2,246 research outputs found
Adenosine pharmacologic stress myocardial perfusion tomographic imaging in patients with significant aortic stenosis Diagnostic efficacy and comparison of clinical, hemodynamic and electrocardiographic variables with 100 age-matched control subjects
AbstractObjectives. This study assessed the safety and diagnostic accuracy of adenosine stress myocardial perfusion scintigraphy for the detection of coronary artery disease using single-photon emission computed tomography (SPECT) in patients with significant aortic stenosis.Background. Exercise cardiac stress testing in patients with significant aortic stenosis is generally avoided because of concerns for safety. In addition, those studies that have analyzed the utility of exercise testing both with and without myocardial thallium-201 scintigraphy for the diagnosis of coronary artery disease have yielded low specificity. Currently, no safe and accurate means exists to noninvasively assess the presence, extent and severity of coronary artery disease in patients with significant aortic stenosis.Methods. The study included 35 patients with moderate to severe aortic stenosis (mean [±SD] aortic valve area 0.84 ± 0.16 cm2, range 0.5 to 1.2; mean maximal instantaneous aortic valve gradient 44.4 ± 15.9 mm Hg, range 20 to 84). All patients underwent a 6-min adenosine infusion (140 μg/kg body weight per min) protocol and either separate acquisition rest thallium-201/stress technetium-99m sestamibi or stress and 4-h redistribution thallium-201 SPECT. Visual 20-segment SPECT analysis used a standard five-point scoring system from 0 (normal tracer uptake) to 4 (absent uptake). The SPECT results were considered abnormal if more than two segments had a stress score ≥2. Hemodynamic, electrocardiographic and clinical responses were compared with those in a reference group of 100 consecutive age-matched patients undergoing adenosine SPECT who did not have aortic stenosis.Results. Hemodynamic responses during adenosine stress testing between the study and control patients demonstrated no significant difference in the net change in systolic blood pressure (18% of baseline vs. 14%, patients with aortic stenosis vs. control subjects), heart rate (21% vs. 19%), rate-pressure product (0% vs. 2%) or incidence of chest pain (23% vs. 35%) or transient second- (9% vs. 9%) or third-degree atrioventricular block (3% vs. 1%). In the 20 patients who had coronary angiography, sensitivity for detection of coronary artery disease was 92% (12 of 13) and specificity was 71% (5 of 7).Conclusions. In this preliminary study, adenosine was found to be well tolerated and diagnostically accurate in patients with moderate to severe aortic stenosis
How short is too short? Constraining zero-range interactions in nucleon-nucleon scattering
We discuss a number of constraints on the effects of zero-range potentials in
quantum mechanics. We show that for such a potential , where
is the momentum of the nucleon in the center of mass frame and is
the S-wave phase shift, must be a monotonically decreasing function of energy.
This implies that the effective range of the potential is non-positive. We also
examine scattering from the sum of two potentials, one of which is a
short-range interaction. We find that if the short-range interaction is of
zero-range then it must be attractive, and the logarithmic derivative of the
radial wave function at the origin must be a monotonically decreasing function
of energy. If the short-range interaction is not of zero range then a
constraint which gives the minimum possible range for it to fit the phase
shifts exists. The implications of these results for effective field theory
treatments of nucleon-nucleon interactions are discussed.Comment: 5 pages, RevTeX. Version accepted for publication in Phys. Lett. B.
