Predicting the effect on pulse wave reflection of different endovascular repair techniques in abdominal aortic aneurysm using 1D patient-specific models

Machine Learning-Based Pulse Wave Analysis for Early Detection of Abdominal Aortic Aneurysms Using In Silico Pulse Waves

An abdominal aortic aneurysm (AAA) is usually asymptomatic until rupture, which is associated with extremely high mortality. Consequently, the early detection of AAAs is of paramount importance in reducing mortality; however, most AAAs are detected by medical imaging only incidentally. The aim of this study was to investigate the feasibility of machine learning-based pulse wave (PW) analysis for the early detection of AAAs using a database of in silico PWs. PWs in the large systemic arteries were simulated using one-dimensional blood flow modelling. A database of in silico PWs representative of subjects (aged 55, 65 and 75 years) with different AAA sizes was created by varying the AAA-related parameters with major impacts on PWs—identified by parameter sensitivity analysis—in an existing database of in silico PWs representative of subjects without AAAs. Then, a machine learning architecture for AAA detection was trained and tested using the new in silico PW database. The parameter sensitivity analysis revealed that the AAA maximum diameter and stiffness of the large systemic arteries were the dominant AAA-related biophysical properties considerably influencing the PWs. However, AAA detection by PW indexes was compromised by other non-AAA related cardiovascular parameters. The proposed machine learning model produced a sensitivity of 86.8 % and a specificity of 86.3 % in early detection of AAA from the photoplethysmogram PW signal measured in the digital artery with added random noise. The number of false positive and negative results increased with increasing age and decreasing AAA size, respectively. These findings suggest that machine learning-based PW analysis is a promising approach for AAA screening using PW signals acquired by wearable devices.

Endograft Specific Haemodynamics After Endovascular Aneurysm Repair: Flow Characteristics of Four Stent Graft Systems

OBJECTIVES

The implication of haemodynamics in the occurrence of complications after endovascular aneurysm repair (EVAR) has been raised in the literature. Different aortic stent graft configurations may lead to different haemodynamic properties. The current study deals with the post-operative haemodynamic variability between four stent graft systems with different structure, material, and type of fixation.

METHODS

Computed tomography data of 32 patients were used, equally distributed among the four endograft groups, namely the AFX, Endurant, Excluder, and Nellix. Velocity, wall shear stress (WSS), and helicity statistics were calculated, in regions around the flow division where disturbances are expected. The haemodynamic data were compared between and within the groups.

RESULTS

The morphology of AAAs pre-operatively did not vary significantly among the four groups. Before the flow division, lowest velocity was observed in Endurant cases and highest in Nellix cases. Endurant induced the lowest peak WSS and Nellix the highest (p = .03). The helicity levels were low in AFX and Nellix cases and high in Endurant and Excluder cases. After the flow division, the trend in the results was preserved. Nellix induced the highest velocity and WSS, followed closely by Excluder and AFX. There was a significant increase of helicity before and after flow division in AFX (p <0.001, R² = 0.09) and Nellix (p <0.001) cases.

CONCLUSIONS

It has been shown that different types of endografts induce variable haemodynamic conditions around the flow division. The parallel limb structure, featured by Nellix, seems to induce favourable flow conditions in terms of velocity and WSS, while helical flow before the flow division is suppressed. High WSS is generally considered to be a desirable flow characteristic in endovascular devices, whereas helicity extremes (very low or high) are potentially a negative sign. Endurant, with the stiffer material and the short neck structure, was associated with the lowest blood velocity and WSS values but preserved high helicity levels. The AFX and Excluder, which include the same material, induced similar haemodynamic conditions.

Effects on Aortoiliac Fluid Dynamics After Endovascular Sealing of Abdominal Aneurysms

OBJECTIVES:

To evaluate the effects on aortoiliac fluid dynamics after the implantation of an endograft based on endovascular aneurysm sealing (EVAS) versus endovascular aneurysm repair (EVAR) strategy.

METHODS:

An adaptive geometrical deformable model was used for aortic lumen segmentation in 8 patients before and after the surgery. Abdominal aneurysms were treated with an endograft based on the EVAS system (Nellix, n = 4) and with a device based on an anatomical fixation technology (n = 4). Pressure, blood velocity, and wall shear stress (WSS) were estimated at different aortic regions using computational fluid dynamics methods. Physiologic inlet/outlet flow values at the abdominal aorta, the celiac trunk, and the mesenteric and the renal arteries were set. Pressure references were set at iliac arteries outlet.

RESULTS:

Maximum aneurysm sizes were similar for both groups in the preoperative scans. The lumen area was lower after EVAR ( P < .05) and EVAS ( P < .01) compared to preoperative aortic lumen sizes. Pressure increase was higher in the proximal abdominal aorta after EVAS compared to EVAR (2.3 ± 0.3 mm Hg vs 0.9 ± 0.3 mm Hg, P < .001). Peak blood velocities inside the endografts were 3-fold higher for EVAS compared to EVAR (54 ± 5 cm/s vs 17 ± 4 cm/s, P < .01). Velocities at the iliac arteries also remained higher for EVAS (38 ± 4 cm/s vs 24 ± 4 cm/s, P < .05). Peak WSS at the iliac arteries remained higher for EVAS compared to EVAR group ( P < .05).

CONCLUSION:

The significant modification of the aortic bifurcation anatomy after EVAS alters aortoiliac fluid dynamics, showing a pressure impact at the renal arteries level and an acceleration of the blood velocity at the iliac region with a concomitant increase in peak WSS.

Waves and fluid-solid interaction in stented blood vessels

This paper focuses on the modelling of fluid–structure interaction and wave propagation problems in a stented artery. Reflection of waves in blood vessels is well documented in the literature, but it has always been linked to a strong variation in geometry, such as the branching of vessels. The aim of this work is to detect the possibility of wave reflection in a stented artery due to the repetitive pattern of the stents. The investigation of wave propagation and possible blockages under time-harmonic conditions is complemented with numerical simulations in the transient regime.

Deformation and dynamic response of abdominal aortic aneurysm sealing

Endovascular sealing is a new technique for the repair of abdominal aortic aneurysms. Commercially available in Europe since 2013, it takes a revolutionary approach to aneurysm repair through minimally invasive techniques. Although aneurysm sealing may be thought as more stable than conventional endovascular stent graft repairs, post-implantation movement of the endoprosthesis has been described, potentially leading to late complications. The paper presents for the first time a model, which explains the nature of forces, in static and dynamic regimes, acting on sealed abdominal aortic aneurysms, with references to real case studies. It is shown that elastic deformation of the aorta and of the endoprosthesis induced by static forces and vibrations during daily activities can potentially promote undesired movements of the endovascular sealing structure.