385 research outputs found
A modeling-based evaluation of isothermal rebreathing for breath gas analyses of highly soluble volatile organic compounds
Isothermal rebreathing has been proposed as an experimental technique for
estimating the alveolar levels of hydrophilic volatile organic compounds (VOCs)
in exhaled breath. Using the prototypic test compound acetone we demonstrate
that the end-tidal breath profiles of such substances during isothermal
rebreathing show characteristics that contradict the conventional pulmonary
inert gas elimination theory due to Farhi. On the other hand, these profiles
can reliably be captured by virtue of a previously developed mathematical model
for the general exhalation kinetics of highly soluble, blood-borne VOCs, which
explicitly takes into account airway gas exchange as major determinant of the
observable breath output.
This model allows for a mechanistic analysis of various rebreathing protocols
suggested in the literature. In particular, it clarifies the discrepancies
between in vitro and in vivo blood-breath ratios of hydrophilic VOCs and yields
further quantitative insights into the physiological components of isothermal
rebreathing.Comment: 21 page
Physiological modeling of isoprene dynamics in exhaled breath
Human breath contains a myriad of endogenous volatile organic compounds
(VOCs) which are reflective of ongoing metabolic or physiological processes.
While research into the diagnostic potential and general medical relevance of
these trace gases is conducted on a considerable scale, little focus has been
given so far to a sound analysis of the quantitative relationships between
breath levels and the underlying systemic concentrations. This paper is devoted
to a thorough modeling study of the end-tidal breath dynamics associated with
isoprene, which serves as a paradigmatic example for the class of low-soluble,
blood-borne VOCs.
Real-time measurements of exhaled breath under an ergometer challenge reveal
characteristic changes of isoprene output in response to variations in
ventilation and perfusion. Here, a valid compartmental description of these
profiles is developed. By comparison with experimental data it is inferred that
the major part of breath isoprene variability during exercise conditions can be
attributed to an increased fractional perfusion of potential storage and
production sites, leading to higher levels of mixed venous blood concentrations
at the onset of physical activity. In this context, various lines of supportive
evidence for an extrahepatic tissue source of isoprene are presented.
Our model is a first step towards new guidelines for the breath gas analysis
of isoprene and is expected to aid further investigations regarding the
exhalation, storage, transport and biotransformation processes associated with
this important compound.Comment: 14 page
Do Gamma-Ray Burst Sources Repeat?
The demonstration of repeated gamma-ray bursts from an individual source
would severely constrain burst source models. Recent reports (Quashnock and
Lamb 1993; Wang and Lingenfelter 1993) of evidence for repetition in the first
BATSE burst catalog have generated renewed interest in this issue. Here, we
analyze the angular distribution of 585 bursts of the second BATSE catalog
(Meegan et al. 1994). We search for evidence of burst recurrence using the
nearest and farthest neighbor statistic and the two-point angular correlation
function. We find the data to be consistent with the hypothesis that burst
sources do not repeat; however, a repeater fraction of up to about 20% of the
observed bursts cannot be excluded.Comment: ApJ Letters, in press, 13 pages, including three embedded figures.
