Biomedical subjects

P Auger

Publications and source records attributed to P Auger.

At least 19 recordsLinked to original sources

PubMed2005

Simulated interactions between a Class III antiarrhythmic drug and a figure 8 reentry.

Ventricular Fibrillation is responsible for a majority of sudden cardiac death, but little is known about how ventricular tachycardia (VT) degenerates into ventricular fibrillation. Several clinical studies focused only on preventing VT with a class III antiarrhythmic drug resulted in many deaths. Our simulations investigate the interactions between an antiarrhythmic drug likely to suppress a VT and a Figure 8 reentry. A parameter AAR is introduced to increase the action potential duration and therefore simulate various Class III drugs. Simulations are ran under several conditions (phases of the reentry, values of AAR, durations). They show that a VT can be suppressed whatever the phase of the reentry but it strongly depends on the duration of the effect. It confirms that a drug which can suppress a reentry can also worsen it. It also shows a great variety of activation patterns and thus the complexity of antiarrhythmic drugs effects. Simulations also demonstrate that suppressing VT is an increasing function of AAR.

Action Potentials↗

PubMed2005

Global production increased by spatial heterogeneity in a population dynamics model.

Spatial and temporal heterogeneity are often described as important factors having a strong impact on biodiversity. The effect of heterogeneity is in most cases analyzed by the response of biotic interactions such as competition of predation. It may also modify intrinsic population properties such as growth rate. Most of the studies are theoretic since it is often difficult to manipulate spatial heterogeneity in practice. Despite the large number of studies dealing with this topics, it is still difficult to understand how the heterogeneity affects populations dynamics. On the basis of a very simple model, this paper aims to explicitly provide a simple mechanism which can explain why spatial heterogeneity may be a favorable factor for production. We consider a two patch model and a logistic growth is assumed on each patch. A general condition on the migration rates and the local subpopulation growth rates is provided under which the total carrying capacity is higher than the sum of the local carrying capacities, which is not intuitive. As we illustrate, this result is robust under stochastic perturbations.

Models, Theoretical↗

PubMed2004

HREM, FIM and tomographic atom probe investigation of Guinier-Preston zones in an Al-1.54at% Cu alloy.

An investigation of Guinier-Preston zones in Al-1.54at% Cu alloy annealed for 30h at 100 degrees C was carried out on the same monocrystal with complementary techniques of high-resolution electron microscopy (HREM) and tomographic atom probe-field ion microscopy (TAP-FIM). HREM results show that majority of GP1 zones are monolayers 1-9nm in size. However, some GP2 zones and particles in an intermediate state of growth between GP1 and GP2 stage were also found. From TAP results it follows that GP1 zones with different Cu concentrations ranging from 40% up to 100% Cu coexist. The residual solid solution is very heterogeneous. In the vicinity of GP particles the Cu content in the matrix falls down to zero, the solid solution in other regions contains from 0.7 to 1at% Cu.

Journal Article↗

PubMed2004

Hardening of aged duplex stainless steels by spinodal decomposition.

Mechanical properties, such as hardness and impact toughness, of ferrite-containing stainless steels are greatly affected by long-term aging at intermediate temperatures. It is known that the alpha-alpha' spinodal decomposition occurring in the iron-chromium-based ferrite is responsible for this aging susceptibility. This decomposition can be characterized unambiguously by atom probe analysis, allowing comparison both with the existing theories of spinodal decomposition and the evolution of some mechanical properties. It is then possible to predict the evolution of hardness of industrial components during service, based on the detailed knowledge of the involved aging process.

Chromium↗

PubMed2003

Do migratory or demographic disruptions rule the population impact of pollution in spatial networks?

Ecotoxicology supplies environmental quality criteria mainly based on the potential effects of contaminants on demographic rates of natural populations. Possible impacts through pollutant-induced disruptions of spatial behaviors are totally neglected. Should it be significant to take into account this "second way"? We developed the example of a hypothetical brown trout population living in a river network. We analyzed how behaviors of toxic avoidance or attraction during the spawning migration alter the impact of pollution. Attraction behaviors basically enhanced the bad effect of pollution. More interesting, avoidance behaviors can weakly lift the asymptotic population growth rate, while if there is density-dependent effects on recruitment, pollutant avoidance can actually lead to a substantial drop in equilibrium size. Our model allowed comparing the relative significance of migratory and demographic disruptions for explaining the population impacts of pollution; we thus stress on the need of increasing efforts to develop knowledge relative to toxicant-induced spatial behaviors and to integrate such effects in the definition of environmental quality criteria.

