Simulation numérique des interactions fluide-structure dans une fistule artério-veineuse sténosée et des effets de traitements endovasculaires

Une fistule artérioveineuse (FAV) est un accès vasculaire permanent créé par voie chirurgicale en connectant une veine et une artère chez le patient en hémodialyse. Cet accès vasculaire permet de mettre en place une circulation extracorporelle partielle afin de remplacer les fonctions exocrines des reins. En France, environ 36000 patients sont atteint d insuffisance rénale chronique en phase terminale, stade de la maladie le plus grave qui nécessite la mise en place d un traitement de suppléance des reins : l hémodialyse. La création et présence de la FAV modifient significativement l hémodynamique dans les vaisseaux sanguins, au niveau local et systémique ainsi qu à court et à plus long terme. Ces modifications de l hémodynamiques peuvent induire différents pathologies vasculaires, comme la formation d anévrysmes et de sténoses. L objectif de cette étude est de mieux comprendre le comportement mécanique et l hémodynamique dans les vaisseaux de la FAV. Nous avons étudié numériquement les interactions fluide-structure (IFS) au sein d une FAV patient-spécifique, dont la géométrie a été reconstruite à partir d images médicales acquises lors d un précédent doctorat. Cette FAV a été créée chez le patient en connectant la veine céphalique du patient à l artère radiale et présente une sténose artérielle réduisant de 80% la lumière du vaisseau. Nous avons imposé le profil de vitesse mesuré sur le patient comme conditions aux limites en entrée et un modèle de Windkessel au niveau des sorties artérielle et veineuse. Nous avons considéré des propriétés mécaniques différentes pour l artère et la veine et pris en compte le comportement non-Newtonien du sang. Les simulations IFS permettent de calculer l évolution temporelle des contraintes hémodynamiques et des contraintes internes à la paroi des vaisseaux. Nous nous sommes demandées aussi si des simulations non couplées des équations fluides et solides permettaient d obtenir des résultats suffisamment précis tout en réduisant significativement le temps de calcul, afin d envisager son utilisation par les chirurgiens. Dans la deuxième partie de l étude, nous nous sommes intéressés à l effet de la présence d une sténose artérielle sur l hémodynamique et en particulier à ses traitements endovasculaires. Nous avons dans un premier temps simulé numériquement le traitement de la sténose par angioplastie. En clinique, les sténoses résiduelles après angioplastie sont considérées comme acceptables si elles obstruent moins de 30% de la lumière du vaisseau. Nous avons donc gonflé le ballonnet pour angioplastie avec différentes pressions de manière à obtenir des degrés de sténoses résiduelles compris entre 0 et 30%. Une autre possibilité pour traiter la sténose est de placer un stent après l angioplastie. Nous avons donc dans un deuxième temps simulé ce traitement numériquement et résolu le problème d IFS dans la fistule après la pose du stent. Dans ces simulations, la présence du stent a été prise en compte en imposant les propriétés mécaniques équivalentes du vaisseau après la pose du stent à une portion de l artère. Dans la dernière partie de l étude nous avons mis en place un dispositif de mesure par PIV (Particle Image Velocimetry). Un moule rigide et transparent de la géométrie a été obtenu par prototypage rapide. Les résultats expérimentaux ont été validés par comparaison avec les résultats des simulations numériques.An arteriovenous fistula (AVF) is a permanent vascular access created surgically connecting a vein onto an artery. It enables to circulate blood extra-corporeally in order to clean it from metabolic waste products and excess of water for patients with end-stage renal disease undergoing hemodialysis. The hemodynamics results to be significantly altered within the arteriovenous fistula compared to the physiological situation. Several studies have been carried out in order to better understand the consequences of AVF creation, maturation and frequent use, but many clinical questions still lie unanswered. The aim of the present study is to better understand the hemodynamics within the AVF, when the compliance of the vascularwall is taken into account. We also propose to quantify the effect of a stenosis at the afferent artery, the incidence of which has been underestimated for many years. The fluid-structure interactions (FSI) within a patient-specific radio-cephalic arteriovenous fistula are investigated numerically. The considered AVF presents an 80% stenosis at the afferent artery. The patient-specific velocity profile is imposed at the boundary inlet, and a Windkessel model is set at the arterial and venous outlets. The mechanical properties of the vein and the artery are differentiated. The non-Newtonian blood behavior has been taken into account. The FSI simulation advantageously provides the time-evolution of both the hemodynamic and structural stresses, and guarantees the equilibrium of the solution at the interface between the fluid and solid domains. The FSI results show the presence of large zones of blood flow recirculation within the cephalic vein, which might promote neointima formation. Large internal stresses are also observed at the venous wall, which may lead to wall remodeling. The fully-coupled FSI simulation results to be costly in computational time, which can so far limit its clinical use. We have investigated whether uncoupled fluid and structure simulations can provide accurate results and significantly reduce the computational time. The uncoupled simulations have the advantage to run 5 times faster than the fully-coupled FSI. We show that an uncoupled fluid simulation provides informative qualitative maps of the hemodynamic conditions in the AVF. Quantitatively, the maximum error on the hemodynamic parameters is 20%. The uncoupled structural simulation with non-uniform wall properties along the vasculature provides the accurate distribution of internal wall stresses, but only at one instant of time within the cardiac cycle. Although partially inaccurate or incomplete, the results of the uncoupled simulations could still be informative enough to guide clinicians in their decision-making. In the second part of the study we have investigated the effects of the arterial stenosis on the hemodynamics, and simulated its treatment by balloon-angioplasty. Clinically, balloon-angioplasty rarely corrects the stenosis fully and a degree of stenosis remains after treatment. Residual degrees of stenosis below 30% are considered as successful. We have inflated the balloon with different pressures to simulate residual stenoses ranging from 0 to 30%. The arterial stenosis has little impact on the blood flow distribution: the venous flow rate remains unchanged before and after the treatment and thus permits hemodialysis. But an increase in the pressure difference across the stenosis is observed, which could cause the heart work load to increase. To guarantee a pressure drop below 5 mmHg, which is considered as the threshold stenosis pressure difference clinically, we find that the residual stenosis degree must be 20% maximum.COMPIEGNE-BU (601592101) / SudocSudocFranceF

