No description available.

Thesis (M. Sc.)--University of Adelaide. / Includes bibliographical references (leaves 152-170).

No description available.

Thesis (Ph.D.)--Tufts University, 2004. / Adviser: David A. Boas. Submitted to the Dept. of Electrical Engineering. Includes bibliographical references (leaves 407-416). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (M.A.)--Georgia State University, 2007. / Title from file title page. David A. Washburn, committee chair; Heather M. Kleider, Eric J. Vanman, committee members. Electronic text (65 p. : ill. (some col.)) : digital, PDF file. Description based on contents viewed May 23, 2008. Includes bibliographical references (p. 58-64).

This overview reviews seventeen publications between 1995 and 2005. CHE Imray was the first author of eleven of the papers, the senior author of four and a major contributor to two of the publications. The overview should be read in conjunction with the full copies of the seventeen publications (Appendix 2). The brain is exquisitely sensitive to oxygen requiring a constant supply of adequately oxygenated blood to function normally. Cerebral oxygen delivery is dynamic, and alters rapidly in response to changes in physiological and pathological stimuli. Interference with cerebral oxygen delivery, either as a result of decreased cerebral blood flow, decreased arterial oxygenation or particulate matter (cerebral microemboli) within the blood can all rapidly result in temporary or permanent loss of function within minutes. The author has used non-invasive cerebral perfusion imaging techniques, initially in the clinical setting (in clinic, at the bedside and in the operating theatre) and later transferring these methods to the field setting at high altitude. As a result of these studies, new insights into cerebral perfusion have been gained. Novel concepts such as 'virtual altitude' and 'partitioning of arterial and venous volumes' have been developed. New equipment has been designed and developed, such as the recumbent, collapsible, portable exercise bike. Finally new clinical treatments have been developed, including an apparently safe way to treat the high-risk group of patients with crescendo transient ischaemic attacks or mini-strokes, greatly reducing the risk of developing a subsequent major stroke. The work submitted for consideration for a PhD by publication represents ten years of investigation in two closely inter-related fields. The aim of the submission is to provide a background to the seventeen publications (Appendix 2) allowing them to be seen in context to existing knowledge. Appendix 3 contains twelve additional communications that have either been published, or accepted for publication after the original list of seventeen publications was submitted to the University of Glamorgan. They confirm the author's ongoing interest and contributions to this field of research.

The technique of transcranial Doppler ultrasound (TCD) was used to assess cerebral blood flow (CBF) in humans. Studies were performed at rest and during dynamic submaximal exercise. In the resting experiments, TCD was combined with the technique of dynamic end-tidal forcing to study the dynamics of the CBF response to step changes in end-tidal (i.e. arterial) PC02 and PO2 In the resting and exercise experiments, the degree of consistency was examined between three indices of CBF that can be extracted from the TCD spectrum. Finally, the ventilatory and the CBF responses to acute isocapnic hypoxia were examined to try to quantify the possible reduction in ventilation that could be attributed to changes in CBF with hypoxia. In the studies performed at rest, during either hypoxia and/or hypercapnia (Chapter 2), the three indices of CBF extracted from the TCD spectrum were all consistent. However, during submaximal exercise (Chapter 5), the indices were less consistent and results suggest that the increase in CBF with exercise that has been reported with TCD needs to be treated with caution. The dynamic studies of the CBF response to step changes in end-tidal PC02 and PO2 in humans revealed that the CBF response to hypercapnia (Chapter 3) is characterised by a significant asymmetry, with a slower on-transient than off-transient, and also by a degree of undershoot following the relief of hypercapnia. The CBF response to hypocapnia (Chapter 4) is also characterised by a significant asymmetry, with a faster on-transient than off-transient. Furthermore, there is a slow progressive adaptation throughout the hypocapnic period. These studies show that the CBF responses to hypercapnia and hypocapnia are much faster than previously been thought. Finally, the work described in Chapter 6 attempts to quantify the possible reduction in ventilation that could be attributed to changes in CBF with hypoxia to determine whether it could be of sufficient magnitude to underlie hypoxic ventilatory decline (HVD). The results suggest that, in awake humans, changes in CBF during acute isocapnic hypoxia are quantitatively insufficient to underlie HVD.

