The aim of this study is to investigate the fluid-structural response to pulsatile Newtonian and non-Newtonian blood flow through an axisymmetric stenosed vessel using FLOTRAN and ANSYS. This is to provide a basic understanding of atherosclerosis. The flow was set to be laminar and follows a sinusoidal waveform. The solid model was set to have isotropic elastic properties. The Fluid-Structural Interaction (FSI) coupling was two-way and iterative. Rigid and Newtonian cases were investigated to provide an understanding on the effects of incorporating FSI into the model. The wall expansion was found to decrease the axial velocity and increase the recirculation effects of the flow. To validate the models and methods used, the results were compared with the study by Lee and Xu [2002] and Ohja et al [1989]. Close comparisons were achieved, suggesting the models used were valid. Two non-Newtonian models were investigated with FSI: Carreau and Power Law models. The Carreau model fluid behaviour was very close to the Newtonian model. The Power Law model produced significant difference in viscosity, velocity and wall shear stress distributions. Pressure distribution for all models was similar. In order to quantify the changes, Importance Factor (IG) was introduced to determine the overall non-Newtonian effects at two regions: the entire flow model and about the vessel wall. The Carreau model showed reasonable values of IG whereas the Power Law model showed excessive values. Transient and geometrical effects were found to affect the Importance Factor. The stress distributions for all models were found to be similar. Highest stress occurred at the shoulders of the stenosis where a stress concentration occurred due to sharp corners of the geometry and large bending moments. The highest stresses were in the axial direction. Notable circumferential stress was found at the ends of the vessel. Carreau model produced slightly higher stresses than the other models. Wall stresses were found to be primarily influenced by internal pressure, rather than wall shear stresses.

<p>Contrast-enhanced magnetic resonance angiography (MRA) is used to obtain images showing the vascular system. To detect stenosis, which is narrowing of for example blood vessels, maximum intensity projection (MIP) is typically used. This technique often fails to demonstrate the stenosis if the projection angle is not suitably chosen. To improve identification of this region a color-coding algorithm could be helpful. The color should be carefully chosen depending on the vessel diameter. </p><p>In this thesis a segmentation to produce a binary 3d-volume is made, followed by a distance transform to approximate the Euclidean distance from the centerline of the vessel to the background. The distance is used to calculate the smallest diameter of the vessel and that value is mapped to a color. This way the color information regarding the diameter would be the same from all the projection angles. </p><p>Color-coded MIPs, where the color represents the maximum distance, are also implemented. The MIP will result in images with contradictory information depending on the angle choice. Looking in one angle you would see the actual stenosis and looking in another you would see a color representing the abnormal diameter.</p>

<p>Randomised controlled trials (RCT) have demonstrated a net benefit of carotid endarterectomy (CEA) in stroke prevention for patients with severe carotid artery stenosis as compared to best medical treatment. Results in routine clinical practice must not be inferior to those in the RCTs. The carotid arteries are clamped during CEA which may impair the cerebral perfusion. </p><p>The aim of this thesis was to assess population-based outcomes from CEA, investigate risk factors for perioperative complications/late mortality and to evaluate effects of carotid clamping during CEA. In the Swedish vascular registry 6182 CEAs were registered during 1994-2003. Data on all CEAs were retrieved, analysed and validated. In the validation process no death or disabling stroke was unreported. The perioperative stroke or death rate was 4.3% for those with symptomatic and 2.1% for asymptomatic stenosis (the latter decreasing over time). Risk factors for perioperative complications were age, indication, diabetes, cardiac disease and contralateral occlusion. Median survival time was 10.8 years for the symptomatic and 10.2 years for the asymptomatic group. </p><p>Tolerance to carotid clamping during CEA under general anaesthesia was evaluated in 62 patients measuring cerebral oximetry, transit time volume flowmetry and stump pressure. High internal carotid artery flow before clamping and low stump pressure was associated with decreased oxygenation after clamping suggesting shunt indication. </p><p>In 18 patients undergoing CEA, jugular bulb blood samples demonstrated significantly altered levels of marker for inflammatory activation (IL-6) and fibrinolytic activity (D-dimer and PAI-1) during carotid clamping as compared to radial artery levels. This indicates a cerebral ischaemia due to clamping although clinically well tolerated. </p><p>In conclusion, the perioperative outcome after CEA in Sweden compared well with the RCTs results. Tolerance to carotid clamping may be evaluated by combining stump pressure and volume flow measurements. Although clinically tolerated clamping may induce a cerebral ischaemic response.</p>