Long term outcomes of ‘Christmas Tree’ banding for haemodialysis access induced distal ischemia: A 13-year experience

Background: The reduction in distal arterial flow following arteriovenous fistula (AVF) creation can cause a perfusion deficit known as haemodialysis access induced distal ischemia (HAIDI). Various techniques have been advocated to treat this difficult problem with varying success. We present the long-term outcomes following a novel banding technique. Methods: 46 patients in this cohort from 2008 to 2021 underwent a novel banding procedure using a Dacron™ patch shaped with one slit-end and saw-tooth edges (resulting in a ‘Christmas-tree’ pattern) to provide a ratchet mechanism to progressively constrict the fistula outflow. Real-time finger perfusion pressure monitoring allowed an accurate reduction in AVF flow whilst increasing distal arterial perfusion pressure. Baseline characteristic were recorded and Kaplan-Meier survival curves were obtained to calculate the post-intervention primary, assisted primary and secondary patency. Results: 29 patients presented with rest pain and 11 presented with tissue loss due to distal ischemia. The post-intervention primary access patency was 100%, 98%, 78% and 61% at 30, 60 and 180 days and 1 year respectively. Complete resolution of symptoms was achieved in 74% (n = 34) of patients and a partial response needing no further intervention was achieved in 11% (n = 5) of patients. A Youden index calculation suggested that digital pressures of 41 mm Hg or lower in an open AVF were highly sensitive for symptomatic hand ischemia whereas pressures greater than 65 mm Hg ruled out distal ischemia. Conclusion: ‘Christmas-tree’ banding with on table finger systolic pressures is not only an efficacious and durable method for treating HAIDI but also preserves fistula patency

Patient-specific computational haemodynamics associated with the surgical creation of an arteriovenous fistula

