Biomedical subjects

S Ellner

Publications and source records attributed to S Ellner.

9 recordsLinked to original sources

PubMed2001

End-tidal carbon dioxide measurements as a prognostic indicator of outcome in cardiac arrest.

PURPOSE: To evaluate the use of end-tidal carbon dioxide values in predicting survival in cardiopulmonary arrest. BACKGROUND: The decision about when to terminate resuscitative efforts for patients with cardiopulmonary arrest is often subjective. End-tidal carbon dioxide values have been suggested as potential objective criteriafor making this decision. METHODS: This study was a cooperative effort of the St Louis chapter of the American Association of Critical-Care Nurses and its members and involved 6 hospitals and an air evacuation service. All adult patients who had a cardiopulmonary arrest were eligiblefor the study. Once a patient with cardiac arrest was intubated, end-tidal carbon dioxide and cardiac rhythms were measured and recorded every 5 minutes for 20 minutes or until resuscitation efforts were terminated. Patients' survival at the time of the arrest, survival 24 hours after the arrest, and discharge status were followed up. RESULTS: A total of 127 patients were enrolled in the study. All but 1 patient with end-tidal carbon dioxide values less than 10 mm Hg died before discharge. End-tidal carbon dioxide values greater than 10 mm Hg were associated with various degrees of survival. Overall survival to discharge was less than 14%, regardless of the end-tidal carbon dioxide value. CONCLUSION: Measurements of end-tidal carbon dioxide can be used to accurately predict nonsurvival of patients with cardiopulmonary arrest. End-tidal carbon dioxide levels should be monitored during cardiopulmonary arrest and should be considered a useful prognostic value for determining the outcome of resuscitative efforts.

Capnography↗

PubMed1996

Patterns of genetic polymorphism maintained by fluctuating selection with overlapping generations.

We study the form of polymorphisms that can be maintained by the joint effects of generation overlap and randomly fluctuating selection, acting on a quantitative trait affecting offspring viability. The genetic system can be single locus or multilocus, haploid or diploid. Selection is assumed to be stabilizing with a randomly fluctuating optimum, and we assume additive allelic effects without epistasis. For fluctuations above a certain threshold, nonzero genetic variance is maintained in an evolutionarily stable population. Our model allows a continuum of alleles with arbitrary effects at each locus. Nonetheless, the genotype distribution in an evolutionarily stable population is discrete--a polymorphism of a few alleles with distinctly different effects--and often involves only a pair of alleles at each locus. The form of the genotype distribution depends on the number of loci affecting the trait, and on the kurtosis of the distribution of the phenotypic optimum theta. If the trait is affected by several loci, the number of polymorphic loci increases with increased variance of fluctuations in selection. For distributions of theta with negative kurtosis (i.e., lower kurtosis than a Gaussian) the number of polymorphic loci increases gradually (0-->1-->2--> ...M), and the genetic variability is in the form of a few diallelic or triallelic loci with alleles of large effect. For distributions with positive kurtosis, the increase is abrupt (0 -> many) and involves many diallelic loci. These results do not fit the conventional multivariate Gaussian or near-Gaussian models for quantitative traits, but may partially explain recent findings that heritable variation in natural populations is often due to genes of large effect.

Diploidy↗

PubMed1996

Environmental fluctuations and the maintenance of genetic diversity in age or stage-structured populations.

The ability of random fluctuations in selection to maintain genetic diversity is greatly increased when generations overlap. This result has been derived previously using genetic models with very special assumptions about the population age structure. Here we explore its robustness in more realistic population models, with very general age structure or physiological structure. For a range of genetic models (haploid, diploid, single and multi-locus) we find that the condition for maintaining genetic diversity generalizes almost without change. Genetic diversity is maintained by selection if a product of the form (generation overlap) x (selection intensity) x (variability in the selection regime) is sufficiently large, where the generation overlap is measured in units of Fisher's reproductive value. This conclusion is based on a local evolutionary stability analysis, which differs from the standard "protected polymorphism" criterion for the maintenance of genetic diversity. Simulation results match the predictions from the local stability analysis, but not those from the protected polymorphism criterion. The condition obtained here for maintaining genetic diversity requires fitness fluctuations that are substantial but well within the range observed in many studies of natural populations.

Aging↗

PubMed1992

A mathematical model for estimation of broiler egg weight loss from physical dimensions and air cell size during incubation.

A formula was derived that will enable hatcheries to estimate percentage weight loss using the width, length, and air cell diameter of the egg. The formula was derived by fitting an equation to the shape of the egg and using this shape equation to obtain estimates of air cell volume and egg volume. These estimates were coupled with the principle that weight = density x volume to give the final formula as a function of air cell diameter divided by egg width. The formula was tested by comparing its estimated values with measured values from eggs set at two different incubation conditions chosen to cover the range of conditions found in commercial hatcheries. As the day of transfer approached, embryo size and distortion of the air cell caused inaccuracies to develop in the results of this equation. However, data suggest that the equation can be used to calculate percentage weight loss until approximately the 16th day of incubation.

Animals↗

PubMed1989

Convergence to stationary distributions in two-species stochastic competition models.