Animal Migration↗

PubMed2003

Quantitative steps in symbiogenesis and the evolution of homeostasis.

The merging of two independent populations of heterotrophs and autotrophs into a single population of mixotrophs has occurred frequently in evolutionary history. It is an example of a wide class of related phenomena, known as symbiogenesis. The physiological basis is almost always (reciprocal) syntrophy, where each species uses the products of the other species. Symbiogenesis can repeat itself after specialization on particular assimilatory substrates. We discuss quantitative aspects and delineate eight steps from two free-living interacting populations to a single fully integrated endosymbiotic one. The whole process of gradual interlocking of the two populations could be mimicked by incremental changes of particular parameter values. The role of products gradually changes from an ecological to a physiological one. We found conditions where the free-living, epibiotic and endobiotic populations of symbionts can co-exist, as well as conditions where the endobiotic symbionts outcompete other symbionts. Our population dynamical analyses give new insights into the evolution of cellular homeostasis. We show how structural biomass with a constant chemical composition can evolve in a chemically varying environment if the parameters for the formation of products satisfy simple constraints. No additional regulation mechanisms are required for homeostasis within the context of the dynamic energy budget (DEB) theory for the uptake and use of substrates by organisms. The DEB model appears to be dosed under endosymbiosis. This means that when each free-living partner follows DEB rules for substrate uptake and use, and they become engaged in an endosymbiotic relationship, a gradual transition to a single fully integrated system is possible that again follows DEB rules for substrate uptake and use.

Animals↗

PubMed2002

Host patch selection induced by parasitism: basic reproduction ratio r(0) and optimal virulence.

The effects of parasites on the behavior of their hosts are well documented. For example, parasites may affect the habitat selection of the host individual. We used variables aggregation methods to investigate the way in which parasites affect the spatial pattern of susceptible hosts. We developed a simple epidemiological model, taking into account both the reproduction processes of hosts (density-dependent birth and death) and infection, considered separately on two different patches, and the migration of susceptible hosts between these two patches. We used the complete model of three equations to generate an aggregated model describing the dynamics of the combined susceptible and infected host populations. We obtained the basic reproduction ratio (R(0)) from the aggregated model, and then studied the effect of the migratory behavior of susceptible hosts on the ability of the parasite to invade the system. We also used the basic reproduction ratio to investigate the evolution of parasite virulence in relation to the migration decisions of susceptible hosts. We found that host investment in avoidance of the infected patch leads to an increase in optimal virulence if host investment is costly.

Animals↗

PubMed2001

Sternal wound infection after heart transplantation: incidence and results with aggressive surgical treatment.

BACKGROUND: Sternal wound infection remains a significant complication. We reviewed the incidence and the treatment of sternal wound infection after heart transplantation. METHODS: Of 226 patients who had a heart transplantation, 20 (8.8%) underwent postoperative wound debridement for superficial or deep sternal wound infection. The incidence and the survival of patients with sternal wound infection were analyzed. RESULTS: The incidence of sternal wound infection was similar among patients treated with four protocols of immunosuppressive drugs: cyclosporine and prednisone (0 of 22; 0%); cyclosporine, prednisone, and azathioprine (2 of 24; 8.3%); cyclosporine, prednisone, azathioprine, and antithymocyte globulin (15 of 139; 10.8%); and cyclosporine, prednisone, mycophenolate mofetil, and antithymocyte globulin (3 of 41; 7.3%) (p = 0.4). Six-month and 5-year survival of patients with sternal wound infection averaged 85% +/- 8% and 74% +/- 10% compared with 92% +/- 2% and 82% +/- 3% in patients without wound infection (p = 0.15). Patients with deep sternal wound infection, debridement, and reconstruction had a 5-year survival averaging 80% +/- 10%. CONCLUSIONS: The incidence of sternal wound infection remains similar between patients treated with the triple drug therapy. Surgical debridement and reconstruction can result in long-term survival after heart transplantation.

