Patient-specific computational flow modelling for assessing hemodynamic changes following fenestrated endovascular aneurysm repair

Advanced Imaging Techniques for Complex Endovascular Aortic Repair: Preoperative, Intraoperative and Postoperative Advancements

BACKGROUND

Endovascular aortic repair (EVAR) requires extensive preoperative, intraoperative, and postoperative imaging for planning, surveillance, and detection of endo-leaks. There have been manyadvancements in imaging modalities to achieve this purpose. This review discussed different imaging modalities used at different stages of treatment of complex EVAR.

METHODS

We conducted a literature review of all the imaging modalities utilized in EVAR by searching various databases.

RESULTS

Preoperative techniques include analysis of images obtained via modified central line using analysis software and intravascular ultrasound. Fusion imaging (FI), carbon dioxide (CO2) angiography, intravascular ultrasound, and Fiber Optic RealShape (FORS) technology have been crucial in obtaining real-time imaging for the detection of endo-leaks during operative procedures. Conventional imaging modalities like computed tomography (CT) angiography (CTA) and magnetic resonance (MR) angiography are still employed for postoperative surveillance along with computational fluid dynamics and contrast-enhanced ultrasound (CEUS). The advancements in artificial intelligence (AI) have been the breakthrough in developing robust imaging applications.

CONCLUSIONS

This review explains the advantages, disadvantages, and side-effect profile of the abovementioned imaging modalities.

Changes in renal-mesenteric duplex ultrasound velocities after fenestrated and branched endovascular aortic aneurysm repair

OBJECTIVE

Stenting of renal and mesenteric vessels may result in changes in velocity measurements due to arterial compliance, potentially giving rise to confusion about the presence of stenosis during follow-up. The aim of our study was to compare preoperative and postoperative changes in peak systolic velocity (PSV, cm/s) after placement of the celiac axis (CA), superior mesenteric artery (SMA) and renal artery (RAs) bridging stent grafts during fenestrated-branched endovascular aortic repair (FB-EVAR) for treatment of complex abdominal aortic aneurysms (AAA) and thoracoabdominal aortic aneurysms.

METHODS

Patients were enrolled in a prospective, nonrandomized single-center study to evaluate FB-EVAR for treatment of complex AAA and thoracoabdominal aortic aneurysms between 2013 and 2020. Duplex ultrasound examination of renal-mesenteric vessels were obtained prospectively preoperatively and at 6 to 8 weeks after the procedure. Duplex ultrasound examination was performed by a single vascular laboratory team using a predefined protocol including PSV measurements obtained with <60° angles. All renal-mesenteric vessels incorporated by bridging stent grafts using fenestrations or directional branches were analyzed. Target vessels with significant stenosis in the preoperative exam were excluded from the analysis. The end point was variations in PSV poststent placement at the origin, proximal, and mid segments of the target vessels for fenestrations and branches.

RESULTS

There were 419 patients (292 male; mean age, 74 ± 8 years) treated by FB-EVAR with 1411 renal-mesenteric targeted vessels, including 260 CAs, 409 SMAs, and 742 RAs. No significant variances in the mean PSVs of all segments of the CA, SMA, and RAs at 6 to 8 weeks after surgery were found as compared with the preoperative values (CA, 135 cm/s vs 141 cm/s [P = .06]; SMA, 128 cm/s vs 125 cm/s [P = .62]; RAs, 90 cm/s vs 83 cm/s [P = .65]). Compared with baseline preoperative values, the PSV of the targeted vessels showed no significant differences in the origin and proximal segment of all vessels. However, the PSV increased significantly in the mid segment of all target vessels after stent placement.

CONCLUSIONS

Stent placement in nonstenotic renal and mesenteric vessels during FB-EVAR is not associated with a significant increase in PSVs at the origin and proximal segments of the target vessels. Although there is a modest but significant increase in velocity measurements in the mid segment of the stented vessel, this difference is not clinically significant. Furthermore, PSVs in stented renal and mesenteric arteries were well below the threshold for significant stenosis in native vessels. These values provide a baseline or benchmark for expected PSVs after renal-mesenteric stenting during FB-EVAR.