Today, invasive coronary angiography is still the gold standard to perform the diagnosis of coronary artery disease. But it is an invasive procedure that carries non negligible morbidity (1.5%) and mortality (0.15%), and results in high costs. Less invasive and more cost-effective techniques are highly desirable. Over the past 15 years, substantial advances have been made in non-invasive cardiac imaging. In the first part of this work, we prospectively evaluated the diagnostic accuracy of 40-slice multidetector CT (MDCT) to detect coronary artery disease prior to cardiac valve surgery in 82 patients. On a per-patient basis, MDCT correctly identified 14/15 patients with (sensitivity 93%) and 60/67 patients without coronary disease (specificity 90%). Performing invasive angiography only in case of abnormal CT might have avoided invasive angiography in 60/82 (73%) patients without coronary disease. Thus, MDCT could be potentially useful in the preoperative evaluation of such patients, allowing to avoid systematic cardiac catheterization in a large number of patients. Magnetic resonance coronary angiography (MRCA) has also emerged as a promising alternative due to the lack of ionizing radiation and absence of iodinated contrast injection. Therefore, we compared diagnostic accuracy of whole-heart MRCA and MDCT, against QCA, to identify >50% stenosis basis in 77 patients. WH-MRCA acquisition failed in a high number of patients. This was caused by an unstable breathing pattern or drift of the diaphragm position. Because of higher success rate, MDCT had higher diagnostic accuracy than WH-MRCA to detect coronary stenosis. Thus MDCT is superior to WH-MRCA, however WH-MRCA can perform as well as CT in interpretable segments with adequate image quality. In the second part of this work, to evaluate whether MDCT and cardiac magnetic resonance (cMR) might allow simultaneous assessment of aortic valve area (AVA), we compared measurements of AVA by MDCT to cMR, transesophageal and transthoracic echocardiography. AVA by MDCT and cMR correlated highly with AVA by other techniques. In our study, we compared 3 planimetric approaches to AVA calculated by the continuity equation using TTE. We did observe excellent correlations between planimetric and continuity equation-derived AVA, but all 3 planimetric measures were found to overestimate continuity equation AVA. A potential explanation for this observation could be that we measure different aortic valve orifices. Indeed planimetric techniques measure the true dimensions of the anatomical orifice, whereas the continuity equation measures the "effective" orifice area. The ability of MDCT and cMR to accurately assess aortic valve area at the time of non-invasive coronary imaging, places these techniques in a strong position for the comprehensive assessment of such patients. However, despite these good results, it must nonetheless be emphasized that to be acceptable in daily clinical practice, a strategy in which invasive coronary angiography would not be performed systematically but rather selectively in only a subset of patients, requires a perfect sensitivity for disease detection in individual patients. Unfortunately, the present work shows that MDCT and WH-MRCA have not yet reached such a level of accuracy. Finally, these tests are not a substitute for other imaging techniques in all cardiovascular conditions. Unlike an echocardiogram machine, the MRI and MDCT scanners cannot be brought to the bedside of an acutely ill patient.

Postoperative heart failure (PHF) remains a major determinant of the outcome after cardiac surgery. However, characteristics of and risk factors for PHF after valve surgery have received little attention. Post-ischaemic disturbances of myocardial metabolism that may contribute to PHF and are amenable to metabolic treatment have been identified early after coronary surgery (CABG). Knowledge derived from these studies may not be applicable to other patient groups. We therefore studied myocardial energy metabolism in 20 elective patients undergoing aortic valve replacement (AVR) for isolated aortic stenosis (AS). The metabolic studies indicated that myocardial oxidative metabolism had not fully recovered when the procedure was completed. Free fatty acids were the only major substrates taken up by the heart. Signs of preoperative and postoperative metabolic adaptation with substantial uptake of glutamate, previously demonstrated in patients with coronary artery disease, were found. Postoperative infusion of glutamate, (2 mL/kg body weight and hour of 0.125 M solution) based on assessment of myocardial glutamate requirements in CABG patients, resulted in a two-fold increase in myocardial glutamate uptake and a seven-fold increase in AV differences across the leg. This was associated with a significant myocardial uptake of lactate and metabolic changes in the leg suggesting mitigation of net amino acid loss and peripheral tissue lipolysis. Characteristics of and risk factors for PHF were evaluated in 398 patients undergoing isolated AVR for AS from 1 January 1995 to 31 December 2000. These were compared with 398 patients, matched for age and sex, undergoing on-pump isolated CABG. Forty-five AVR and 47 CABG patients fulfilled criteria for PHF and these were studied in detail. PHF usually presented at weaning from cardiopulmonary bypass. After CABG it was closely associated with preoperative ischaemic events and intraoperatively acquired myocardial infarction. Potential causes and eliciting events of PHF after AVR for AS were obvious only in one-third of the patients. Risk factors for PHF after AVR for AS indicated either pre-existing myocardial dysfunction, increased right or left ventricular after-load, or intraoperatively acquired myocardial injury. PHF was associated with high early mortality after CABG, whereas the consequences of PHF after AVR for AS became evident only with time, resulting in a 42% five-year mortality. Although PHF had a different temporal impact on late mortality after CABG and AVR for AS, it emerged as the statistically most significant risk factor for mortality occurring within 5 years from surgery both after AVR for AS and after CABG. Potential implications of our findings include needs for greater focus on preoperative surveillance of patients with AS for optimal timing of surgery, mitigation of intraoperatively acquired myocardial injury and tailoring of treatment for PHF. Furthermore, the findings have implications for long-term follow up of AS patients after surgery.

