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ABSTRACT Purpose: The primary aim of the study (i.e. Chapter 3) was to compare the magnitude of retinal vascular reactivity in arterioles of varying diameter in healthy, young subjects. The secondary aims were to determine: a) if there are any order effects in terms of provoking vasoconstriction or vasodilation first; and b) the repeatability of the vascular reactivity measurements. An additional aim (i.e. Chapter 4) was to determine the effect of healthy aging on the relationship between retinal vascular reactivity and vessel diameter. Method: The sample comprised 10 healthy, young subjects (mean age 26.5 years, SD 4.04) and 7 healthy, older subjects (mean age 55.43 years, SD 5.41). Each subject from the young age group attended for three sessions. The first session was used to determine eligibility and select hemodynamic measurement sites. At sessions 2 and 3, O2 and CO2 were sequentially administered to the subjects using a face mask and sequential re-breathing circuit (to maintain standardized hyperoxia and hypercapnia). The order of vasoconstriction and vasodilation was varied across sessions 2 and 3. The design of the protocol was simplified for the subjects from the older age group. Each subject from the older group attended for one visit. O2 and CO2 were administered to the subjects using a face mask and sequential re-breathing circuit. The order of gas provocation was varied among the subjects (i.e. hyperoxia or hypercapnia first). For both groups, measurements of vessel diameter, centerline blood velocity and derived blood flow were acquired at each condition (i.e. baseline, during stabilized vasoconstriction, vasodilation, and recovery) at two discrete measurement sites along the supero-temporal arteriole. Results: The results of the repeated measures ANOVA showed a significant difference between the narrow and wide measurement sites for the younger group for flow (p??? 0.0003) and a significant influence of inspired gas provocation on flow for both protocols (p<0.0001). In addition, the interaction of measurement site and inspired gas provocation was significant (p<0.0001). The magnitude of retinal vascular reactivity showed a significantly greater blood flow response for the wide measurement site (p<0.0001). O2 provocation resulted in vasoconstriction that was still present up to 10 minutes after cessation of the stimulus (order effect of O2; p???0.046). No such order effect was apparent for CO2 provocation (order effect of CO2; p=0.352). The group mean blood flow Coefficient of Repeatability (COR) for the narrow measurement site was 0.74 ??l/min (relative to group mean flow of 4.85 ??l/min ?? SD 1.31) and for the wide measurement site was 1.49 ??l/min (relative to group mean flow of 11.29 ??l/min ?? SD 3.55). There was no difference between the young and the older age groups in retinal vascular reactivity for both the narrow (two-tailed Student t-test, p=0.8692) and wide (two-tailed Student t-test, p=0.2795) measurement sites. Conclusion: This study demonstrated that the magnitude of retinal vascular reactivity was greater for arteriolar measurement sites with wider baseline vessel diameters. In addition, it demonstrated that hyperoxic provocation resulted in a persistent vasoconstriction up to 10 minutes after cessation of the stimulus. The study demonstrated that the repeatability of retinal blood flow measurements in absolute terms is lower for smaller diameter vessels. Finally, this study also suggests that age does not affect the relationship between retinal vascular reactivity and vessel diameter.

The purpose of this investigation was to study the effects of decreasing barometric pressure upon the pressure-flow relationships in a peripheral vascular bed in an attempt at better delineation of the autoregulatory mechanisms. A decrease in barometric pressure does influence the transmural pressure and could theoretically affect smooth muscle tone. An evaluation of the extent of the transmural effect is essential to understanding vascular dynamics at altitudes.

Alzheimer’s Dementia (AD) is among the most common diseases in the Geriatric population, and its prevalence is expected to quadruple by 2047.Vascular Dementia (VaD) is the second most frequent cause of dementia, with studies indicating VaD accounts for 10-20% of dementia cases across the globe. A diagnostic model differentiating AD and VaD would be clinically and scientifically valuable, considering the treatment approaches for these conditions are different. Although there are differences between AD and VaD on their neuropsychological profiles, a diagnostic model that successfully differentiates AD and VaD on neuropsychological testing has not been developed, despite previous attempts. Our study addresses this gap in the literature by examining two diagnostic models used to predict the conversion of AD from mild cognitive impairment, and a third model was proposed to differentiate AD from VaD. We conducted ROC Analyses using the variables LM II Standard Score, Animals Total, and CDRS Sum based on a previous diagnostic model. The sensitivity and specificity for the diagnosis of mild VaD were calculated for all possible scores of each test measure. The Animals Total cutoff score of 7 achieved excellent sensitivity and specificity, receiving 96% and 92%, respectively. In this sample, patients who could name at least seven animals under 60 seconds were highly likely to be diagnosed with VaD. LM II Scaled Score also achieved statistical significance (p <0.001) and a cutoff score of 4 received 96% sensitivity and 77% specificity. Patients who achieved an LM II Scaled Score of 4 or higher were highly likely to be diagnosed with VaD.