332 research outputs found
Altered blood gas tensions of oxygen and carbon dioxide confound coronary reactivity to apnea
Purpose: Arterial blood gases change frequently during anesthesia and intensive care. Apnea can occur during diagnostic exams and airway and surgical interventions. While the impact of blood gas levels on coronary blood flow is established, their confounding effect on coronary vasoreactivity in response to an apneic stimulus, especially in coronary artery disease, is not known.
Methods: Six anesthetized control swine and eleven swine with coronary artery stenosis were examined. Nine different blood gas levels from a combination of arterial partial pressure of oxygen (70, 100, and 300 mmHg) and carbon dioxide (30, 40, and 50 mmHg) were targeted. Apnea was induced by halting controlled positive pressure ventilation for 3–30s, while the left descending coronary artery flow was measured and reported relative to apnea duration, and at the adjusted mean (12s).
Results: At normoxemic-normocapnic blood gas levels, apnea increased coronary blood flow in proportion to the duration of apnea in the control (r = 0.533, p < 0.001) and stenosed groups (r = 0.566, p < 0.001). This culminated in a 42% (95% CI: 27–58) increase in controls (p < 0.001) and, to a lesser extent, 27% (15–40) in the presence of coronary artery stenosis (p < 0.001). Vasoreactivity was augmented by mild-hypoxemic levels [81% (65–97), and 66% (53–79) increase in flow respectively, p < 0.001 vs. normoxemia], but markedly reduced during hyperoxia (7.5% (−8.2–23) and 0.3% (−12–13), respectively, p < 0.001 vs. normoxemia).
Conclusion: Alterations of blood oxygen and carbon dioxide affect coronary vascular reactivity induced by apnea in swine, which was attenuated further in the presence of coronary stenosis. Especially hyperoxia significantly reduces coronary blood flow and blunts coronary vascular reactivity
Torsion, an alternative to dark matter?
We confront Einstein-Cartan's theory with the Hubble diagram. An affirmative
answer to the question in the title is compatible with today's supernovae data.Comment: 14 pp, 3 figures. Version 2 matches the version published in Gen.
Rel. Grav., references added. Version 3 corrects a factor 3 in Cartan's
equations to become
Dynamics of electrons in the quantum Hall bubble phases
In Landau levels N > 1, the ground state of the two-dimensional electron gas
(2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for
small filling of the partially filled Landau level, into a succession of bubble
states with increasing number of guiding centers per bubble as the filling
increases, to a modulated stripe state near half filling. In this work, we show
that these first-order phase transitions between the bubble states lead to
measurable discontinuities in several physical quantities such as the density
of states and the magnetization of the 2DEG. We discuss in detail the behavior
of the collective excitations of the bubble states and show that their spectra
have higher-energy modes besides the pinned phonon mode. The frequencies of
these modes, at small wavevector k, have a discontinuous evolution as a
function of filling factor that should be measurable in, for example, microwave
absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40
Fermion confinement induced by geometry
We consider a five-dimensional model in which fermions are confined in a
hypersurface due to an interaction with a purely geometric field. Inspired by
the Rubakov-Shaposhnikov field-theoretical model, in which massless fermions
can be localized in a domain wall through the interaction of a scalar field, we
show that particle confinement may also take place if we endow the
five-dimensional bulk with a Weyl integrable geometric structure, or if we
assume the existence of a torsion field acting in the bulk. In this picture,
the kind of interaction considered in the Rubakov-Shaposhnikov model is
replaced by the interaction of fermions with a geometric field, namely a Weyl
scalar field or a torsion field. We show that in both cases the confinement is
independent of the energy and the mass of the fermionic particle. We generalize
these results to the case in which the bulk is an arbitrary n-dimensional
curved space.Comment: 8 page
Robustness and Generalization
We derive generalization bounds for learning algorithms based on their
robustness: the property that if a testing sample is "similar" to a training
sample, then the testing error is close to the training error. This provides a
novel approach, different from the complexity or stability arguments, to study
generalization of learning algorithms. We further show that a weak notion of
robustness is both sufficient and necessary for generalizability, which implies
that robustness is a fundamental property for learning algorithms to work
Convective Systems over the South China Sea: Cloud-Resolving Model Simulations
