850 research outputs found
Cardiac magnetic resonance imaging assessment of aortic stenosis to improve clinical care
Background
Aortic stenosis is the commonest valve disease requiring intervention in the
developed world. Current guideline-based management strategies are based
on historical observational data or expert opinion and may leave many patients
with irreversible myocardial damage and adverse outcomes following valve
intervention. The aims of this thesis are to investigate novel cardiac magnetic
resonance techniques and how they can be applied to improve our decision
making around the timing of valve intervention.
Methods and Results
Cardiac magnetic resonance imaging can detect two forms of myocardial
fibrosis non-invasively; diffuse fibrosis using T1 mapping and replacement
fibrosis with the late gadolinium enhancement technique. I devised a novel
measure of diffuse fibrosis, the indexed extracellular volume (iECV) and
showed that these techniques can be used to divide patients into three
categories according to the type and amount of fibrosis present: no fibrosis,
diffuse fibrosis and replacement fibrosis. Moreover, I demonstrated that there
was evidence of increasing left ventricular decompensation across these three
groups.
How fibrosis and left ventricular hypertrophy change over time has not been
well studied in patients with aortic stenosis. Using serial imaging scans, I
showed that hypertrophy and diffuse fibrosis gradually progress over time,
whilst replacement fibrosis accumulates rapidly once first established.
Following valve replacement, cellular hypertrophy regresses faster than
diffuse fibrosis, but replacement fibrosis appears permanent and irreversible.
I then proceeded to investigate T1 mapping measures in a large international
multicentre cohort of patients with aortic stenosis scheduled for valve
replacement. I showed that extracellular volume-based T1 mapping measures
were comparable across centres and therefore confirmed that multicentre
studies are feasible. Extracellular volume fraction was associated with a
decompensating ventricle and emerged as a powerful independent predictor
of all-cause mortality in this group.
Finally, I investigated the use of novel hybrid magnetic resonance and positron
emission tomography imaging in patients with aortic stenosis, showing that
this technique is feasible and well-tolerated. I tested novel attenuation
correction and motion correction methods and showed that this technique can
offer multiparametric imaging of valve, myocardium and coronary arteries in a
single scan.
Conclusion
I have defined the longitudinal changes in hypertrophy and myocardial fibrosis
in aortic stenosis and validated extracellular volume measures as prognostic
markers in this group. Moreover, I have described novel magnetic resonance
and positron emission tomography techniques and their potential to aid the
clinical assessment of patients with aortic stenosis
Global Strings and the Aharonov-Bohm Effect
When a fermion interacts with a global vortex or cosmic string a solenoidal
"gauge" field is induced. This results in a non-trivial scattering
cross-section. For scalars and non-relativistic fermions the cross-section is
similar to that of Aharonov and Bohm, but with corrections. A cosmological
example is compared to one in liquid He-A and important differences are
discovered.Comment: 11 pages, DAMTP 93-5
Asthma in Vermont Dairy Farmers
Introduction. Although 5.4% of the Vermont population participates in agriculture as an occupation, little data exists on the prevalence of asthma in Vermont dairy farmers, due to inadequate sample sizes. Previous studies have shown dairy farmers are at risk of respiratory illness due to unique exposures intrinsic to their occupation. We conducted a study to assess the prevalence of asthma in dairy farmers in Vermont, to understand rates among this population and potential occupational risks.Methods. We distributed a paper survey modeled after previously-validated surveys, such as the BRFSS, to farmers at Vermont Farmer Bureau meetings, farmers markets, and individual farmers through Cabot Creamery. Out of 309 distributed surveys, we received 176 completed surveys for a response rate of 57%.Results. Self-reported asthma rate in dairy farmers was 21% (22% in dairy only farmers), with 90% of these cases reported as confirmed by a doctor. Of non-dairy farmers, 11% self-reported experiencing asthma. Farming activities associated with exacerbation of asthma symptoms were milking, prepping or cleaning bedding, and haying. 31% of dairy-only farmers reported symptom exacerbations due to these occupational triggers.Conclusions. The prevalence of asthma in Vermont dairy farmers is one of the highest reported rates in any Vermont occupation. Our data suggest that certain occupational exposures may increase risk of asthma and warrant further study; certain farming practices were associated with exacerbation of respiratory symptoms in farmers diagnosed with asthma. These findings and further research can assist in development of health care and preventive health measures for farmers.https://scholarworks.uvm.edu/comphp_gallery/1238/thumbnail.jp
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The Testing of an Alternator
"Before a company which manufactures electrical
machines allow them to leave the factory, each must
undergo certain tests to make sure that there are no
faults of design or workmanship and also ofttimes that they meet the specifications of the buyer. Four important
tests are made, e.g. (1) no load and (2) load
characteristics, (3) short circuit and (4) efficiency
tests. To make a test cf a large machine loaded
would take considerable rower. In practice the load
characteristic is determined from the no-load and the
short circuit curves."--p.
A Global Analog of Cheshire Charge
It is shown that a model with a spontaneously broken global symmetry can
support defects analogous to Alice strings, and a process analogous to Cheshire
charge exchange can take place. A possible realization in superfluid He-3 is
pointed out.Comment: 24 pages (figures 1-4 included as uu-encoded tar files), CALT-68-1865
(Revised version: an expression (eq. 17) for global charge density is
corrected; some typos and sign mismatches are removed.
Causality - Complexity - Consistency: Can Space-Time Be Based on Logic and Computation?
The difficulty of explaining non-local correlations in a fixed causal
structure sheds new light on the old debate on whether space and time are to be
seen as fundamental. Refraining from assuming space-time as given a priori has
a number of consequences. First, the usual definitions of randomness depend on
a causal structure and turn meaningless. So motivated, we propose an intrinsic,
physically motivated measure for the randomness of a string of bits: its length
minus its normalized work value, a quantity we closely relate to its Kolmogorov
complexity (the length of the shortest program making a universal Turing
machine output this string). We test this alternative concept of randomness for
the example of non-local correlations, and we end up with a reasoning that
leads to similar conclusions as in, but is conceptually more direct than, the
probabilistic view since only the outcomes of measurements that can actually
all be carried out together are put into relation to each other. In the same
context-free spirit, we connect the logical reversibility of an evolution to
the second law of thermodynamics and the arrow of time. Refining this, we end
up with a speculation on the emergence of a space-time structure on bit strings
in terms of data-compressibility relations. Finally, we show that logical
consistency, by which we replace the abandoned causality, it strictly weaker a
constraint than the latter in the multi-party case.Comment: 17 pages, 16 figures, small correction
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