Acute Disease↗

PubMed2001

Linear discrete population models with two time scales in fast changing environments I: autonomous case.

In this work we consider a structured population with groups and subgroups of individuals. The intra-group dynamics is assumed to be fast in comparison with the inter-group dynamics. We study linear discrete models where the slow dynamics is represented by a single matrix and the fast dynamics is described by means of the first k terms of a converging sequence of different matrices. The number k can be interpreted as the ratio between the two time scales. The aim of this work is to extend aggregation techniques to the case of fast changing environments. The main idea of aggregation is to build up a new system, with lower dimension, that summarizes the information concerning the fast process. This "aggregated" system provides essential information on the original one. It is shown that the asymptotic behavior of the original system can be approximated by the asymptotic behavior of the aggregated system when the ratio between the two time scales is large enough. We present an example of an age structured population in a patchy environment. The migration process is assumed to be fast in comparison with the demographic process. Numerical simulations illustrate that the asymptotic growth rate and the stable age distribution of the population in the original and the aggregated systems are getting closer as the ratio k increases.

Animals↗

PubMed2001

Predictive value of Candida colonization index in 131 patients undergoing two different cardiovascular surgical procedures.

BACKGROUND: Nosocomial candidemia is an important infection because of its increasing incidence and its high fatality rate. Candidiasis involves multiple risk factors. In this work, we study the degree of Candida colonization in cardiovascular surgical patients by taking into account the number of sites colonized and the density of growth. METHODS: Eleven (11) selected variables (age, sex, weight, diabetes, number of antibiotics, duration of antibiotics, length of hospitalization, length of stay in surgical intensive care unit, duration of surgery, temperature during surgery, and number of bypass) were considered to predict the perioperative variation of the colonization index (CI) by Candida in 131 cardiovascular surgical patients. These patients were divided into two groups: group A, coronary artery bypass grafting with extracorporeal circulation (72 patients) and group B, coronary artery bypass grafting without extracorporeal circulation (59 patients). RESULTS: One thousand and forty-eight fungal cultures were obtained from four different body sites and 162 isolated were identified. Candida albicans accounted for 74% of the strains in group A and 97% in group B. The statistical analysis (two-way anova) shows that group A patients with an increased CI have received significantly more antibiotics than those with a stable CI (1.50+/-0.83 vs 1.08+/- 0.40, p=0.0055). CONCLUSIONS: Epidemiological data obtained from this study show that coronary artery bypass grafting with extracorporeal circulation procedure is associated with an increase in the use of antibiotics and subsequently a higher risk a Candida colonization-infection.

Aged↗

PubMed2000

Dynamics of a feline virus with two transmission modes within exponentially growing host populations.

Feline panleucopenia virus (FPLV) was introduced in 1977 on Marion Island (in the southern Indian Ocean) with the aim of eradicating the cat population and provoked a huge decrease in the host population within six years. The virus can be transmitted either directly through contacts between infected and healthy cats or indirectly between a healthy cat and the contaminated environment: a specific feature of the virus is its high rate of survival outside the host. In this paper, a model was designed in order to take these two modes of transmission into account. The results showed that a mass-action incidence assumption was more appropriate than a proportionate mixing one in describing the dynamics of direct transmission. Under certain conditions the virus was able to control the host population at a low density. The indirect transmission acted as a reservoir supplying the host population with a low but sufficient density of infected individuals which allowed the virus to persist. The dynamics of the infection were more affected by the demographic parameters of the healthy hosts than by the epidemiological ones. Thus, demographic parameters should be precisely measured in field studies in order to obtain accurate predictions. The predicted results of our model were in good agreement with observations.

Animals↗

PubMed2000

Immediate and long-term results of valve replacement for native and prosthetic valve endocarditis.