Convergence of Phase-Averaged, Transitional Flow in an Abdominal Aortic Aneurysmal Model

Abdominal aortic aneurysm can exhibit transitional flow characteristics in laminar flow regimes. To report transitional flow characteristics, we examined the convergence of phase-averaged solutions by executing blood flow simulations of a patient-specific abdominal aortic aneurysmal model for 257 cardiac cycles with periodic, pulsatile boundary conditions. The phase-averaged solutions were computed by averaging the solutions over various numbers of cardiac cycles and compared against the ones averaged over 124 cycles. The phase-averaged solutions reported small differences when they were averaged over a large number of cardiac cycles. The instantaneous solutions, however, failed to exhibit fluctuations reported in the phase-averaged solutions. To study transitional blood flows in the aneurysmal region, we need to report phase-averaged solutions as they exhibit nonperiodic, disturbed flow characteristics. Additionally, when reporting phase-averaged solutions, it is preferred to compute an average over a large number of cardiac cycles to be able to represent flow structures of the converged phase-averaged solutions.

A systematic review of cardiac in-silico clinical trials

Computational models of the heart are now being used to assess the effectiveness and feasibility of interventions through in-silico clinical trials (ISCTs). As the adoption and acceptance of ISCTs increases, best practices for reporting the methodology and analysing the results will emerge. Focusing in the area of cardiology, we aim to evaluate the types of ISCTs, their analysis methods and their reporting standards. To this end, we conducted a systematic review of cardiac ISCTs over the period of 1 January 2012-1 January 2022, following the preferred reporting items for systematic reviews and meta-analysis (PRISMA). We considered cardiac ISCTs of human patient cohorts, and excluded studies of single individuals and those in which models were used to guide a procedure without comparing against a control group. We identified 36 publications that described cardiac ISCTs, with most of the studies coming from the US and the UK. In 75% of the studies, a validation step was performed, although the specific type of validation varied between the studies. ANSYS FLUENT was the most commonly used software in 19% of ISCTs. The specific software used was not reported in 14% of the studies. Unlike clinical trials, we found a lack of consistent reporting of patient demographics, with 28% of the studies not reporting them. Uncertainty quantification was limited, with sensitivity analysis performed in only 19% of the studies. In 97% of the ISCTs, no link was provided to provide easy access to the data or models used in the study. There was no consistent naming of study types with a wide range of studies that could potentially be considered ISCTs. There is a clear need for community agreement on minimal reporting standards on patient demographics, accepted standards for ISCT cohort quality control, uncertainty quantification, and increased model and data sharing.

Near-wall hemodynamic changes in subclavian artery perfusion induced by retrograde inner branched thoracic endograft implantation

Objective

Left subclavian artery (LSA)-branched endografts with retrograde inner branch configuration (thoracic branch endoprosthesis [TBE]) offer a complete endovascular solution when LSA preservation is required during zone 2 thoracic endovascular aortic repair. However, the hemodynamic consequences of the TBE have not been well-investigated. We compared near-wall hemodynamic parameters before and after the TBE implantation using computational fluid dynamic simulations.

Methods

Eleven patients who had undergone TBE implantation were included. Three-dimensional aortic arch geometries were constructed from the pre- and post-TBE implantation computed tomography images. The resulting 22 three-dimensional aortic arch geometries were then discretized into finite element meshes for computational fluid dynamic simulations. Inflow boundary conditions were prescribed using normal physiological pulsatile circulation. Outlet boundary conditions consisted of Windkessel models with previously published values. Blood flow, modeled as Newtonian fluid, simulations were performed with rigid wall assumptions using SimVascular's incompressible Navier-Stokes solver. We compared well-established hemodynamic descriptors: pressure, flow rate, time-averaged wall shear stress (TAWSS), the oscillatory shear index (OSI), and percent area with an OSI of >0.2. Data were presented on the stented portion of the LSA.

