Intracoronary ultrasound

Knowledge of the characteristics of the atherosclerotic plaque (eccentricity, composition, effect of initial dilatation or ablation) and of the flow modifications induced by a coronary stenosis would establish more precisely the severity of the lesion under evaluation, improve the planning and guidance of therapeutic interventions, and facilitate the detection of subsequent complications. The miniaturization of the ultrasound catheters a11d the development of Doppler probes with guidewire technology have permitted the application of these techniques during diagnostic or therapeutic procedures in the Cardiac Catheterization Laboratory. In this thesis, the application of catheter-based ultrasound probes for the assessment of function and morphology of the coronary arterial tree is addressed. In particular, after a general introduction to the physical principles and probe teclmology in catheterbased diagnostic intracoronary teclmiques (Chapter 1), Chapter II and Ill discuss the application of intracoronary ultrasound for the quantitative assessment of arterial dimensions and the reconstruction of three-dimensional images of arterial segments. Chapter IV, V and VI report the results of experimental studies carried out to validate the accuracy of intravascular ultrasound for the assessment of wall morphology and pathology and of acute changes of arterial compliance and to study the effects of changes of the angle of incidence of the ultrasound beam. Using Doppler guidewires~ changes in flow velocity distal to a stenosis can be studied, providing information on the functional characteristics of the stenosis nnder assessment complementary to the morphological evaluation obtained with twodimensional ultrasound imaging. In Chapter VII and VIII the physical and teclmical background of intracoronary Doppler is discussed and the advantages of a spectral analysis of the Doppler signal are established based on the comparison with the results obtained with a conventional zero-crossing detector. Based on the limitations of the conventional indexes of stenosis severity in the assessment of the results of coronary interventions (Chapter X), alternative indexes based on flow velocity measurements are proposed. Chapter IX explores the possibility of using the maximal velocity of the stenotic jet in the evaluation of the severity of a stenosis based on the continuity equation. Chapters X, XI and XII report the results obtained using a simultaneous assessment of pressure gradient and flow velocity, allowing the examination of stenosis hemodynamics as in an isolated hydraulic conduit. In Chapter XIII the possibility to study the relation between coronary pressure and flow velocity is tested in arteries with and without coronary stenoses. The potential usefulness of this approach, validated in animal models, for the assessment of an impaired coronary conductance due to the presence of a flow limiting stenosis or to the presence of a reduced maximal vasodilatation of the distal resistance vessels is discussed. In the fmal chapter of this thesis, the response of large conduit arteries and resistance vessels to endothelium-dependent vasodilators such as acetylcholine is assessed using quantitative angiography and intracoronary Doppler. ix

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