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V2494 cyg: A unique FU ori type object in the cygnus OB7 complex
A photometric and spectral study of the variable star V2494 Cyg in the L 1003 dark cloud is presented. The brightness of the star, formerly known as HH 381 IRS, increased by 2.5 mag in R (probably in the 1980s) and since then has remained nearly constant. Since the brightness increase, V2494 Cyg has illuminated a bipolar cometary nebula. The stellar spectrum has several features typical of the FU Ori (FUor) type, plus it exhibits very strong Ha and forbidden emissionlines with high-velocity components. These emission lines originate in the Herbig-Haro (HH) jet near the star. The kinematic age of the jet is consistent with it forming at the time of the outburst leading to the luminosity increase. V2494 Cyg also produces a rather extended outflow; it is the first known FUor with both an observed outburst and a parsec-sized HH flow. The nebula, illuminated by V2494 Cyg, possesses similar morphological and spectral characteristics to Hubble's variable nebula (R Monocerotis/NGC 2261). © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Testing the Dipole and Quadrupole Moments of Galactic Models
If gamma-ray bursts originate in the Galaxy, at some level there should be a
galactic pattern in their distribution on the sky. We test published galactic
models by comparing their dipole and quadrupole moments with the moments of the
BATSE 3B catalog. While many models have moments that are too large, several
models are in acceptable or good agreement with the data.Comment: 5 pages, LaTex using Revtex macro aipbook.sty and psfig. To appear in
the Proceedings of the 3rd Huntsville Symposium on Gamma-Ray Bursts, AIP,
eds. C. Kouveliotou, M. S. Briggs, G. J. Fishma
Physiological modeling of isoprene dynamics in exhaled breath
Human breath contains a myriad of endogenous volatile organic compounds
(VOCs) which are reflective of ongoing metabolic or physiological processes.
While research into the diagnostic potential and general medical relevance of
these trace gases is conducted on a considerable scale, little focus has been
given so far to a sound analysis of the quantitative relationships between
breath levels and the underlying systemic concentrations. This paper is devoted
to a thorough modeling study of the end-tidal breath dynamics associated with
isoprene, which serves as a paradigmatic example for the class of low-soluble,
blood-borne VOCs.
Real-time measurements of exhaled breath under an ergometer challenge reveal
characteristic changes of isoprene output in response to variations in
ventilation and perfusion. Here, a valid compartmental description of these
profiles is developed. By comparison with experimental data it is inferred that
the major part of breath isoprene variability during exercise conditions can be
attributed to an increased fractional perfusion of potential storage and
production sites, leading to higher levels of mixed venous blood concentrations
at the onset of physical activity. In this context, various lines of supportive
evidence for an extrahepatic tissue source of isoprene are presented.
Our model is a first step towards new guidelines for the breath gas analysis
of isoprene and is expected to aid further investigations regarding the
exhalation, storage, transport and biotransformation processes associated with
this important compound.Comment: 14 page
The Local Environment of the FUor-like Objects AR 6A and 6B
We present new 12CO J=3-2 and HCN J=3-2 molecular line maps of the region
surrounding the young star AR 6 using the 15 metre James Clerk Maxwell
Telescope. AR 6 was previously found to be a double source with both components
exhibiting several characteristics of FU Orionis (FUor) eruptive variable
stars. Our data indicates that AR 6, like FU Orionis itself, does not possess a
CO outflow and likewise, does not show evidence for large amounts of molecular
g as in its circumstellar environment. We conclude that from the near-IR to the
sub-mm, AR 6 is similar to FU Orionis in several respects. We interpret the
lack of significant dust and molecular gas in the circumstellar environment of
AR 6, together with the large near-IR thermal excess, as evidence that the
sources have exhausted their natal envelopes, that they have at least small hot
circumstellar disks, and that they are more evolved than Class I protostars.
This, in itself, suggests that, since FUor eruptions have also been observed in
stars with large dust mass envelopes (e.g. V346 Nor) and with CO outflows (e.g.