Results

TBE implantation was associated with a small decrease in peak LSA pressure (153 mm Hg; interquartile range [IQR], 151-154 mm Hg vs 159 mm Hg; IQR, 158-160 mm Hg; P = .005). No difference was observed in peak LSA flow rates before and after implantation: 40.4 cm³/ (IQR, 39.5-41.6 cm³/s) vs 41.3 cm³/s (IQR, 37.2-44.8 cm³/s; P = .59). There was a significant postimplantation increase in TAWSS (15.2 dynes/cm² [IQR, 12.2-17.7 dynes/cm²] vs 6.2 dynes/cm² [IQR, 5.7-10.3 dynes/cm²]; P = .003), leading to decreases in both the OSI (0.088 [IQR, 0.063 to -0.099] vs 0.1 [IQR, 0.096-0.16]; P = .03) and percentage of area with an OSI of >0.2 (10.4 [IQR, 5.8-15.8] vs 15.7 [IQR, 10.7-31.9]; P = .13). Neither LSA side branch angulation (median, 81°, IQR, 77°-109°) nor moderate compression (16%-58%) seemed to have an impact on the pressure, flow rate, TAWSS, or percentage of area with an OSI of >0.2 in the stented LSA.

Conclusions

The implantation of TBE produces modest hemodynamic disturbances that are unlikely to result in clinically relevant changes.

Patient-specific modeling of hemodynamic characteristics associated with the formation of visceral artery aneurysms at uncommon locations

Objective

Hemodynamic characteristics play critical roles in aneurysm initiation and growth. This study aims to explore the effect of common hemodynamic parameters on the formation of visceral artery aneurysms (VAAs), especially those from the pancreaticoduodenal arteries or other uncommon locations, using real patients' models.

Methods

Three-dimension vessel models of 14 VAAs from 13 patients were selected and constructed from computed tomography angiography (CTA) images. Aneurysms were manually removed to perform computational fluid dynamics (CFD) simulations of the models before aneurysm formation. Flow field characteristics were obtained and compared at the aneurysm forming and para-aneurysm areas. Aneurysm forming models were categorized into high-wall-shear stress (WSS) and low-WSS groups according to WSS value at aneurysm forming versus para-aneurysm areas.

Results

Computational fluid dynamics analysis revealed that the high WSS group had significantly higher WSSmax (P = 0.038), higher time average WSS (TAWSS) (P = 0.011), higher WSS gradient (WSSG) (p = 0.036), as well as lower oscillatory shear index (OSI) (P = 0.022) compared to the low WSS group. Significant higher WSSmax (P = 0.003), TAWSS (P = 0.003), WSSG (P = 0.041) and lower OSI (P = 0.021) was observed at the aneurysm forming site compared to both upstream and downstream areas.

Conclusion

Both local increase and decrease of WSS and WSS gradient were observed for the visceral artery aneurysm forming area. Computational fluid dynamics analysis could shed light on the pathogenesis of visceral artery aneurysms at uncommon vessel locations.

Large Fenestrations Versus Scallops for the SMA During Fenestrated EVAR: Does it Matter?

OBJECTIVE

FEVAR is an established customized treatment for aortic aneurysms with three current commercially available configurations for the superior mesenteric artery (SMA) - a single-wide scallop, large fenestration, or small fenestration, with the scallop or large fenestration most utilized. Outcomes comparing SMA single-wide scallops to large fenestrations with the ZFEN device are scarce. As large fenestrations have the benefit of extending the proximal seal zone compared to scalloped configurations, we sought to determine the differences in seal zone and sac regression outcomes between the two SMA configurations.

METHODS

We retrospectively reviewed our prospectively maintained complex EVAR database and included all patients treated with the Cook ZFEN device with an SMA scallop or large fenestration configuration at its most proximal build. All first post-operative CT scans (1-30 days) were analyzed on TeraRecon to determine precise proximal seal zone lengths, and standard follow-up anatomic and clinical metrics were tabulated.

RESULTS

A total of 234 consecutive ZFEN patients from 2012-2021 were reviewed, and 137 had either a scallop or large fenestration for the SMA as the proximal-most configuration (72 scallops and 65 large fenestrations) with imaging available for analysis. Mean follow-up was 35 months. Mean proximal seal zone length was 19.5±7.9 mm for scallop vs 41.7±14.4 mm for large fenestration groups (P<.001). There was no difference in sac regression between scallop and large fenestration at one year (10.1±10.9 mm vs 11.0±12.1, P = 0.63). Overall, 30-day mortality (1.3% vs 2.5%, P=.51) and all-cause three-year mortality (72.5% vs 81.7%, P=.77) were not significantly different. Reinterventions within 30 days were primarily secondary to renal artery branch occlusions, with only one patient in the scallop group requiring reintervention for an SMA branch occlusion.

