Predicting the Outcome of TEVAR for Aortic Dissection

Lead Research Organisation: Imperial College London
Department Name: Department of Chemical Engineering

Abstract

Aortic dissection is a potentially life threatening disease that begins with an initial tear in the inner most layer of the aortic wall, the intima. When blood passes through this tear the layers of the aortic wall separate creating a secondary, unwanted, channel of blood, known as the false lumen. Secondary and/or reentry tears can form in the distal thoracic aorta, abdominal aorta or the iliac arteries below the bifurcation. The reasoning for why a dissection occurs is not completely understood, however the most common risk factors are uncontrolled hypertension and atherosclerosis. Dissection can also occur due to predisposing factors (Salameh and Ratchford, 2016). As the aorta is the primary vessel for supplying oxygenated blood to the body a fault in the system can lead to a range of complications, including ischemia (and ultimately major organ failure), aortic aneurysm and rupture. The severity of aortic dissection and the resulting complications leads to a high mortality rate. Stanford Type A dissection has a mortality rate of 1% per hour from onset with a rate of 50% by day 3, while Stanford Type B has a lower, but still significant, 30-day rate of 10% (and up to 70% for the highest-risk groups) (Frank J. Criado, 2011). Typically, a Type A dissection requires immediate surgical treatment, while Type B dissections are normally treated using either endovascular or medical therapies, with the decision for which dependent on other complications the patient may be exhibiting (Salameh and Ratchford, 2016).
Given the importance of thrombosis to patient prognosis, it is highly desirable to be able to predict the thrombosis process when treatment is administered. A mathematical thermodynamics-based model (Menichini and Xu, 2016) was developed in idealized models to predict the thrombosis process in Type B dissections. It has since been applied to patient specific geometries to investigate the accuracy of the model (Menichini et al., 2016), and produced promising results with model outputs being comparable to in vivo data collected from the patients. Looking at the second year and onwards of the current PhD, one of the objectives will be to critically analyse this model. This will include carrying out an in-depth sensitivity analysis of the assumptions made, which may lead to the expansion of the model - for example, one desirable factor to investigate and potentially include is fluid-structure interactions to simulate the non-rigid wall of the aorta. There have also been numerous kinetic-based models produced, incorporating the complex biological and chemical mechanisms involved in thrombosis formation (Anand et al., 2006; Biasetti et al., 2012; ?; Tolenaar et al., 2014; Filipovic et al., 2008; Leiderman and Fogelson, 2011). An in-depth review of these models and comparison to the previously discussed hemodynamic model will be conducted to determine key differences in results produced by all models.

Publications

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Armour CH (2020) Location of Reentry Tears Affects False Lumen Thrombosis in Aortic Dissection Following TEVAR. in Journal of endovascular therapy : an official journal of the International Society of Endovascular Specialists

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 30/09/2021
1966212 Studentship EP/N509486/1 01/10/2017 30/09/2021 Chloe Harriet Armour
 
Description Type B aortic dissection is a life threating disease of the aorta in which a tear (or multiple tears) forms in the aortic wall, creating two separate channels of blood flow - the true and false lumen. It can be treated in two different ways with many different potential outcomes, however the primary objective is to achieve complete thrombosis of the false lumen. One method of treatment is thoracic endovascular aortic repair (TEVAR), in which a stent graft is inserted into the dissected aorta to cover the entry tear to stop blood flow into the false lumen. The overall aim of this award is to study various stages of type B aortic dissection using computational fluid dynamics (CFD), in a hope to understand reasons for late complications and explore ways to improve outcomes. The first objective focused on assessing the influence of the location of additional re-entry tears relative to the deployed stent in TEVAR. The study found that the greater the distance between the tear and the stent, the greater the amount of false lumen thrombosis (FLT) occurred. The results showed that in some cases a longer stent graft can be deployed to improve FLT.
The second objective was to assess the influence of additional re-entry tears in type B dissections, as the vast majority of patients present with more than one tear. This was done through a controlled animal study, in collaboration with the Department of Vascular Surgery at Zhongshan Hospital, Shanghai. This study allowed us to acquire a complete set of comprehensive data on the progression of dissection at multiple stages with varying number of tears. The study showed the strong influence of additional tears on flow distribution and pressure throughout the aorta - the pressure difference between the true and false lumen reduced as the number of tears increased. A potential scenario which can occur either pre- or post-TEVAR is a high pressure false lumen - a very undesirable situation as it would likely lead to expansion of the false lumen, and ultimately aortic rupture. The results of this study showed the potential benefits of re-intervention to create additional tears to stabilise the condition.
The use of CFD to study aortic dissection is a growing field, with the potential to provide detailed insight into the disease on which doctors can make important treatment decisions. To ensure the results are accurate and reliable, the third objective of this award focused on assessing the influence of inlet boundary condition (which is set to mimic the blood flow pumping from the heart) on type B dissection simulations. Several patient-specific and generic inlet profiles were tested, and the study showed the importance of certain patient-specific details, including the need for a patient-specific stroke volume and flow waveform to accurately model the velocity and pressure, respectively, throughout the aorta.
Exploitation Route Results from the first objective described above have been accepted for publication in the Journal of Endovascular Therapy which is an official publication of the International Society of Endovascular Specialists. Results from the second objective are being prepared for submission to a high impact journal in the field of cardiothoracic surgery and interventions. These publications will stimulate more relevant research and assist doctors in adjusting and refining treatment decisions. For the biomechanics and biomedical engineering research community, the methodology developed and validated in this study can be replicated to simulate and analyse a vast range of scenarios possible with type B dissection. The computational methods are particularly beneficial to computational hemodynamics researchers by providing useful guidelines for patient-specific analysis of type B dissections.
Sectors Healthcare,Pharmaceuticals and Medical Biotechnology