Endovascular repair of the aneurysm of the abdominal aorta by Chimney technique: a focus on the hemodynamics

Abdominal aortic aneurysm (AAA) is a pathological, localized and permanent dilatation of abdominal aorta that leads to vascular wall weakening due to the loss of mechanical properties caused by collagen and elastin degradation. If vessel’s diameter dilatation is larger than 3 centimeters the patient is subjected to a surveillance protocol and if needed he has to be treated to prevent aneurysm rupture and patient’s death. AAA can be treated by two techniques: open surgical repair (OSR), performed by an incision in the patient abdominal area, and endovascular repair (EVAR), performed by two incision in the groin area. The aim of the techniques is to exclude the aneurysm from the blood flow. EVAR has many advantages: (1) it is minimally invasive; (2) does not require total anesthesia; (3) recovery times are short and (4) presents a low mortality and morbidity rate. Some patients are not suitable for this technique due to their complex anatomical features; thrombus formation risk and device migration do not guarantee a long term stability of the device. In this study we focus on patients treated by Chimney EVAR (ChEVAR), it is a kind of EVAR that involves the insertion of a main channel within secondary stent graft are located to supply the renal and mesenteric arteries. The aim of the work is to analyze two pre and post operative patient-specific models treated with ChEVAR and study their hemodynamic behavior using Computational Fluid Dynamic (CFD). Starting from patient-specific Computer Angiography Tomography (CTA) scan images a 3D model is segmented by the software Materialise Mimics, then a computational grid is carried out in ICEM CFD. The fluid dynamic simulations are carried out by the commercial software ANSYS Fluent. Simulations have been performed imposing a velocity profile in inlet section and two different outlet pressure conditions: a pressure profile and a zero pressure condition. On each model near-wall hemodynamic descriptors are analyzed such as: Time Averaged Wall Shear Stress (TAWSS); Oscillatory Shear index (OSI) and Relative Residence Time (RRT); moreover pressure and Wall Shear Stress (WSS) related to significant instants of the cardiac cycle are investigated. The trend of hemodynamics descriptors is different from pre- to post-operative simulations: both for the two boundary outlet conditions TAWSS increases in post operative situations encouraging the presence of direct flow towards the distal position. The area that undergoes the greatest transformation regarding OSI is the aneurysmal area where post operatively OSI decreases. The results referred to Relative Residence Time are in line with those observed in OSI, therefore RRT decreases especially in the aneurysmal sac from pre-operative to post-operative. Analyzing the instantaneous quantities different situations are revealed: pre-operatively, considering a zero pressure in outlets, the pressure decreases between systole and diastole. In the post operative case the pressure decreases from proximal to distal area restoring the physiological pressure trend. In the case of outlet pressure profile, the pressure does not reach values close to zero, it remains elevated. WSS after the operation positively increases reducing the risk of thrombus formation. ChEVAR reported positive results in the treatment of AAA. Future works may include a larger number of patients, other parameters could be investigated to study the risk of migration and thrombus formation.