Despite arteriovenous fistulae (AVF) being the preferred vascular access for haemodialysis, high primary failure rates (30-70%) and low one-year patency rates (40-70%) hamper their use. Furthermore, AVF creation has been associated with haemodynamic changes causing maladaptive cardiac remodelling leading to cardiovascular (CV) complications. In this study, we present a new workflow for characterising the haemodynamic profile prior to and following surgical creation of a successful left radiocephalic AVF in a 20-year-old end-stage kidney disease patient. The reconstructed vasculature was generated using multiple ferumoxytol-enhanced magnetic resonance angiography (FeMRA) datasets. Computational fluid dynamics (CFD) simulations utilising a scale-resolving turbulence model were completed to investigate the changes in the proximal haemodynamics following AVF creation, in addition to the post-AVF juxta-anastomosis flow patterns, which is impractical to obtain in-vivo. Following AVF creation, a significant 2-3-fold increase in blood flow rate was induced downstream of the left subclavian artery. This was validated through comparison with post-AVF patient-specific phase-contrast data. Proximal to the anastomosis, the increased flow rate yielded an increase in time-averaged wall shear stress (WSS), which is a key marker of adaptive vascular remodelling. In the juxta-anastomosis region, the success of the AVF was discussed with respect to the National Kidney Foundation's vascular access guidelines, where the patient-specific AVF met the flow rate and geometry criterion. The AVF venous diameter exceeded 6mm and the venous flow rate surpassed 600mL/min. This workflow may potentially be significant clinically when applied to multi-patient cohorts, with population-wide patient-specific conclusions being ascertained for the haemodynamic assessment of AVFs and improved surgical planning

The use of patient-specific modelling in the assessment of a clinical indicator for arteriovenous fistula failure

The arteriovenous fistula (AVF) is a surgically-made vascular structure connecting an artery to a vein. It is the optimal form of vascular access for haemodialysis-dependent end-stage renal disease patients. However, AVF are prone to access dysfunction through the formation of stenoses, which compromise the structure’s utility. To date, a plethora of clinical models are used to predict AVF formation failure based on patient factors and other models predicting late AVF failure by assessing haemodynamics and quantifying disturbed flow behaviours and wall shear stress metrics with stenosis formation. That said, inconsistencies were identified in the correlation between these metrics and diseased AVFs. This thesis aims to assess the suitability of another haemodynamic-related metric, resistance, derived from pressure drop and flow rates through patient-specific CFD modelling, for diagnosing and predicting AVF failure. A three-dimensional ultrasound scanning system was used to obtain patient-specific geometry and flow profiles, used for CFD models which were then analysed, with resistance calculated for each patient. The significance of patient-specific CFD modelling was demonstrated in its usefulness to generate a patient-targeted indicator of diseased AVF. To study the effectiveness of resistance as a metric, the relationship between CFD-derived resistance and the potential for AVF failure was evaluated, starting with classification of resistance results among patients who had undergone treatment for stenosis. An exploratory study into the suitability of CFD-derived resistance and its association with patients’ AVF conditions was further conducted by classifying data from a larger patient dataset and fitting the classified data to a multilevel regression model. CFD-derived resistance was found to be higher at the proximal vein of problematic AVF, however this figure was 76% lower among patients who had undergone stenosis treatment. Meanwhile, no correlation was found between resistance at the proximal artery and patency status. An area under curve of 92.1% was found from the receiver operating characteristic analysis, noting an outstanding discrimination of the classification. CFD-derived resistance appears to be a promising metric in the assessment of a suitable diagnostic marker for AVF failure. This research concludes with aspirations for clinical implementation of a related system, alongside routine surveillance of AVF

The use of infrared thermal imaging as a marker of tissue perfusion and predictor of arteriovenous fistula outcomes

The gold standard of vascular access is the arteriovenous fistula (AVF). Unfortunately it is associated with high rates of failing to mature. Therefore the ability to predict AVF outcomes would change clinical practice. Predictive markers of AVF outcomes were assessed in chapter 2. The literature and our study showed numerous contradictions. In chapter 3 we assessed a multifactorial approach with a systematic review on predictive models of maturation. The review found few models and the disparity between each one limits the development of a unified model. Recent development in infrared thermal imaging (IRTI) technology has made it portable and easy to use. In Chapter 4, we proved that IRTI is a valid and user-friendly method of measuring skin temperature and is comparable to traditional methods of thermometry. IRTI can be used to quantity reactive hyperaemia following a vascular occlusion test (chapter 5). In Chapter 6 we showed that IRTI is an accurate tool in predicting AVF outcome. It was shown to have superiority to intra-operative thrill and other independent patient factors. In conclusion IRTI has a definite role in patients with vascular access. There is also potential for its use in patients with other conditions such as peripheral vascular disease

Progress in Hemodialysis

Hemodialysis (HD) represents the first successful long-term substitutive therapy with an artificial organ for severe failure of a vital organ. Because HD was started many decades ago, a book on HD may not appear to be up-to-date. Indeed, HD covers many basic and clinical aspects and this book reflects the rapid expansion of new and controversial aspects either in the biotechnological or in the clinical field. This book revises new technologies and therapeutic options to improve dialysis treatment of uremic patients. This book consists of three parts: modeling, methods and technique, prognosis and complications