CONCLUSIONS

Despite attaining longer proximal seal lengths, large SMA fenestrations were not associated with a difference in sac regression compared to scalloped SMA configurations at one-year follow up. There were no significant differences in reinterventions or overall long-term survival between the two SMA strategies.

Reintervention Does Not Impact Long-term Survival After Fenestrated Endovascular Aneurysm Repair

OBJECTIVES

Fenestrated endovascular aneurysm repair (FEVAR) is increasingly used in the treatment of juxtarenal aortic aneurysms and short-neck infrarenal aneurysms. Reinterventions (REIs) occur frequently, contributing to patient morbidity and resource utilization. We sought to determine if REIs impact long-term survival after FEVAR.

METHODS

A single-institution retrospective review of all Cook ZFEN repairs was performed. Patients with ≥6 months follow-up and without adjunctive branch modifications were included. REI was defined as any aneurysm, device, target branch, or access-related intervention after the index procedure. REIs were categorized by early (<30 days) or late (≥30 days), indication (branch, endoleak, limb, access-related, or other), and target branch/device component. Patients were stratified into REI vs No REI and Branch REI vs Non-Branch REI.

RESULTS

Of 219 consecutive ZFEN from 2012-2021, 158 patients met inclusion criteria. Forty-one (26%) patients underwent a total of 51 REIs (10 early, 41 late) over a mean follow-up of 33.9 months. The most common indication for REI was branch-related 61% (31/51), with the renal arteries most frequently affected 51% (26/51). The only differences found in baseline, aneurysm, or device characteristics were a higher mean SVS comorbidity score (9.6 vs 7.9, P=.04) and larger suprarenal neck angle (23.3 vs 17.1 degrees, P=.04) in No REI, while REI had larger mean proximal seal zone diameter (26.3 vs 25.1 mm, P=.03) and device diameter (31.9 vs 30.0 mm, P=.002) than No REI. Technical success and operative characteristics were similar between groups, except for longer mean fluoroscopy time (74.9 vs 60.8 min, P=.01) and longer median length of stay (2 vs 2 days, P=.006) in REI. While the rate of early major adverse events (<30 days) was higher in REI (24.4% vs 6.0%, P=.001), 30-day mortality was not statistically different (4.9% vs 0.9%, P=.10). On Kaplan-Meier analysis, freedom from REI at 1- and 5-years was 85.7% and 62.6%, respectively, in the overall cohort. There was no difference in estimated 5-year survival between REI and No REI (62.8% vs 63.5%, log-rank P=.87) and Branch REI and Non-Branch REI (71.8% vs 49.9%, log-rank P=.16). In multivariate analysis, REI did not predict mortality; age, the SVS comorbidity score, and preoperative maximum aneurysm diameter each increased the hazard of death (HR 1.07 95% CI 1.02-1.12, P=.007; HR 1.10, 95% CI 1.01-1.18, P=.02; HR 1.05, 95% CI 1.02-1.08, P=.003 respectively).

CONCLUSIONS

Following ZFEN, 26% of patients required a total of 51 REIs with most occurring ≥30 days and 61% being branch-related, with no influence on 5-year survival. Age, comorbidity, and baseline aneurysm diameter independently predicted mortality. FEVAR mandates lifelong surveillance and protocols to maintain branch patency. Despite their relative frequency, REIs do not influence 5-year post-procedural survival.

Patient-specific changes in aortic hemodynamics is associated with thrombotic risk after fenestrated endovascular aneurysm repair with large diameter endografts

Background

The durability of fenestrated endovascular aneurysm repair (fEVAR) has been threatened by thrombotic complications. In the present study, we used patient-specific computational fluid dynamic (CFD) simulation to investigate the effect of the endograft diameter on hemodynamics after fEVAR and explore the hypothesis that diameter-dependent alterations in aortic hemodynamics can predict for thrombotic events.