Two sets of sufficient conditions are given for convergence to stationary distributions, for some general models of two species competing in a randomly varying environment. The models are nonlinear stochastic difference equations which define Markov chains. One set of sufficient conditions involves strong continuity and phi-irreducibility of the transition probability for the chain. The second set has a much weaker irreducibility condition, but is only applicable to monotonic models. The results are applied to a stochastic two-species Ricker model, and to Chesson's "lottery model with vacant space", to illustrate how the assumptions can be checked in specific models.

Ecology↗

PubMed1986

Effects of noise on some dynamical models in ecology.

We investigate effects of random perturbations on the dynamics of one-dimensional maps (single species difference equations) and of finite dimensional flows (differential equations for n species). In particular, we study the effects of noise on the invariant measure, on the "correlation" dimension of the attractor, and on the possibility of detecting the nonlinear deterministic component by applying reconstruction techniques to the time series of population abundances. We conclude that adding noise to maps with a stable fixed-point obscures the underlying determinism. This turns out not to be the case for systems exhibiting complex periodic or chaotic motion, whose essential properties are more robust. In some cases, adding noise reveals deterministic structure which otherwise could not be observed. Simulations suggest that similar results hold for flows whose attractor is almost two-dimensional.

Animals↗

PubMed1985

ESS germination strategies in randomly varying environments. I. Logistic-type models.

ESS germination strategies are studied in a model of annual plant population dynamics in a randomly varying environment. The possible strategies are different values of the annual germination fraction G, either constant over time or varying in response to a "cue" correlated with upcoming environmental conditions. The model generalizes D. Cohen's model (1966, J. Theor. Biol. 12, 119-129; 1968, J. Ecol. 56, 219-228) by allowing density-dependent per capita seed yields. ESSs are characterized in terms of the resulting harmonic mean growth rate of population density. The ESS criterion cannot be solved analytically, but qualitative relationships between the value of the ESS and other population parameters are obtained, and environments in which 100% germination is an ESS are identified. Some explicit predictions of the theory are summarized and compared with ideas of M. Westoby (1981, Amer. Nat. 118, 882-885). The results of this study are compared with those of Cohen (op. cit.) in a companion paper.

Biological Evolution↗

PubMed1985

ESS germination strategies in randomly varying environments. II. Reciprocal Yield-Law models.

The study of ESS germination fractions in S. Ellner (1985, Theor. Pop. Biol. 27, 000-000) is applied and extended in the case of annuals obeying the Reciprocal Yield Law. The effects of parameter changes on the value of the ESS germination fraction are determined in some limiting cases by analyses of approximations. Numerical solutions of the ESS criterion are used to check the robustness of the conclusions and the accuracy of the approximations. In general, the ESS germination fraction decreases with an increased survivorship of buried seeds and with increased "variability" of seed yields. However, different measures of "variability" are appropriate in different circumstances. To avoid the possibility of conflicting predictions depending on the measure of variability, it is suggested that tests of the theory be limited to co-occurring species, and to variability due to climatic fluctuations. The ESS theory based on the Reciprocal Yield Law is compared with D. Cohen's (1966, J. Theor. Biol. 12, 119-129; 1968, J. Ecol. 56, 219-228) density-independent theory of "optimal" germination. The theories differ qualitatively and quantitatively regarding the influence of mean yield, seed survivorship, and the frequency of favorable years on the predicted germination fraction.

Biological Evolution↗

PubMedDate not supplied

A multicenter 12-month evaluation of single-tooth implants restored 3 weeks after 1-stage surgery.

The time-intensive, multi-step process of dental implant therapy limits patient acceptance. This 3-year prospective multicenter study sought to determine the safety of an expedited therapy that consisted of loading unsplinted maxillary anterior single-tooth implants 3 weeks after 1-stage surgical placement, and determination of the peri-implant cortical bone and mucosal responses to the expedited procedure. Fifty-two patients missing 1 or 2 maxilliary anterior teeth were enrolled in a study approved by the Institutional Committee on Human Subjects Research and based on strict inclusion and exclusion criteria. Astra Tech ST implants placed in a 1-stage procedure were restored 3 weeks later with ST abutments and a provisional crown (baseline); 7 to 9 weeks later, a porcelain-fused-to-metal or all-ceramic crown was cemented. Radiographic and clinical examinations were made at baseline and at 6 and 12 months. Implant survival was recorded. Cortical bone responses and peri-implant mucosal responses were evaluated. Fifty-eight implants were placed. During the 3-week period after implant placement, 4 patients were dismissed because of smoking cigarettes (a protocol deviation), and 1 patient was excluded because of deviation in loading time. Of the remaining 53 implants, 2 failed before definitive crown cementation. The resultant 96.2% survival rate was independent of implant length, tooth position, and bone quality/quantity. The mean change in marginal bone level was 0.4 mm at 12 months. The number of surfaces with plaque decreased from 3.4% at baseline to 0.5% at 12 months. The surfaces with inflammation also decreased. A mean gain in papilla length of 0.61 mm occurred, and a gain in buccal gingiva (x = 0.34 mm) was observed. A high success rate with positive tissue responses was achieved for maxillary anterior unsplinted single-tooth implants placed in a 1-stage surgery and restored at 3 weeks. This 2-component system is suited to a single-stage, rapid loading protocol for esthetic single-tooth replacement.

Adaptation, Physiological↗