Basic research study
Low wall shear stress predominates at sites of abdominal aortic aneurysm rupture

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Objective

Aortic diameter as the primary criterion in the decision to repair abdominal aortic aneurysms (AAAs) has drawbacks as some rupture below size thresholds, whereas others reach extreme size without rupture. Predictions of static aortic wall stress have also failed to reliably predict rupture potential. The objective of this study was to computationally assess blood flow characteristics at the site of infrarenal AAA rupture. On the basis of the finite element literature correlating rupture location with high static local wall stress, we hypothesized that a computational fluid dynamics approach would also demonstrate rupture at regions of high pressure and wall shear stress (WSS).

Methods

Three-dimensional AAA geometry was generated from computed tomography angiography images of seven ruptured AAAs. Aortic blood flow velocity, pressure, and WSS were computationally determined. Flow characteristics at the site of rupture were determined and compared across all cases.

Results

AAA size at the time of rupture was 8.3 ± 0.9 cm. Only three of the seven AAAs ruptured at the site of maximal diameter. Blood flow velocity in the aneurysmal aorta showed dominant flow channels with zones of recirculation, where low WSS predominated. Regardless of aneurysm size or configuration, rupture occurred in or near these flow recirculation zones in all cases. WSS was significantly lower and thrombus deposition was more abundant at the site of rupture.

Conclusions

This computational study was the first to assess blood flow characteristics at the site of infrarenal AAA rupture in realistic aortic geometries. In contradiction to our initial hypothesis, rupture occurred not at sites of high pressure and WSS but rather at regions of predicted flow recirculation, where low WSS and thrombus deposition predominated. These findings raise the possibility that this flow pattern may lead to thrombus deposition, which may elaborate adventitial degeneration and eventual AAA rupture.

Clinical Relevance

Aortic diameter as the primary criterion in the decision to repair abdominal aortic aneurysms (AAAs) has drawbacks as some AAAs rupture below size thresholds, whereas others grow to large sizes without rupture. Computational fluid dynamics was used to model blood flow in ruptured human AAAs. Rupture occurred in regions of low wall shear stress, where flow recirculation and thrombus deposition predominated, suggesting that prolonged interaction of circulating factors with the vascular endothelium may lead to wall degeneration. An understanding of the interplay between wall shear stress and vascular remodeling will ultimately lead to a better understanding of AAA development, growth, and rupture potential.

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Author conflict of interest: none.

The editors and reviewers of this article have no relevant financial relationships to disclose per the JVS policy that requires reviewers to decline review of any manuscript for which they may have a conflict of interest.