Methods

A single-institutional retrospective study was performed of patients who had undergone fEVAR for juxtarenal aortic aneurysms. The patients were stratified into large diameter (34-36 mm) and small diameter (24-26 mm) endograft groups. Patient-specific CFD simulations were performed using three-dimensional paravisceral aortic models created from computed tomographic images with allometrically scaled boundary conditions. Aortic time-averaged wall shear stress (TAWSS) and residence time (RT) were computed and correlated with future thrombotic complications (eg, renal stent occlusion, development of significant intraluminal graft thrombus).

Results

A total of 36 patients (14 with a small endograft and 22 with a large endograft) were included in the present study. The patients treated with large endografts had experienced a higher incidence of thrombotic complications compared with small endografts (45.5% vs 7.1%; P = .016). Large endografts were associated with a lower postoperative aortic TAWSS (1.45 ± 0.76 dynes/cm² vs 3.16 ± 1.24 dynes/cm²; P < .001) and longer aortic RT (0.78 ± 0.30 second vs 0.34 ± 0.08 second; P < .001). In the large endograft group, a reduction >0.39 dynes/cm² in aortic TAWSS demonstrated discriminatory power for thrombotic complications (area under the receiver operating characteristic curve, 0.77). An increased aortic RT of ≥0.05 second had similar accuracy for predicting thrombotic complications (area under the receiver operating characteristic curve, 0.78). The odds of thrombotic complications were significantly higher if patients had met the hemodynamic threshold changes in aortic TAWSS (odds ratio, 7.0; 95% confidence interval, 1.1-45.9) and RT (odds ratio, 8.0; 95% confidence interval, 1.13-56.8).

Conclusions

Patient-specific CFD simulation of fEVAR in juxtarenal aortic aneurysms demonstrated significant endograft diameter-dependent differences in aortic hemodynamics. A postoperative reduction in TAWSS and an increased RT correlated with future thrombotic events after large-diameter endograft implantation. Patient-specific simulation of hemodynamics provides a novel method for thrombotic risk stratification after fEVAR.

The durability of fenestrated endovascular aneurysm repair (fEVAR) has been threatened by thrombotic complications. Using patient-specific computational flow simulation, the present retrospective study of 36 patients with juxtarenal aortic aneurysms treated with fEVAR identified several endograft diameter-dependent changes in aortic hemodynamics associated with thrombotic complications. A postoperative reduction in aortic wall shear stress and increased particle residence time correlated with the development of intraluminal graft thrombus and renal stent occlusion in patients treated with large diameter (>34 mm) endografts. These computationally estimated hemodynamic parameters could provide a novel method for patient-specific risk stratification for adverse events after fEVAR.

Patient-specific computational modeling of endovascular aneurysm repair: State of the art and future directions

Endovascular aortic repair (EVAR) has become the preferred intervention option for aortic aneurysms and dissections. This is because EVAR is much less invasive than the alternative open surgery repair. While in-hospital mortality rates are smaller for EVAR than open repair (1%-2% vs. 3%-5%), the early benefits of EVAR are lost after 3 years due to larger rates of complications in the EVAR group. Clinicians follow instructions for use (IFU) when possible, but are left with personal experience on how to best proceed and what choices to make with respect to stent-graft (SG) model choice, sizing, procedural options, and their implications on long-term outcomes. Computational modeling of SG deployment in EVAR and tissue remodeling after intervention offers an alternative way of testing SG designs in silico, in a personalized way before intervention, to ultimately select the strategies leading to better outcomes. Further, computational modeling can be used in the optimal design of SGs in cases of complex geometries. In this review, we address some of the difficulties and successes associated with computational modeling of EVAR procedures. There is still work to be done in all areas of EVAR in silico modeling, including model validation, before models can be applied in the clinic, but much progress has already been made. Critical to clinical implementation are current efforts focusing on developing fast algorithms that can achieve (near) real-time solutions, as well as ways of dealing with inherent uncertainties related to patient aortic wall degradation on an individualized basis. We are optimistic that EVAR modeling in the clinic will soon become a reality to help clinicians optimize EVAR interventions and ultimately reduce EVAR-associated complications.