Thesis (Ph. D.)--University of Canterbury, 2009. / Typescript (photocopy). "1 April 2009." Includes bibliographical references (leaves [263]-274). Also available via the World Wide Web.

published_or_final_version / Medicine / Master / Doctor of Medicine

Orthostatic vasovagal syncope is the sudden loss of consciousness resulting from a temporary impairment of cerebral blood flow, within approximately an hour of standing. Patients who suffer from this problem have "vasovagal syndrome". The purpose of this thesis was to devise a method to detect the syndrome following the assumption of upright position. Data from 106 syncopal patients undergoing head-up tilt table testing (HUT) were acquired, including electrical activity of the heart (electrocardiogram), blood pressure, oxygen saturation, and cerebral perfusion parameters from near-infrared spectroscopy (NIRS). The data set was examined with the aim of generating automatic diagnoses. Comparison of the rate-pressure product (blood pressure multiplied by heart rate) during the time of syncope with a recommended threshold, in addition to comparison with monitoring the fall of systolic blood pressure during prolonged tilt, yielded an 84% accuracy rate for vasovagal syndrome. The thesis reviewed the techniques used on the aforementioned time series by previous researchers, emphasising the concepts underlying "time-frequency analysis", a method for analysing nonstationary signals. Since even healthy patients experience time-varying frequency information in their haemodynamics, a transform known as the Smoothed Pseudo-Wigner Ville Distribution (SPWVD) is well suited to their analysis. This distribution was applied to RR tachograms, plots of heart period against time. After the smoothing parameters of the SPWVD were chosen based on artificial data, the optimised transform was then applied to a second artificial tachogram to calculate the LF/HF (low- to high-frequency) ratio, an indicator of heart rate variability. The computed LF/HF ratio tracked the expected value within an error margin of 3.6%. Finally, by applying the same transform to clinical data, it was proved to offer better resolution than an alternative known as the Lomb periodogram. Classical techniques from the literature predicting vasovagal syncope were found to fail on the current data set: out of 29 tests, only two yielded statistically significant differences between the two patient groups. These were compared with the author's time-frequency analysis of RR tachograms, linear regression of heart rate, and examination of NIRS oscillations and changes on tilt. Of these, the ICFV during time period P3 was found to perform best (negative predictive value: 0.86). A linear classifier was used to combine the best four predictors; it achieved an overall accuracy of 0.88. Following the data-driven approach, an analytical modelling approach was undertaken. In order to define an appropriate model that traded off simplicity with comprehensiveness, the mechanisms of vasovagal syncope were reviewed. A model of orthostasis was developed, validated, and used toward parameter estimation from patient data. Three parameters (baroreceptor operating point, cardiac effectiveness, and baroreflex gain) were gleaned from the supine baseline recording to "normalise" the model for a given patient, before four new parameters (sympathetic and parasympathetic gains at the sino-atrial node, peripheral vasoconstriction gain, and total blood volume) were estimated from the data collected in the upright position. The expectation was that this approach would improve feature extraction (and hence prediction accuracy) as well as the clinical interpretation of the results. However, the modelling approach was found to offer no significant improvement upon the data-driven signal processing results: a linear classifier on the four post-tilt parameters yielded a negative predictive value of just 0.69. This result may have been due to inaccuracies in the time series data owing to instrumentation error. It is also possible that the modelling approach was not able to provide the quality of feature extraction necessary for predicting vasovagal syncope in the elderly. Finally, methods to predict syncope during mid- to late HUT were examined. Using information derived from heart rate and baroreflex sensitivity, a technique was developed to ease patient comfort by terminating the test approximately 2 minutes before syncope was expected to occur.