717 research outputs found
Systematic review of the safety and efficacy of foam sclerotherapy for venous disease of the lower limbs
Background: Foam sclerotherapy is a potential treatment for lower limb venous disease. Methods: A systematic review, with no restriction on study design, to assess the safety and efficacy of foam sclerotherapy. Results: 69 studies were included. For serious adverse events including pulmonary embolism and deep vein thrombosis, the median event rates were less than 1%. Median rate for visual disturbance was 1.4%. Median rates for some other adverse events were more common, including headache (4.2%), thrombophlebitis (4.7%), matting/skin staining/pigmentation (17.8%) and pain at the site of injection (25.6%). Median rate for complete occlusion of treated veins was 87.0% and for recurrence or development of new veins was 8.1%. Evidence from meta-analysis for complete occlusion suggests that foam sclerotherapy is associated with a lower rate compared with surgery (RR 0.86, 95% CI 0.67 to 1.10) and a higher rate compared with liquid sclerotherapy (RR 1.39, 95% CI 0.91 to 2.11). However, there was substantial heterogeneity across the studies in the meta-analysis. Conclusion: Serious adverse events were rare. There is insufficient evidence to reliably compare the effectiveness of foam sclerotherapy with other minimally invasive therapies or surgery. Evidence from high quality randomised controlled trials is required.This manuscript is based on a systematic review commissioned and funded by the National Institute for Health and Clinical Excellence (NICE) through its Interventional Procedures Programme. The Health Services Research Unit is supported by a core grant from the Chief Scientist Office of the Scottish Executive Health Department
Semiclassical theory of transport in a random magnetic field
We study the semiclassical kinetics of 2D fermions in a smoothly varying
magnetic field . The nature of the transport depends crucially on
both the strength of the random component of and its mean
value . For , the governing parameter is ,
where is the correlation length of disorder and is the Larmor radius
in the field . While for the Drude theory applies, at
most particles drift adiabatically along closed contours and are
localized in the adiabatic approximation. The conductivity is then determined
by a special class of trajectories, the "snake states", which percolate by
scattering at the saddle points of where the adiabaticity of their
motion breaks down. The external field also suppresses the diffusion by
creating a percolation network of drifting cyclotron orbits. This kind of
percolation is due only to a weak violation of the adiabaticity of the
cyclotron rotation, yielding an exponential drop of the conductivity at large
. In the regime the crossover between the snake-state
percolation and the percolation of the drift orbits with increasing
has the character of a phase transition (localization of snake states) smeared
exponentially weakly by non-adiabatic effects. The ac conductivity also
reflects the dynamical properties of particles moving on the fractal
percolation network. In particular, it has a sharp kink at zero frequency and
falls off exponentially at higher frequencies. We also discuss the nature of
the quantum magnetooscillations. Detailed numerical studies confirm the
analytical findings. The shape of the magnetoresistivity at is
in good agreement with experimental data in the FQHE regime near .Comment: 22 pages REVTEX, 14 figure
Variation of elastic scattering across a quantum well
The Drude scattering times of electrons in two subbands of a parabolic
quantum well have been studied at constant electron sheet density and different
positions of the electron distribution along the growth direction. The
scattering times obtained by magnetotransport measurements decrease as the
electrons are displaced towards the well edges, although the lowest-subband
density increases. By comparing the measurements with calculations of the
scattering times of a two-subband system, new information on the location of
the relevant scatterers and the anisotropy of intersubband scattering is
obtained. It is found that the scattering time of electrons in the lower
subband depends sensitively on the position of the scatterers, which also
explains the measured dependence of the scattering on the carrier density. The
measurements indicate segregation of scatterers from the substrate side towards
the quantum well during growth.Comment: 4 pages, 4 figure
Quantum magneto-oscillations in a two-dimensional Fermi liquid
Quantum magneto-oscillations provide a powerfull tool for quantifying
Fermi-liquid parameters of metals. In particular, the quasiparticle effective
mass and spin susceptibility are extracted from the experiment using the
Lifshitz-Kosevich formula, derived under the assumption that the properties of
the system in a non-zero magnetic field are determined uniquely by the
zero-field Fermi-liquid state. This assumption is valid in 3D but, generally
speaking, erroneous in 2D where the Lifshitz-Kosevich formula may be applied
only if the oscillations are strongly damped by thermal smearing and disorder.