L1551 IRS5), FUor events probably occur at many different stages in the early,
formative phase of a star's life, and lends support to the idea that FUor
outbursts are repetitive like their shorter-lived relatives occurring in EXor
eruptive variables. Finally, we show that, being part of the 'Spokes' young
stellar cluster, AR 6 is unlike many FUors which typically are located in more
sparsely populated regions.Comment: Accepted for publication in The Astronomical Journal (October 2008
A mathematical model for breath gas analysis of volatile organic compounds with special emphasis on acetone
Recommended standardized procedures for determining exhaled lower respiratory
nitric oxide and nasal nitric oxide have been developed by task forces of the
European Respiratory Society and the American Thoracic Society. These
recommendations have paved the way for the measurement of nitric oxide to
become a diagnostic tool for specific clinical applications. It would be
desirable to develop similar guidelines for the sampling of other trace gases
in exhaled breath, especially volatile organic compounds (VOCs) which reflect
ongoing metabolism. The concentrations of water-soluble, blood-borne substances
in exhaled breath are influenced by: (i) breathing patterns affecting gas
exchange in the conducting airways; (ii) the concentrations in the
tracheo-bronchial lining fluid; (iii) the alveolar and systemic concentrations
of the compound. The classical Farhi equation takes only the alveolar
concentrations into account. Real-time measurements of acetone in end-tidal
breath under an ergometer challenge show characteristics which cannot be
explained within the Farhi setting. Here we develop a compartment model that
reliably captures these profiles and is capable of relating breath to the
systemic concentrations of acetone. By comparison with experimental data it is
inferred that the major part of variability in breath acetone concentrations
(e.g., in response to moderate exercise or altered breathing patterns) can be
attributed to airway gas exchange, with minimal changes of the underlying blood
and tissue concentrations. Moreover, it is deduced that measured end-tidal
breath concentrations of acetone determined during resting conditions and free
breathing will be rather poor indicators for endogenous levels. Particularly,
the current formulation includes the classical Farhi and the Scheid series
inhomogeneity model as special limiting cases.Comment: 38 page
The Random Quadratic Assignment Problem
Optimal assignment of classes to classrooms \cite{dickey}, design of DNA
microarrays \cite{carvalho}, cross species gene analysis \cite{kolar}, creation
of hospital layouts cite{elshafei}, and assignment of components to locations
on circuit boards \cite{steinberg} are a few of the many problems which have
been formulated as a quadratic assignment problem (QAP). Originally formulated
in 1957, the QAP is one of the most difficult of all combinatorial optimization
problems. Here, we use statistical mechanical methods to study the asymptotic
behavior of problems in which the entries of at least one of the two matrices
that specify the problem are chosen from a random distribution .
Surprisingly, this case has not been studied before using statistical methods
despite the fact that the QAP was first proposed over 50 years ago
\cite{Koopmans}. We find simple forms for and , the
costs of the minimal and maximum solutions respectively. Notable features of
our results are the symmetry of the results for and
and the dependence on only through its mean and standard deviation,
independent of the details of . After the asymptotic cost is determined for
a given QAP problem, one can straightforwardly calculate the asymptotic cost of
a QAP problem specified with a different random distribution
Structure and Composition of Isolated Core-Shell(In,Ga)N/GaNRods Based on Nanofocus X-Ray Diffraction and Scanning Transmission Electron Microscopy
Nanofocus x-ray diffraction is used to investigate the structure and local strain field of an isolated
ðIn; GaÞN=GaN core-shell microrod. Because the high spatial resolution of the x-ray beam is only
80 × 90 nm2, we are able to investigate several distinct volumes on one individual side facet. Here, we find a
drastic increase in thickness of the outer GaN shell along the rod height. Additionally, we performed highangle
annular dark-field scanning-transmission-electron-microscopy measurements on several rods from the
same sample showing that (In,Ga)N double-quantum-well and GaN barrier thicknesses also increase
strongly along the height. Moreover, plastic relaxation is observed in the top part of the rod. Based on the
experimentally obtained structural parameters, we simulate the strain-induced deformation using the finiteelement
method, which serves as the input for subsequent kinematic scattering simulations. The simulations
reveal a significant increase of elastic in-plane relaxation along the rod height. However, at a certain height,
the occurrence of plastic relaxation yields a decrease of the elastic strain. Because of the experimentally
obtained structural input for the finite-element simulations, we can exclude unknown structural influences
on the strain distribution, and we are able to translate the elastic relaxation into an indium concentration
which increases by a factor of 4 from the bottom to the height where plastic relaxation occurs
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