The two-dimensional version of the Goddard Cumulus Ensemble (GCE) model is used to simulate two South China Sea Monsoon Experiment (SCSMEX) convective periods [18–26 May (prior to and during the monsoon onset) and 2–11 June (after the onset of the monsoon) 1998]. Observed large-scale advective tendencies for potential temperature, water vapor mixing ratio, and horizontal momentum are used as the main forcing in governing the GCE model in a semiprognostic manner. The June SCSMEX case has stronger forcing in both temperature and water vapor, stronger low-level vertical shear of the horizontal wind, and larger convective available potential energy (CAPE). The temporal variation of the model-simulated rainfall, time- and domain-averaged heating, and moisture budgets compares well to those diagnostically determined from soundings. However, the model results have a higher temporal variability. The model underestimates the rainfall by 17 % to 20 % compared to that based on soundings. The GCE model-simulated rainfall for June is in very good agreement with the Tropical Rainfall Measuring Mission (TRMM), precipitation radar (PR), and the Global Precipitation Climatology Project (GPCP). Overall, the model agrees better with observations for the June case rather than the May case. The model-simulated energy budgets indicate that the two largest terms for both cases are net condensatio
Cartan's spiral staircase in physics and, in particular, in the gauge theory of dislocations
In 1922, Cartan introduced in differential geometry, besides the Riemannian
curvature, the new concept of torsion. He visualized a homogeneous and
isotropic distribution of torsion in three dimensions (3d) by the "helical
staircase", which he constructed by starting from a 3d Euclidean space and by
defining a new connection via helical motions. We describe this geometric
procedure in detail and define the corresponding connection and the torsion.
The interdisciplinary nature of this subject is already evident from Cartan's
discussion, since he argued - but never proved - that the helical staircase
should correspond to a continuum with constant pressure and constant internal
torque. We discuss where in physics the helical staircase is realized: (i) In
the continuum mechanics of Cosserat media, (ii) in (fairly speculative) 3d
theories of gravity, namely a) in 3d Einstein-Cartan gravity - this is Cartan's
case of constant pressure and constant intrinsic torque - and b) in 3d Poincare
gauge theory with the Mielke-Baekler Lagrangian, and, eventually, (iii) in the
gauge field theory of dislocations of Lazar et al., as we prove for the first
time by arranging a suitable distribution of screw dislocations. Our main
emphasis is on the discussion of dislocation field theory.Comment: 31 pages, 8 figure
Calcium phosphates and silicon: exploring methods of incorporation
Background: Bioinorganics have been explored as additives to ceramic bone graft substitutes with the aim to improve their performance in repair and regeneration of large bone defects. Silicon (Si), an essential trace element involved in the processes related to bone formation and remodeling, was shown not only to enhance osteoblasts proliferation but also to stimulate the differentiation of mesenchymal stem cells (MSCs) and preosteoblasts into the osteogenic lineage. In this study, the added value of Si to calcium phosphate (CaP) coatings was evaluated. Methods: Tissue culture plastic well plates were coated with a thin CaP layer to which traces amounts of Si were added, either by adsorption or by incorporation through coprecipitation. The physicochemical and structural properties of the coatings were characterized and the dissolution behavior was evaluated. The adsorption/incorporation of Si was successfully achieved and incorporated ions were released from the CaP coatings. Human MSCs were cultured on the coatings to examine the effects of Si on cell proliferation and osteogenic differentiation. For the statistical analysis, a one-way ANOVA with Bonferroni post-hoc test was performed. Results: The results showed that human MSCs (hMSCs) responded to the presence of Si in the CaP coatings, in a dosedependent manner. An increase in the expression of markers of osteogenic differentiation by human MSCs was observed as a result of the increase in Si concentration. Conclusions: The incorporation/adsorption of Si into CaP coatings was successfully achieved and hMSCs responded with an increase in osteogenic genes expression with the increase of Si concentration. Furthermore, hMSCs cultured on CaP-I coatings expressed higher levels of ALP and OP, indicating that this may be the preferred method of incorporation of bioinorganics into CaPsPortuguese Foundation for Science and Technology (FCT) for providing Ana I.
Rodrigues her PhD scholarship (Grant No. SFRH/BD/69962/2010). This work was
partially supported by national funds through the FCT under the scope of the
project OSTEOSYNTHESIS project (PTDC/CTM-BIO/0814/2012) and by the European
Regional Development Fund (FEDER) through the “COMPETE” - Operational
Programme for Competitiveness factors (FCOMP-01-0124-FEDER-028491).info:eu-repo/semantics/publishedVersio
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