In this work, the effects of interactions and disorder on the amplitude of
magneto-oscillations in 2D are studied. It is found that the effective mass
diverges logarithmically with decreasing temperature signaling a deviation from
the Fermi-liquid behavior. It is also shown that the quasiparticle lifetime due
to inelastic interactions does not enter the oscillation amplitude, although
these interactions do renormalize the effective mass. This result provides a
generalization of the Fowler-Prange theorem formulated originally for the
electron-phonon interaction.Comment: 4 pages, 1 figur
On the Theory of Metal-Insulator Transitions in Gated Semiconductors
It is shown that recent experiments indicating a metal-insulator transition
in 2D electron systems can be interpreted in terms of a simple model, in which
the resistivity is controlled by scattering at charged hole traps located in
the oxide layer. The gate voltage changes the number of charged traps which
results in a sharp change in the resistivity. The observed exponential
temperature dependence of the resistivity in the metallic phase of the
transition follows from the temperature dependence of the trap occupation
number. The model naturally describes the experimentally observed scaling
properties of the transition and effects of magnetic and electric fields.Comment: 4 two-column pages, 4 figures (included in the text
Effects of posture and venous insufficiency on endothelial-dependent and -independent cutaneous vasodilation in the perimalleolar region
Objectives: To assess the effects of posture, endothelial function and venous insufficiency on cutaneous microvascular vasodilator function in the gaiter area, in particular defining factors which may affect microangiopathy and ulcer formation.
Methods: Endothelial-dependent and âindependent vasodilator responses to incremental-doses of acetylcholine (Ach) and sodium nitroprusside (SNP) were evaluated in the perimalleolar region in the supine and standing positions in middle-aged patients with isolated superficial venous insufficiency (ISVI) (n=25) and health controls (n=28) using laser Doppler fluximetry (LDF) and iontophoresis of vasodilators.
Results: The venoarteriolar reflex (vasoconstriction on standing) was equally present in both groups, and reduced the vasodilator responses to SNP in the upright position (e.g for patients with ISVI, peak SNP response was 82+11 PU [standing] vs 123+15 PU [supine]). The presence of ISVI had no effect on endothelial vasodilator function in the supine position, but on standing cutaneous reactivity to Ach was significantly reduced (e.g peak Ach response 69+8 PU [ISVI] vs 109+11 PU [controls], p<0.003).
Conclusions: Upright posture impairs cutaneous endothelial-dependent vasodilation in the gaiter area of patients with ISVI. This may be of clinical and prognostic utility in identifying which patients with uncomplicated ISVI are at highest risk of tissue breakdown and ulcer formation in the gaiter area
Apparent Metallic Behavior at B = 0 of a two-dimensional electron system in AlAs
We report the observation of metallic-like behavior at low temperatures and
zero magnetic field in two dimensional (2D) electrons in an AlAs quantum well.
At high densities the resistance of the sample decreases with decreasing
temperature, but as the density is reduced the behavior changes to insulating,
with the resistance increasing as the temperature is decreased. The effect is
similar to that observed in 2D electrons in Si-MOSFETs, and in 2D holes in SiGe
and GaAs, and points to the generality of this phenomenon
Thermodynamic Signature of a Two-Dimensional Metal-Insulator Transition
We present a study of the compressibility, K, of a two-dimensional hole
system which exhibits a metal-insulator phase transition at zero magnetic
field. It has been observed that dK/dp changes sign at the critical density for
the metal-insulator transition. Measurements also indicate that the insulating
phase is incompressible for all values of B. Finally, we show how the phase
transition evolves as the magnetic field is varied and construct a phase
diagram in the density-magnetic field plane for this system.Comment: 4 pages, 4 figures, submitted to Physical Review Letters; version 1
is identical to version 2 but didn't compile properl
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