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.

Objectives: This study evaluated the safety and hospital impact of transition from a routine to a selective policy of postoperative transfer to the intensive care unit (ICU) for elective open abdominal aortic aneurysm (AAA) repair.Methods: This retrospective study included all open elective AAA repairs from August 8 2010, to December 1, 2014, performed in our center. The study was approved by the Institutional Review Board, and informed consent was waived. Patients were identified through our prospective database, and electronic records were reviewed to extract patient characteristics, operative details, and postoperative complications. Patients operated on before March 13, 2012, were routinely sent to the ICU after operation (group A). Patients treated after this date were sent directly to an intermediate care unit with a ratio of one nurse for three patients (group B), unless otherwise determined preoperatively by the surgeon or after intraoperative complications. We evaluated the safety of our change in practice, looking at complications and mortality rate, length of stay, and transfer from an intermediate care unit to the ICU.Results: The study included 310 patients (266 men, 44 women), with a mean age 70 of years, and a mean AAA diameter 65 mm. Group A and B included 118 and 192 patients, respectively. The postoperative mortality rate was similar in each group (1%). ICU admission in group B was spared in 78% (149 of 192) of patients. Only two patients (1%) from the intermediate care unit were subsequently admitted to the ICU. There was no increase in mortality in group B (0.5%) compared with group A (0.8%) during hospital stay. Hospital lengths of stay were similar between groups group A (8.6 days) and group B (8.0 days; P ¼ NS).Conclusions: Our results confirm the safety of a selective ICU pathway after open elective AAA repair, with most patients sent directly to an intermediate care unit.

Direct numerical simulations were performed on four patient-specific abdominal aortic aneurysm (AAA) geometries and the resulting pulsatile blood flow dynamics were compared to aneurysm shape and correlated with intraluminal thrombus (ILT) deposition. For three of the cases, turbulent vortex structures impinged/sheared along the anterior wall and along the posterior wall a zone of recirculating blood formed. Within the impingement region the AAA wall was devoid of ILT and remote to this region there was an accumulation of ILT. The high wall shear stress (WSS) caused by the impact of vortexes is thought to prevent the attachment of ILT. WSS from impingement is comparable to peak-systolic WSS in a normal-sized aorta and therefore may not damage the wall. Expansion occurred to a greater extent in the direction of jet impingement and the wall-normal force from the continuous impact of vortexes may contribute to expansion. It was shown that the impingement region has low oscillatory shear index (OSI) and recirculation zones can have either low or high OSI. No correlation could be identified between OSI and ILT deposition since different flow dynamics can have similar OSI values.

Objective We have previously demonstrated that human abdominal aortic aneurysm (AAA) rupture occurs in zones of low wall shear stress where flow recirculation and intraluminal thrombus (ILT) deposition are increased. Matrix metalloproteinase-9 (MMP-9) is involved in the pathogenesis of AAA via its lytic effect on collagen and elastin. We hypothesize that flow-mediated ILT deposition promotes increased local inflammatory and MMP-9 activity that leads to AAA wall degeneration. The purpose of this study was to examine the correlation between predicted pulsatile flow dynamics and regional differences in MMP-9, elastin, collagen, and ILT deposition in human AAA. Methods Full-thickness aortic tissue samples were collected from 24 patients undergoing open AAA repair. Control infrarenal aortic tissue was obtained from 6 patients undergoing aortobifemoral bypass. Full-thickness aortic tissue and ILT were assessed for MMP-9 levels using a cytokine array assay. Histologic and immunohistochemical assessment of inflammation, collagen and elastin content, and MMP-9 levels were also measured. Three-dimensional AAA geometry was generated from computed tomography angiogram (CTA) images using Mimics software and computational fluid dynamics was used to predict pulsatile aortic blood flow. Results The majority of AAA showed eccentric ILT deposition which was correlated with predicted recirculation blood flow (R 2 = –0.17; P < .05). The regions of high ILT were associated with significant increases in inflammation and loss of elastin and collagen compared with regions of low ILT, or with control tissue. MMP-9 was significantly higher in areas of high ILT deposition compared with areas devoid of ILT. Tissue MMP-9 was correlated with the thickness of ILT deposition (R 2 = 0.46; P < .05), and was also present in high levels in thick compared with thin ILT. Conclusions We have shown a correlation between flow-mediated ILT deposition with increased tissue levels of MMP-9 activity, increased inflammatory infiltrate, and decreased elastin and collagen content in stereotactically sampled human AAA, suggesting that ILT deposition is associated with local increases in proteolytic activity that may preferentially weaken and promote rupture at selected regions.

Background: Abdominal aortic aneurysm (AAA) rupture has an associated mortality of 90%. AAAs are repaired when they meet size criteria, become symptomatic, or rupture. Use of aortic diameter as the primary criterion in the decision to intervene fails to take into consideration that AAAs rupture at sizes below operative thresholds or reach extreme size without rupture. We have previously shown that AAAs rupture at sites of low wall shear stress (WSS) where flow recirculation and intraluminal thrombus (ILT) tend to be more abundant. This study examined the fate of ILT deposition in AAA growth. We hypothesized that AAA expansion would be associated with increasing ILT deposition in sites of flow recirculation. Methods: A total of six patients with serial images of AAA growth over three time points were studied. Aortic measurements and sites of ILT deposition were recorded. Three-dimensional AAA geometry was generated from computed tomography angiography images. Predicted aortic blood flow velocity, localized pressure variation, and WSS profiles were correlated with AAA growth and ILT deposition. This study was carried out with biomedical ethics approval. Results: AAA growth was associated with increasing ILT deposition in most cases. The site of maximal ILT deposition strongly correlated with regions of flow recirculation and low WSS but did not correlate with the region of maximal aortic expansion. Interestingly, in some cases, the recirculation zone changed location with AAA growth, and this was associated with similar change in location of ILT deposition. Conclusions: This study has shown that ILT increases with increasing AAA size in most aneurysms and that deposition of ILT occurs at sites with low WSS and flow recirculation. An understanding of the alterations in WSS in pulsatile flow and its effect on vascular endothelium will lead to a better understanding of AAA development and growth and may ultimately lead to better prediction of AAA rupture potential.