Minor changes to the text have been made in order to clarify the scope of the
pape
Non-perturbative regularization and renormalization: simple examples from non-relativistic quantum mechanics
We examine several zero-range potentials in non-relativistic quantum
mechanics. The study of such potentials requires regularization and
renormalization. We contrast physical results obtained using dimensional
regularization and cutoff schemes and show explicitly that in certain cases
dimensional regularization fails to reproduce the results obtained using cutoff
regularization. First we consider a delta-function potential in arbitrary space
dimensions. Using cutoff regularization we show that for the
renormalized scattering amplitude is trivial. In contrast, dimensional
regularization can yield a nontrivial scattering amplitude for odd dimensions
greater than or equal to five. We also consider a potential consisting of a
delta function plus the derivative-squared of a delta function in three
dimensions. We show that the renormalized scattering amplitudes obtained using
the two regularization schemes are different. Moreover we find that in the
cutoff-regulated calculation the effective range is necessarily negative in the
limit that the cutoff is taken to infinity. In contrast, in dimensional
regularization the effective range is unconstrained. We discuss how these
discrepancies arise from the dimensional regularization prescription that all
power-law divergences vanish. We argue that these results demonstrate that
dimensional regularization can fail in a non-perturbative setting.Comment: 19 pages, LaTeX, uses epsf.te
Global Stability of a Premixed Reaction Zone (Time-Dependent Liñan’s Problem)
Global stability properties of a premixed, three-dimensional reaction zone are considered. In the nonadiabatic case (i.e., when there is a heat exchange between the reaction zone and the burned gases) there is a unique, spatially one-dimensional steady state that is shown to be unstable (respectively, asymptotically stable) if the reaction zone is cooled (respectively, heated) by the burned mixture. In the adiabatic case, there is a unique (up to spatial translations) steady state that is shown to be stable. In addition, the large-time asymptotic behavior of the solution is analyzed to obtain sufficient conditions on the initial data for stabilization. Previous partial numerical results on linear stability of one-dimensional reaction zones are thereby confirmed and extended
Recent topics of mesic atoms and mesic nuclei -- mesic nuclei exist ?--
We study -meson production in nuclei to investigate the in-medium
modification of the -meson spectral function at finite density. We
consider (), () and () reactions to produce a
-meson inside the nucleus and evaluate the effects of the medium
modifications to reaction cross sections. The structures of the bound states,
-mesic nuclei, are also studied. For strong absorptive interaction cases,
we need to know the spectrum shape in a wide energy region to deduce the
properties of .Comment: Talk given at EXA08, Vienna, September 2008. To be published in the
Proceedings, Hyperfine Interactions. 6 pages, 6 figure
Schwarzschild Geometry Emerging from Matrix Models
We demonstrate how various geometries can emerge from Yang-Mills type matrix
models with branes, and consider the examples of Schwarzschild and
Reissner-Nordstroem geometry. We provide an explicit embedding of these branes
in R^{2,5} and R^{4,6}, as well as an appropriate Poisson resp. symplectic
structure which determines the non-commutativity of space-time. The embedding
is asymptotically flat with asymptotically constant \theta^{\mu\nu} for large
r, and therefore suitable for a generalization to many-body configurations.
This is an illustration of our previous work arXiv:1003.4132, where we have
shown how the Einstein-Hilbert action can be realized within such matrix
models.Comment: 21 pages, 1 figur
Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
We report total abundances and related parameters for the full sample of the
FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results
are presented for the "second half" of the survey involving 15 lines of sight
to supplement data for the first 23 lines of sight already published. We assess
the correlations between molecular hydrogen and various extinction parameters
in the full sample, which covers a broader range of conditions than the initial
sample. In particular, we are now able to confirm that many, but not all, lines
of sight with shallow far-UV extinction curves and large values of the
total-to-selective extinction ratio, = / -- characteristic
of larger than average dust grains -- are associated with particularly low
hydrogen molecular fractions (). In the lines of sight with large
, there is in fact a wide range in molecular fractions, despite the
expectation that the larger grains should lead to less H formation.
However, we see specific evidence that the molecular fractions in this
sub-sample are inversely related to the estimated strength of the UV radiation
field and thus the latter factor is more important in this regime. We have
provided an update to previous values of the gas-to-dust ratio, (H)/, based on direct measurements of (H) and (H I).
Although our value is nearly identical to that found with Copernicus data, it
extends the relationship by a factor of 2 in reddening. Finally, as the new
lines of sight generally show low to moderate molecular fractions, we still
find little evidence for single monolithic "translucent clouds" with 1.Comment: 35 pages, 5 tables, 7 figures, accepted for publication in The
Astrophysical Journal Supplements Serie
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