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Large artery stiffness is predictive of adverse cardiovascular events and all cause mortality. Artery structure and function are determinants of artery stiffness. This thesis presents a series of in-vivo experimental studies of effect of structural and functional changes on large artery stiffness. Improved analysis methods were developed for measurement of arterial stiffness indexes, Pulse Wave Velocity (PWV) and pressure wave re ection. These were applied in studies of acute in ammation, active and passive changes in systemic pressures, aortic elastic laminae defects, and aortic calcification in rats using a novel, high fidelity, dual pressure sensing technique of measuring aortic rat PWV. Findings indicated that acute in ammation does not increase large artery stiffness, and that localised effects altering arterial structure do not manifest in in-vivo changes in large artery stiffness. The functional component of stiffness was investigated using graded systemic infusion of vasoconstrictor agents (angiotensin-II, noradrenaline, and Endothelin-1 (ET-1)) in the in-vivo ovine iliac artery. There was a markedly greater dose dependency of pressure independent change in PWV (angiotensin-II) compared to direct endothelial effects (ET-1), although blocking of ET-1 receptors produced marked changes in iliac blood ow. A similar experiment in the human iliac artery found that the B-antagonist and nitric oxide (NO) donor, x Structural and functional effects on large artery stiffness nebivolol, potentially causes a decrease in regional functional stiffness. An additional study in human subjects directly measured the decrease in forearm arterial stiffness during reactive hyperaemia following different periods of ischaemia. The findings precluded the use of this method in measuring brachial artery structural stiffness with maximal smooth muscle relaxation. Increasing periods of ischaemia had a bi-phasic relationship with changes in arterial stiffness, the first phase linked to endogenous nitric oxide release. This finding is of importance in the clinical quantification of endothelial dysfunction. These findings in basic research of arterial haemodynamics provide new quantitative contributions to the in-vivo experimental investigation of the aetiology of large artery stiffness related to structure and function of endothelial and medial wall properties. This can lead to potential clinical applications and techniques for assessment of cardiovascular risk.

Saphenous veins are widely used as graft material in coronary artery bypass surgery. Biomechanical testing may help identify veins with the optimum chance of longevity. In this thesis, the author has constructed an experimental apparatus to measure circumferential and axial stress-strain data for such arteries and veins. Three different experimental techniques were used to obtain the dimensions of the vessels during pressure-diameter tests; the traditional method of photographic and direct measurement of the external dimensions along with the assumption of incompressibility to approximate inner dimensions; second the use of intravascular ultrasound to obtain the inner and outer dimensions of the vessels; and third the use of intravascular ultrasound for internal dimensions and photography for the outer dimensions. Three forms of mathematical expression were fitted to the circumferential and axial stress-strain data. The sensitivity of the resulting material parameters was investigated with regard to conversion and reference data variations, and data from repeated experiments, experiments fitting more than one axial strain, and experiments testing the variation in elastic properties were compared. The combination of the ultrasound experimental technique and exponential form of mathematical expression was found to be the most robust. Using this combination, a significant difference between material parameters of pig arteries and human saphenous veins was found . While the data indicate that material parameters of healthy and diseased veins may also differ significantly, the number of healthy veins in this study was not sufficient to be conclusive.

Atherosclerotic plaque caps are composed of a composite soft tissue material that becomes subjected to cyclic loading under stenotic flow conditions. The cyclic loading causes the plaque cap to fatigue and eventually fail. The hypothesis of this work is that arteries and plaque caps may fatigue which may be predicted by a stress vs. number of cycles (S-N) curve. The S-N curve has not been determined for almost any biological soft tissue. The Specific Aim of the thesis is to quantify an S-N curve for normal arterial soft tissue collected from cyclic tension testing. Coronary arteries from porcine hearts will be tested as a material that closely models the plaque cap in non-linear elastic behavior. The S-N curve will be developed through failure testing with multiple cycles at stresses between 0.5 and 5 MPa.

Thesis (M. Med. Sc.)--University of Hong Kong, 2009. / Includes bibliographical references (p. 37-42).

published_or_final_version / Pharmacology and Pharmacy / Master / Master of Medical Sciences

published_or_final_version / Pharmacology and Pharmacy / Master / Master of Medical Sciences

Hypertension is accompanied by structural and mechanical abnormalities in resistance arteries. The effects of resveratrol, a phenolic phytoalexin found naturally in various foods, on systolic blood pressure and resistance artery structure and stiffness were assessed in spontaneously hypertensive rats (SHRs). Vascular geometry and mechanical properties of pressurized mesenteric resistance arteries were calculated from media and lumen dimensions measured using pressure myography. Compared to normotensive Wistar-Kyoto (WKY) rats, resistance arteries from SHRs displayed remodeling with narrowed lumen diameters (246.2±21.0 vs. 308.1±14.3 μm, p<0.05), thickened media widths (39.8±4.6 vs. 28.5±2.7 μm, p<0.05) and augmented media-to-lumen ratios (17.7±2.6 vs. 9.3±1.0, p<0.05). Calculations of remodeling and growth indices revealed that SHR vessels underwent mostly eutrophic remodeling. Systolic blood pressure was elevated in 20-week-old SHR versus WKY rats (219±6 vs. 155±6 mmHg, p<0.01) and was unaffected by resveratrol (2.5 mg/Kg/d). In SHRs, resveratrol treatment attenuated eutrophic remodeling and normalized increased vessel compliance (p<0.01) as determined by a restorative leftward shift in the stress-strain curve of SHR arteries (p<0.01). Resveratrol treatment restored stiffness in SHRs (4.2±0.4 vs. 6.6±0.5, p<0.05) through the normalization of vessel geometry. Immunoblotting revealed that resveratrol negated typical pronounced ERK1/2 signaling in SHR arteries. Thus, the results of this study suggest that resveratrol restores vascular mechanical properties in SHRs and attenuates remodeling. Furthermore the attenuation of remodeling in SHR arteries with resveratrol treatment is associated with the inhibition of ERK1/2 activity.

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