Numerical study on patient-specific haemodynamics subjected to embolisation and wall-distensibility

Computational fluid dynamics (CFD) simulations have been peformed to investigate the hemodynamics of patient-specific cerebral aneurysm treated with endovascular coils; and arteriovenous fistula (AVF) using Star-CCM+. Fluid-structure interactions (FSI) between the elastic vessel walls and the blood flow within were also taken into account to provide a more realistic environment and better understanding of haemodynamic effects on wall remodelling. The blood in both studies was modelled as non-Newtonian fluid and comprises of three phases to fully incorporate the effects of shear-thinning and distributions of blood cells, respectively. The use of a less invasive ultrasonic imaging texhnique for CFD simulations is shown to be a viable alternative to magnetic resonance imaging (MRI). This has proven to be beneficial especially for haemodialysis patients who require fistula check-up on a regular basis. Excessively enlarged sections of arteries, called aneurysms, are vulnerable to vessel wall degradation. When blood flows into a cerebral aneurysm, it causes abnormal haemodynamic changes, which increases the risk of aneurysm rupture and strokes. Patients diagnosed with a cerebral aneurysm are therefore treated by stenting the parent artery or aneurysmal coiling to achieve occlusion. Despite high coiling packing density, aneurysm may recanalise, which consequently leads to aneurysm recurrence. Our understanding of the relationship between coiling density and aneurysmal occlusion and aneurysm recurrence in a non-Newtonian environment are limited. The effects of coil packing density on aneurysmal haemodynamics and the mechanism behind aneurysmal recurrence are discussed in this thesis. In the present aneurysm study, the aneurysm dome was embolised with seven different coil configurations of different packing densities. A time-dependent passive scalar was added to the multiphase blood inflow to represent medical dyes which allows for the visualisation of blood flow penetrating into the coils. The observed relationship between passive scalar visualisations, white blood cells distribution, and hemodynamic quantities will be beneficial for clinical evaluation of aneurysm occlusion. It is shown that a packing density of 31% (7 coils) is the optimal coil density that can supress the aneurysmal volume-averaged velocity and wall shear stress. Furthermore, the temporal variation in streamwise velocity inside the aneurysm dome does not nescessarily decrease with coiling packing density during peak systole. Local packing density, distribution of red and white blood cells, and wall compliance have been correlated with aneurysm recurrence. Circumferential wall shear stress, radial wall displacement, adhesion of white blood cells on the wall, and whole blood velocity magnitude in the six and seven coils cases are compared. These two coiling cases are chosen to represent an event of aneurysm recurrence as unexpected increase in the mean inflow into the aneurysm is observed despite higher coil packing densities. To the best of the author’s knowledge, the present aneurysm study is the first to investigate the effects of coil packing densities and blood cells distribution on non-Newtonian aneurysmal flow reduction and aneurysm recurrence in both rigid and compliant cerebral aneurysms. Additionally, the effects of aneurysmal haemodynamics on coil movement and vice versa are investigated in a study featuring a compliant coil in a rigid aneurysm. Over-estimation in aneurysmal velocity and wall shear stress at multiple locations by the rigid coil model has been observed. An arteriovenous fistula (AVF) is a connection between a brachial artery and vein that is surgically created to provide haemodialysis patients with matured vascular access points. AVF maturation failure, however, often occurs and its underlying mechanisms still remain controversial. The present AVF study investigates the effects of the compliant wall and non-Newtonian blood viscosity in an end-to-end AVF. Four simulations were performed to compare Newtonian and non-Newtonian haemodynamics in both rigid and wall-compliant fistulas. Different ranges of wall shear stress parameters corresponding to certain endothelial changes are compared among the four cases. It is found that the effects of wall compliance is more significant than that of non-Newtonian rheology. Furthermore, non-Newtonian effects are more clear when the AVF walls are compliant. Volumetric quantities like flow recirculations and helicity, which are related to abnormal endothelial changes, are also found to be overestimated by the rigid wall assumption. The study also investigates the effects of multiphase haemodynamics on inward wall remodelling and thus AVF maturation failure. Low and oscillating wall shear stress index is introduced as a tool for predicting the risk of maturation failure. Wall shear stresses, both directional and magnitude, red blood cell viscosity, flow recirculations are correlated with wall remodelling and endothelial damages derived from von-Mises stresses