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Aortic Dissection

For a compete overview of the state-of-the-art in aortic dissection, as well as some new data from VascLab - please see our recent paper in the Annals of Biomedical Engineering.

An aortic dissection occurs when a tear on the intimal surface allows blood to enter the media of the aortic wall and propagate to form a false lumen (see image below-right). Dissection is one of three conditions of acute aortic syndrome which also includes penetrating aortic ulcers and intramural haematomas. How these three conditions interact is currently a source of research, with no clear pathways discovered.

The majority of aortic dissections occur in the upper region of the (thoracic) aorta. Dissections can be classified as either Stanford Type A, if they involve the ascending aorta or Stanford Type B if they occur distal to the origin of the left subclavian artery. Incidence rates vary widely, most likely due to misdiagnosis. Recent data from the UK suggests a rise in aortic dissections and data from Australia shows about a 25% increase over the last decade. This is in stark contrast to most other types of CVD (including AAA) that show falling rates.  

Aortic dissection is notoriously tricky to diagnose and medical imaging is the only conclusive method – often taking many hours of imaging and tests. Of paramount importance is that the rate of death increases by 1-3% for every hour that passes undiagnosed. Even after diagnosis, up to 20% of patients will die either during or immediately after surgery. If untreated, the risk of death is about 25% in the first 24hrs, 50% in the first 48hrs and 75% in the first week. Very few patients will survive longer than 1 month with an undiagnosed dissection. In the most recent guidelines from the ACC/AHA, it is acknowledged that the collaboration of multiple specialities may help with diagnosis and identification of those at high-risk. Furthermore, medical imaging is expensive and unfortunately not always covered by insurance. Therefore, if imaging is to be performed, it is important to utilise the datasets.

This is where patient-specific modelling (PSM) can help.

Vascular mechanics plays an obvious role in aortic dissection. The structural integrity of the layered aortic wall influences both the location of the tear (often called the entry tear) and also the propagation of the tear. Regions experiencing local instabilities of the aortic wall that may also be experiencing increased wall stress are regions that may fail – resulting in a tear. It is not uncommon for there to be multiple-tears along the aorta like in the image on the left. Using the finite element method we can compute the tissue stress in these complex geometries (see image below). Additionally, using follow-up image data we can see if computer models predict which geometries are likely to dissect further.

We can also use computational fluid dynamics (CFD) to study way the blood is flowing through the aorta and passing into the false lumen. This also reveals the pressure loading on the aortic wall due to the pumping blood. These extremely complex geometries represent very challenging and interesting CFD simulations.