1,526 research outputs found
Should we use early less invasive hemodynamic monitoring in unstable ICU patients?
In the previous issue of Critical Care, Takala and colleagues presented the results of a multicenter study to investigate whether the early presence of less invasive hemodynamic monitoring improves outcome in patients admitted with hemodynamic instability to the intensive care unit. The authors' results suggest that it makes no difference. We discuss these findings and compare them to the literature on early goal-directed therapy in which monitors are used early but with a protocol
Topological thermal instability and length of proteins
We present an analysis of the effects of global topology on the structural
stability of folded proteins in thermal equilibrium with a heat bath. For a
large class of single domain proteins, we computed the harmonic spectrum within
the Gaussian Network Model (GNM) and determined the spectral dimension, a
parameter describing the low frequency behaviour of the density of modes. We
find a surprisingly strong correlation between the spectral dimension and the
number of amino acids of the protein. Considering that larger spectral
dimension value relate to more topologically compact folded state, our results
indicate that for a given temperature and length of the protein, the folded
structure corresponds to the less compact folding compatible with thermodynamic
stability.Comment: 15 pages, 6 eps figures, 2 table
Macroscopic evidence of microscopic dynamics in the Fermi-Pasta-Ulam oscillator chain from nonlinear time series analysis
The problem of detecting specific features of microscopic dynamics in the
macroscopic behavior of a many-degrees-of-freedom system is investigated by
analyzing the position and momentum time series of a heavy impurity embedded in
a chain of nearest-neighbor anharmonic Fermi-Pasta-Ulam oscillators. Results
obtained in a previous work [M. Romero-Bastida, Phys. Rev. E {\bf69}, 056204
(2004)] suggest that the impurity does not contribute significantly to the
dynamics of the chain and can be considered as a probe for the dynamics of the
system to which the impurity is coupled. The () entropy, which measures
the amount of information generated by unit time at different scales of
time and of the observable, is numerically computed by methods of nonlinear
time-series analysis using the position and momentum signals of the heavy
impurity for various values of the energy density (energy per degree
of freedom) of the system and some values of the impurity mass . Results
obtained from these two time series are compared and discussed.Comment: 7 pages, 5 figures, RevTeX4 PRE format; to be published in Phys. Rev.
Female fertility and environmental pollution
A realistic picture of our world shows that it is heavily polluted everywhere. Coastal regions and oceans are polluted by farm fertilizer, manure runoff, sewage and industrial discharges, and large isles of waste plastic are floating around, impacting sea life. Terrestrial ecosystems are contaminated by heavy metals and organic chemicals that can be taken up by and accumulate in crop plants, and water tables are heavily contaminated by untreated industrial discharges. As deadly particulates can drift far, poor air quality has become a significant global problem and one that is not exclusive to major industrialized cities. The consequences are a dramatic impairment of our ecosystem and biodiversity and increases in degenerative or man-made diseases. In this respect, it has been demonstrated that environmental pollution impairs fertility in all mammalian species. The worst consequences are observed for females since the number of germ cells present in the ovary is fixed during fetal life, and the cells are not renewable. This means that any pollutant affecting hormonal homeostasis and/or the reproductive apparatus inevitably harms reproductive performance. This decline will have important social and economic consequences that can no longer be overlooked
Refolding dynamics of stretched biopolymers upon force quench
Single molecule force spectroscopy methods can be used to generate folding
trajectories of biopolymers from arbitrary regions of the folding landscape. We
illustrate the complexity of the folding kinetics and generic aspects of the
collapse of RNA and proteins upon force quench, using simulations of an RNA
hairpin and theory based on the de Gennes model for homopolymer collapse. The
folding time, , depends asymmetrically on and
where () is the stretch (quench) force, and
is the transition mid-force of the RNA hairpin. In accord with
experiments, the relaxation kinetics of the molecular extension, , occurs
in three stages: a rapid initial decrease in the extension is followed by a
plateau, and finally an abrupt reduction in that occurs as the native
state is approached.
The duration of the plateau increases as decreases
(where is the time in which the force is reduced from to ).
Variations in the mechanisms of force quench relaxation as is altered
are reflected in the experimentally measurable time-dependent entropy, which is
computed directly from the folding trajectories. An analytical solution of the
de Gennes model under tension reproduces the multistage stage kinetics in
. The prediction that the initial stages of collapse should also be a
generic feature of polymers is validated by simulation of the kinetics of
toroid (globule) formation in semiflexible (flexible) homopolymers in poor
solvents upon quenching the force from a fully stretched state. Our findings
give a unified explanation for multiple disparate experimental observations of
protein folding.Comment: 31 pages 11 figure
Defining and identifying communities in networks
The investigation of community structures in networks is an important issue
in many domains and disciplines. This problem is relevant for social tasks
(objective analysis of relationships on the web), biological inquiries
(functional studies in metabolic, cellular or protein networks) or
technological problems (optimization of large infrastructures). Several types
of algorithm exist for revealing the community structure in networks, but a
general and quantitative definition of community is still lacking, leading to
an intrinsic difficulty in the interpretation of the results of the algorithms
without any additional non-topological information. In this paper we face this
problem by introducing two quantitative definitions of community and by showing
how they are implemented in practice in the existing algorithms. In this way
the algorithms for the identification of the community structure become fully
self-contained. Furthermore, we propose a new local algorithm to detect
communities which outperforms the existing algorithms with respect to the
computational cost, keeping the same level of reliability. The new algorithm is
tested on artificial and real-world graphs. In particular we show the
application of the new algorithm to a network of scientific collaborations,
which, for its size, can not be attacked with the usual methods. This new class
of local algorithms could open the way to applications to large-scale
technological and biological applications.Comment: Revtex, final form, 14 pages, 6 figure
Comparison of voter and Glauber ordering dynamics on networks
We study numerically the ordering process of two very simple dynamical models
for a two-state variable on several topologies with increasing levels of
heterogeneity in the degree distribution. We find that the zero-temperature
Glauber dynamics for the Ising model may get trapped in sets of partially
ordered metastable states even for finite system size, and this becomes more
probable as the size increases. Voter dynamics instead always converges to full
order on finite networks, even if this does not occur via coherent growth of
domains. The time needed for order to be reached diverges with the system size.
In both cases the ordering process is rather insensitive to the variation of
the degreee distribution from sharply peaked to scale-free.Comment: 12 pages, 12 figure
Short period attractors and non-ergodic behavior in the deterministic fixed energy sandpile model
We study the asymptotic behaviour of the Bak, Tang, Wiesenfeld sandpile
automata as a closed system with fixed energy.
We explore the full range of energies characterizing the active phase. The
model exhibits strong non-ergodic features by settling into limit-cycles whose
period depends on the energy and initial conditions. The asymptotic activity
(topplings density) shows, as a function of energy density , a
devil's staircase behaviour defining a symmetric energy interval-set over which
also the period lengths remain constant. The properties of -
phase diagram can be traced back to the basic symmetries underlying the model's
dynamics.Comment: EPL-style, 7 pages, 3 eps figures, revised versio
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