recommended for treatment increases by <10% but about 620/1689 (37%) additional events can be averted. A 20% threshold decreases the net number of patients recommended for treatment by about 10% but averts 204/1689 (12%) more disease events than current treatment. CONCLUSIONS: Implementing treatment guidelines that use treatment thresholds based on absolute risk could significantly improve the efficiency of drug treatment to lower blood pressure in primary care.

Adult↗