BACKGROUND: The objective of the present study was to compare current results of prosthetic valve replacement following acute infective native valve endocarditis (NVE) with that of prosthetic valve endocarditis (PVE). Prosthetic valve replacement is often necessary for acute infective endocarditis. Although valve repair and homografts have been associated with excellent outcome, homograft availability and the importance of valvular destruction often dictate prosthetic valve replacement in patients with acute bacterial endocarditis. METHODS: A retrospective analysis of the experience with prosthetic valve replacement following acute NVE and PVE between 1988 and 1998 was performed at the Montreal Heart Institute. RESULTS: Seventy-seven patients (57 men and 20 women, mean age 48 +/- 16 years) with acute infective endocarditis underwent valve replacement. Fifty patients had NVE and 27 had PVE. Four patients (8%) with NVE died within 30 days of operation and there were no hospital deaths in patients with PVE. Survival at 1, 5, and 7 years averaged 80% +/- 6%, 76% +/- 6%, and 76% +/- 6% for NVE and 70% +/- 9%, 59% +/- 10%, and 55% +/- 10% for PVE, respectively (p = 0.15). Reoperation-free survival at 1, 5, and 7 years averaged 80% +/- 6%, 76% +/- 6%, and 76% +/- 6% for NVE and 45% +/- 10%, 40% +/- 10%, and 36% +/- 9% for PVE (p = 0.003). Five-year survival for NVE averaged 75% +/- 9% following aortic valve replacement and 79% +/- 9% following mitral valve replacement. Five-year survival for PVE averaged 66% +/- 12% following aortic valve replacement and 43% +/- 19% following mitral valve replacement (p = 0.75). Nine patients underwent reoperation during follow-up: indications were prosthesis infection in 4 patients (3 mitral, 1 aortic), dehiscence of mitral prosthesis in 3, and dehiscence of aortic prosthesis in 2. CONCLUSIONS: Prosthetic valve replacement for NVE resulted in good long-term patient survival with a minimal risk of reoperation compared with patients who underwent valve replacement for PVE. In patients with PVE, those who needed reoperation had recurrent endocarditis or noninfectious periprosthetic dehiscence.

Adult↗

PubMed2000

Emergence of individual behaviour at the population level. Effects of density-dependent migration on population dynamics.

The aim of this work is to study the effects of different individual behaviours on the overall growth of a spatially distributed population. The population can grow on two spatial patches, a source and a sink, that are connected by migrations. Two time scales are involved in the dynamics, a fast one corresponding to migrations and a slow one associated with the local growth on each patch. Different scenarios of density-dependent migration are proposed and their effects on the population growth are investigated. A general discussion on the use of aggregation methods for the study of integration of different ecological levels is proposed.

Animals↗

PubMed2000

Methods of aggregation of variables in population dynamics.

In ecology, we are faced with modelling complex systems involving many variables corresponding to interacting populations structured in different compartmental classes, ages and spatial patches. Models that incorporate such a variety of aspects would lead to systems of equations with many variables and parameters. Mathematical analysis of these models would, in general, be impossible. In many real cases, the dynamics of the system corresponds to two or more time scales. For example, individual decisions can be rapid in comparison to growth of the populations. In that case, it is possible to perform aggregation methods that allow one to build a reduced model that governs the dynamics of a lower dimensional system, at a slow time scale. In this article, we present a review of aggregation methods for time continuous systems as well as for discrete models. We also present applications in population dynamics. A first example concerns a continuous time model of a single population distributed on a system of two connected patches (a logistic source and a sink), by fast migration. It is shown that under a certain condition, the total equilibrium population can be larger than the carrying capacity of the logistic source. A second example concerns a discrete model of a population distributed on two patches, still a source and a sink, connected by fast migration. The use of aggregation methods permits us to conclude that density-dependent migration can stabilize the total population.

Animals↗

PubMed2000

Colonization of vineyards by phytoseiid mites: their dispersal patterns in the plot and their fate.

The effect of wind and woody margins on the dispersal and population dynamics of phytoseiid mites was studied in a vine plot for a period of two years. Mites were sampled in the plot and in the surrounding vegetation (crops and natural vegetation) in order to determine phytoseiid mite abundance. The surrounding vegetation was considered to be a reservoir of phytoseiids from where the vine plot could be invaded. Directional and non-directional soil and aerial traps were placed in the plot to determine predatory mite exchange between the two areas. Colonization of the plot occurred in two stages: first, mite migration into the plot, followed by their establishment. The two-year study partially clarified the first of these two stages. Kampimodromus aberrans was the main species caught in the aerial traps. Phytoseiid mite dispersal within the vine plot seemed to be affected by both wind (direction, intensity and regularity) and phytoseiid mite density in the woody margin. However, the woody margin had a large effect only over a short distance. Some observations pointed towards an effect of other reservoir areas but it was not possible to characterize these. The population density of the phytoseiid mites in the plot increased from 1996 to 1998, but these increases are much smaller than one would expect on the basis of the number of mites migrating by air in the plot. Moreover, blocks where most mites were trapped were not the blocks where densities of phytoseiid mites on vine leaves were the largest. It therefore seems likely that not all migrants were able to develop. Their settlement pattern was not determined and this could constitute a potential research focus for the future.