Randomised controlled trials (RCT) have demonstrated a net benefit of carotid endarterectomy (CEA) in stroke prevention for patients with severe carotid artery stenosis as compared to best medical treatment. Results in routine clinical practice must not be inferior to those in the RCTs. The carotid arteries are clamped during CEA which may impair the cerebral perfusion. The aim of this thesis was to assess population-based outcomes from CEA, investigate risk factors for perioperative complications/late mortality and to evaluate effects of carotid clamping during CEA. In the Swedish vascular registry 6182 CEAs were registered during 1994-2003. Data on all CEAs were retrieved, analysed and validated. In the validation process no death or disabling stroke was unreported. The perioperative stroke or death rate was 4.3% for those with symptomatic and 2.1% for asymptomatic stenosis (the latter decreasing over time). Risk factors for perioperative complications were age, indication, diabetes, cardiac disease and contralateral occlusion. Median survival time was 10.8 years for the symptomatic and 10.2 years for the asymptomatic group. Tolerance to carotid clamping during CEA under general anaesthesia was evaluated in 62 patients measuring cerebral oximetry, transit time volume flowmetry and stump pressure. High internal carotid artery flow before clamping and low stump pressure was associated with decreased oxygenation after clamping suggesting shunt indication. In 18 patients undergoing CEA, jugular bulb blood samples demonstrated significantly altered levels of marker for inflammatory activation (IL-6) and fibrinolytic activity (D-dimer and PAI-1) during carotid clamping as compared to radial artery levels. This indicates a cerebral ischaemia due to clamping although clinically well tolerated. In conclusion, the perioperative outcome after CEA in Sweden compared well with the RCTs results. Tolerance to carotid clamping may be evaluated by combining stump pressure and volume flow measurements. Although clinically tolerated clamping may induce a cerebral ischaemic response.

Phase-contrast magnetic resonance imaging (PC-MRI) is a powerful tool for measuring blood flow and has a wide range of cardiovascular applications. Simulation of PC-MRI from numerical flow data would be useful for addressing the data quality of PC-MRI measurements and to study and understand different artifacts. It would also make it possible to optimize imaging parameters prior to the PC-MRI measurements and to evaluate different methods for measuring wall shear stress. Based on previous studies a PC-MRI simulation tool was developed. An Eulerian-Lagrangian approach was used to solve the problem. Computational fluid dynamics (CFD) data calculated on a fix structured mesh (Eulerian point of view) were used as input. From the CFD data spin particle trajectories were computed. The magnetization of the spin particle is then evaluated as the particle travels along its trajectory (Lagrangian point of view). The simulated PC-MRI data were evaluated by comparison with PC-MRI measurements on an in vitro phantom. Results indicate that the PC-MRI simulation tool functions well. However, further development is required to include some of the artifacts. Decreasing the computation time will make more accurate and powerful simulations possible. Several suggestions for improvements are presented in this report.

No description available.

Atherosclerosis is a disease of the cardiovascular system where a stenosis may develop in an artery which is an abnormal narrowing in the blood vessel that adversely affects the blood flow. Due to the constriction of the blood vessel, the flow is disturbed, forming a jet and recirculation downstream of the stenosis. Dynamic pressure fluctuations on the inner wall of the blood vessel leads to the vibration of the vessel structure and acoustic energy is propagated through the surrounding tissue that can be detected on the skin surface. Acoustic energy radiating from the interaction of blood flow and stenotic blood vessel carries valuable information from a diagnostic perspective. In this study, a constricted blood flow is modeled by using ADINA finite element analysis software together with the blood vessel in the form of a thin cylindrical shell with an idealized blunt constriction. The flow is considered as incompressible and Newtonian. Water properties at indoor temperature are used for the fluid model. The diameter of the modeled vessel is 6.4 mm with 87% area reduction at the throat of the stenosis. The flow is investigated for Reynolds numbers 1000 and 2000. The problem is handled in three parts which are rigid wall Computational Fluid Dynamics (CFD) solution, structural analysis of fluid filled cylindrical shell, and Fluid Structure Interaction (FSI) solutions of fluid flow and vessel structure. The pressure fluctuations and consequential vessel wall vibrations display broadband spectral content over a range of several hundred Hz with strong fluid-structural coupling. Maximum dynamic pressure and vibration amplitudes are observed around the reattachment point of the flow near the exit of the stenosis and this effect gradually decreases along downstream of flow. Results obtained by the numerical simulations are compared with relevant studies in the literature and it is concluded that ADINA can be used to investigate these types of problems involving high frequency pressure fluctuations of the fluid and the resulting vibratory motion of the surrounding blood vessel structure.

名古屋大学博士学位論文 学位の種類 : 博士(医学)(論文) 学位授与年月日:平成3年9月14日 安田公氏の博士論文として提出された