Kinetic Bhatnagar-Gross-Krook model for fast reactive mixtures and its hydrodynamic limit.

A recently proposed consistent Bhatnagar-Gross-Krook-type approach for reversible bimolecular chemical reactions, well suited to deal with collision dominated gas mixtures in which mechanical and chemical relaxation times are of the same order of magnitude (fast reactions), is discussed. The model recovers essential features of the chemical process such as mass action law at equilibrium and reactive H theorem. The hydrodynamic limit, at both Euler and Navier-Stokes levels, is derived by a Chapman-Enskog procedure, in terms of the relevant hydrodynamic variables, and compared to the corresponding limits holding in the nonreactive and in the slowly reactive cases. In particular, results show that reactive corrections to transport coefficients cannot be neglected for fast reactions.

Effect of input resistance voltage-dependency on DC estimate of membrane capacitance in cardiac myocytes.

The measure of membrane capacitance (C(m)) in cardiac myocytes is of primary importance as an index of their size in physiological and pathological conditions, and for the understanding of their excitability. Although a plethora of very accurate methods has been developed to access C(m) value in single cells, cardiac electrophysiologists still use, in the majority of laboratories, classical direct current techniques as they have been established in the early days of cardiac cellular electrophysiology. These techniques are based on the assumption that cardiac membrane resistance (R(m)) is constant, or changes negligibly, in a narrow potential range around resting potential. Using patch-clamp whole-cell recordings, both in current-clamp and voltage-clamp conditions, and numerical simulations, we document here the voltage-dependency of R(m), up to -45% of its resting value for 10-mV hyperpolarization, in resting rat ventricular myocytes. We show how this dependency makes classical protocols to misestimate C(m) in a voltage-dependent manner (up to 20% errors), which can dramatically affect C(m)-based calculations on cell size and on intracellular ion dynamics. We develop a simple mechanistic model to fit experimental data and obtain voltage-independent estimates of C(m), and we show that accurate estimates can also be extrapolated from the classical approach.

Modelling of predator-prey trophic interactions. Part I: two trophic levels.

A class of lumped parameter models to describe the local dynamics in a controlled environment of a two-trophic chain is considered. The class is characterized by a trophic function (functional response of predator to the abundance of prey) depending on the ratio of prey biomass x and a linear function of predator biomass y: f(qx/[(1-rho)k + rhoy]), where q is the efficiency of the predation process, k is a reference biomass, and rho (0 < or = rho < or = 1) specifies the predation model. The trophic function is defined only by some properties determining its shape. A stability analysis of the models has been performed by taking the parameters q and rho as bifurcation parameters: the regions in the (rho,q) plane of existence and stability of nonnegative equilibrium states and limit cycles are determined. This analysis shows that the behaviour of the models is qualitatively similar for 0 < or = rho < 1 (in particular the null state is always a saddle point), while the value rho=1 gives rise to some kind of structural instability of the system (in particular the null state becomes an attractor for sufficiently high predation efficiency).

[The efficacy of oral treatment with flecainide for paroxysmal atrial fibrillation: correlation with plasma concentration]. / Efficacia del trattamento orale con flecainide nella fibrillazione atriale parossistica: correlazione con le concentrazioni plasmatiche.

In the acute treatment of paroxysmal atrial fibrillation several drugs can be used. The aim of our work was to assess the efficacy of a single oral dose of flecainide in the conversion to sinus rhythm by correlating this data with flecainide plasma concentration. We have considered 37 patients affected by paroxysmal atrial fibrillation (for more than 8 hours) randomly assigned to the following two groups: group A, 19 patients, mean age 44.4 +/- 1.9 years) treated with flecainide (200 mg) and control group B (18 patients, mean age 46.6 +/- 1.8 years). This was done in order to point out any possible overlap between pharmacological and spontaneous conversion to sinus rhythm. In all patients, the following were performed: a Holter recording (524 hours) to evaluate the time of conversion to sinus rhythm (t-conversion to sinus rhythm), a determination of flecainide plasma concentration (after 150 flecainide administration) an Rx, an Echo-2D/Doppler test and an estimation of thyroid function. The Rx, the Echo-2D/Doppler and the endocrinological data in the 2 groups did not show any significant differences. We obtained a conversion to sinus rhythm in all but one of the group A patients (time of conversion to sinus rhythm 162 +/- 83 min) and in just 5 group B patients (time of conversion to sinus rhythm 1118 +/- 125 min) (time of conversion to sinus rhythm A vs B p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)