Agriculture↗

PubMed2000

The dynamics of a fish stock exploited in two fishing zones.

This work presents a specific stock-effort dynamical model. The stocks correspond to two populations of fish moving and growing between two fishery zones. They are harvested by two different fleets. The effort represents the number of fishing boats of the two fleets that operate in the two fishing zones. The bioeconomical model is a set of four ODE's governing the fishing efforts and the stocks in the two fishing areas. Furthermore, the migration of the fish between the two patches is assumed to be faster than the growth of the harvested stock. The displacement of the fleets is also faster than the variation in the number of fishing boats resulting from the investment of the fishing income. So, there are two time scales: a fast one corresponding to the migration between the two patches, and a slow time scale corresponding to growth. We use aggregation methods that allow us to reduce the dimension of the model and to obtain an aggregated model for the total fishing effort and fish stock of the two fishing zones. The mathematical analysis of the model is shown. Under some conditions, we obtain a stable equilibrium, which is a desired situation, as it leads to a sustainable harvesting equilibrium, keeping the stock at exploitable densities.

Animals↗

PubMed1999

A discrete model with density dependent fast migration.

The aim of this work is to develop an approximate aggregation method for certain non-linear discrete models. Approximate aggregation consists in describing the dynamics of a general system involving many coupled variables by means of the dynamics of a reduced system with a few global variables. We present discrete models with two different time scales, the slow one considered to be linear and the fast one non-linear because of its transition matrix depends on the global variables. In our discrete model the time unit is chosen to be the one associated to the slow dynamics, and then we approximate the effect of fast dynamics by using a sufficiently large power of its corresponding transition matrix. In a previous work the same system is treated in the case of fast dynamics considered to be linear, conservative in the global variables and inducing a stable frequency distribution of the state variables. A similar non-linear model has also been studied which uses as time unit the one associated to the fast dynamics and has the non-linearity in the slow part of the system. In the present work we transform the system to make the global variables explicit, and we justify the quick derivation of the aggregated system. The local asymptotic behaviour of the aggregated system entails that of the general system under certain conditions, for instance, if the aggregated system has a stable hyperbolic fixed point then the general system has one too. The method is applied to aggregate a multiregional Leslie model with density dependent migration rates.

Ecology↗

PubMed1999

Behavioral choices based on patch selection: a model using aggregation methods.

The aim of this work is to study the influence of patch selection on the dynamics of a system describing the interactions between two populations, generically called 'population N' and 'population P'. Our model may be applied to prey-predator systems as well as to certain host-parasite or parasitoid systems. A situation in which population P affects the spatial distribution of population N is considered. We deal with a heterogeneous environment composed of two spatial patches: population P lives only in patch 1, while individuals belonging to population N migrate between patch 1 and patch 2, which may be a refuge. Therefore they are divided into two patch sub-populations and can migrate according to different migration laws. We make the assumption that the patch change is fast, whereas the growth and interaction processes are slower. We take advantage of the two time scales to perform aggregation methods in order to obtain a global model describing the time evolution of the total populations, at a slow time scale. At first, a migration law which is independent on population P density is considered. In this case the global model is equivalent to the local one, and under certain conditions, population P always gets extinct. Then, the same model, but in which individuals belonging to population N leave patch 1 proportionally to population P density, is studied. This particular behavioral choice leads to a dynamically richer global system, which favors stability and population coexistence. Finally, we study a third example corresponding to the addition of an aggregative behavior of population N on patch 1. This leads to a more complicated situation in which, according to initial conditions, the global system is described by two different aggregated models. Under certain conditions on parameters a stable limit cycle occurs, leading to periodic variations of the total population densities, as well as of the local densities on the spatial patches.

Animals↗