Measurement of the Retinal Arteriolar Response to a Hyperoxic Provocation
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The endothelium is a monolayer of cells that lines the inner surface of blood vessels; it plays an important role in local blood flow regulation by secreting vasoactive factors, which modulate vascular tone, in response to various agonists and stimuli. Non-invasive stimulation methods can be used to provoke vessels to react (i.e. constricting or dilating), allowing assessment of endothelial function. Such vascular reactivity can be directly visualised in the retinal circulation. This thesis describes a series of experiments performed to investigate retinal vascular reactivity in a number of different cohorts. This was achieved by measuring the diameter of retinal arterioles, from image sequences obtained using a high-resolution confocal scanning laser ophthalmoscope (cSLO), before and during oxygen breathing and calculating the change in diameter in response to the hyperoxic stimulus. First we describe the image processing techniques used to register, and subsequently average, the image sequences. This is followed by a description of the vessel tracking and vessel diameter measurement algorithms implemented. In a group of young healthy subjects we investigate whether a reactivity measurement taken from a single, arbitrary measurement site can be considered as representative of the retinal vasculature as a whole. We present results from a group of otherwise healthy smokers, detailing the chronic and acute effects of smoking. The results obtained from healthy older persons are compared to those obtained from the young subjects, to investigate the impact of healthy aging on the magnitude of vasoconstriction. Finally we detail a small clinical study performed to assess the reactivity response in various clinical conditions, such as hypertension, and we discuss the limitations and difficulties of using our experimental protocol and setup in a clinical setting.