3,849 research outputs found
Improving customer satisfaction through the management of perceptions of waiting
Thesis (M.S.)--Massachusetts Institute of Technology, Sloan School of Management, 1989.Includes bibliographical references (leaves 112-113).by Karen L. Katz and Blaire R. Martin.M.S
An injection and mixing element for delivery and monitoring of inhaled nitric oxide
Background Inhaled nitric oxide (NO) is a selective pulmonary vasodilator used primarily in the critical care setting for patients concurrently supported by invasive or noninvasive positive pressure ventilation. NO delivery devices interface with ventilator breathing circuits to inject NO in proportion with the flow of air/oxygen through the circuit, in order to maintain a constant, target concentration of inhaled NO. Methods In the present article, a NO injection and mixing element is presented. The device borrows from the design of static elements to promote rapid mixing of injected NO-containing gas with breathing circuit gases. Bench experiments are reported to demonstrate the improved mixing afforded by the injection and mixing element, as compared with conventional breathing circuit adapters, for NO injection into breathing circuits. Computational fluid dynamics simulations are also presented to illustrate mixing patterns and nitrogen dioxide production within the element. Results Over the range of air flow rates and target NO concentrations investigated, mixing length, defined as the downstream distance required for NO concentration to reach within ±5 % of the target concentration, was as high as 47 cm for the conventional breathing circuit adapters, but did not exceed 7.8 cm for the injection and mixing element. Conclusion The injection and mixing element has potential to improve ease of use, compatibility and safety of inhaled NO administration with mechanical ventilators and gas delivery devices
Variability in uptake efficiency for pulsed versus constant concentration delivery of inhaled nitric oxide
BACKGROUND: Nitric oxide (NO) is currently administered using devices that maintain constant inspired NO concentrations. Alternatively, devices that deliver a pulse of NO during the early phase of inspiration may have use in optimizing NO dosing efficiency and in extending application of NO to long-term use by ambulatory, spontaneously breathing patients. The extent to which the amount of NO delivered for a given pulse sequence determines alveolar concentrations and uptake, and the extent to which this relationship varies with breathing pattern, physiological, and pathophysiological parameters, warrants investigation. METHODS: A mathematical model was used to analyze inhaled nitric oxide (NO) transport through the conducting airways, and to predict uptake from the alveolar region of the lung. Pulsed delivery was compared with delivery of a constant concentration of NO in the inhaled gas. RESULTS: Pulsed delivery was predicted to offer significant improvement in uptake efficiency compared with constant concentration delivery. Uptake from the alveolar region depended on pulse timing, tidal volume, respiratory rate, lung and dead space volume, and the diffusing capacity of the lung for NO (D(L)NO). It was predicted that variation in uptake efficiency with breathing pattern can be limited using a pulse time of less than 100 ms, with a delay of less than 50 ms between the onset of inhalation and pulse delivery. Nonlinear variation in uptake efficiency with D(L)NO was predicted, with uptake efficiency falling off sharply as D(L)NO decreased below ~50-60 ml/min/mm Hg. Gas mixing in the conducting airways played an important role in determining uptake, such that consideration of bulk convection alone would lead to errors in assessing efficiency of pulsed delivery systems. CONCLUSIONS: Pulsed NO delivery improves uptake efficiency compared with constant concentration delivery. Optimization of pulse timing is critical in limiting intra- and inter-subject variability in dosing
A Real Space Renormalization Group Approach to Field Evolution Equations
A new operator formalism for the reduction of degrees of freedom in the
evolution of discrete partial differential equations (PDE) via real space
Renormalization Group is introduced, in which cell-overlapping is the key
concept. Applications to 1+1-dimensional PDEs are presented for linear and
quadratic equations which are first order in time.Comment: 8 pages, 10 ps figures. Accepted for publication in Phys. Rev.
Density functional study of the actinide nitrides
The full potential all electron linearized augmented plane wave plus local
orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K,
has been used to systematically investigate the structural, electronic, and
magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu,
Am). The theoretical formalism used is the generalized gradient approximation
to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional. Each compound has been studied at six levels
of theory: non-magnetic (NM), non-magnetic with spin-orbit coupling (NM+SOC),
ferromagnetic (FM), ferromagnetic with spin-orbit coupling (FM+SOC),
anti-ferromagnetic (AFM), and anti-ferromagnetic with spin-orbit coupling
(AFM+SOC). The structural parameters, bulk moduli, densities of states, and
charge distributions have been computed and compared to available experimental
data and other theoretical calculations published in the literature. The total
energy calculations indicate that the lowest energy structures of AcN, ThN, and
PaN are degenerate at the NM+SOC, FM+SOC, and AFM+SOC levels of theory with
vanishing total magnetic moments in the FM+SOC and AFM+SOC cases, making the
ground states essentially non-magnetic with spin-orbit interaction. The ground
states of UN, NpN, PuN, and AmN are found to be FM+SOC at the level of theory
used in the present computations. The nature of the interactions between the
actinide metals and nitrogen atom, and the implications on 5f electron
delocalization and localization are discussed in detail.Comment: 5 tables, 12 figure
Sub-micrometer epitaxial Josephson junctions for quantum circuits
We present a fabrication scheme and testing results for epitaxial
sub-micrometer Josephson junctions. The junctions are made using a
high-temperature (1170 K) "via process" yielding junctions as small as 0.8 mu m
in diameter by use of optical lithography. Sapphire (Al2O3) tunnel-barriers are
grown on an epitaxial Re/Ti multilayer base-electrode. We have fabricated
devices with both Re and Al top electrodes. While room-temperature (295 K)
resistance versus area data are favorable for both types of top electrodes, the
low-temperature (50 mK) data show that junctions with the Al top electrode have
a much higher subgap resistance. The microwave loss properties of the junctions
have been measured by use of superconducting Josephson junction qubits. The
results show that high subgap resistance correlates to improved qubit
performance
A First Estimate of the Baryonic Mass Function of Galaxies
We estimate the baryonic (stellar+cold gas) mass function of galaxies in the
local Universe by assigning a complete sample of Two Micron All Sky Survey and
Sloan Digital Sky Survey galaxies a gas fraction based on a statistical sample
of the entire population, under the assumption of a universally-applicable
stellar initial mass function. The baryonic mass function is similar to the
stellar mass function at the high mass end, and has a reasonably steep
faint-end slope owing to the typically high cold gas fractions and low stellar
mass-to-light ratios characteristic of low-mass galaxies. The Schechter
Function fit parameters are phi* = 0.0108(6) per cubic Mpc per dex of mass, M*
= 5.3(3)x10^10 solar masses, and alpha = -1.21(5), with formal error estimates
given in parentheses (for a Hubble constant of 100 km/s per Mpc). We show that
the neutral and molecular hydrogen mass functions derived using this indirect
route are in agreement with direct estimates, validating our indirect method.
Integrating under the baryonic mass function and incorporating all sources of
uncertainty, we find that the baryonic (stellar+cold gas) mass density implied
by this estimate is Omega_cold baryon = 0.0024+0.0007-0.0014, or 8% +4% -5% of
the Big Bang nucleosynthesis expectation.Comment: ApJ Letters, accepted. 4 pages, 2 embedded figure
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