Characterization of retrograde collateral (type II) endoleak using a new canine model

Preoperative risk factors for aneurysm sac expansion caused by type 2 endoleak after endovascular aneurysm repair

Purpose

The objective of this study was to investigate the preoperatively definable risk factors which predict the aneurysm sac expansion caused by persistent type 2 endoleak after endovascular aneurysm repair.

Methods

Between 2008 and 2014, retrospective analysis was performed to examine the predictive risk factors for aneurysm sac enlargement caused by persistent type 2 endoleak, which was defined as a continuous endoleak present for more than six months. Aneurysm sac expansion was diagnosed if the maximum transverse diameter increased by 5 mm or more compared with the preoperative measurement.

Results

During the study period, endovascular aneurysm repair was performed in 211 patients with abdominal aortic aneurysm and common iliac artery aneurysm. Sac enlargement for type 2 endoleaks was observed in 20 patients (9.5%). The presence of more than five patent lumbar arteries flowing into aneurysm sac in the preoperative computed tomography (hazard ratio, 3.37; 95% confidence interval, 1.24–10.8; p = 0.017) was a predictive factor for sac expansion caused by persistent type 2 endoleak on Cox regression analysis. The presence of a patent inferior mesenteric artery was not associated with the sac expansion caused by persistent type 2 endoleak.

Conclusions

The presence of more than five lumbar arteries flowing into the aneurysm sac was a preoperative risk factor for sac expansion caused by persistent type 2 endoleak.

Basic Science Review: Characterization of Endoleak Following Endovascular Repair of Abdominal Aortic Aneurysms

Aneurysm models have been developed to study the pathobiology of abdominal aortic aneurysm and to evaluate the efficacy of endovascular therapy. The purpose of this review is to describe the use and limitations of current animal and experimental models for the characterization of endoleak following endovascular repair of abdominal aortic aneurysms.

Endovascular Repair of Abdominal Aortic Aneurysm: Follow-up with Noninvasive Vascular Elastography in a Canine Model

PURPOSE

To assess the ability of noninvasive vascular elastography (NIVE) to help characterize endoleaks and thrombus organization in a canine model of abdominal aortic aneurysm after endovascular aneurysm repair with stent-grafts, in comparison with computed tomography (CT) and pathologic examination findings.

MATERIALS AND METHODS

All protocols were approved by the Animal Care Committee in accordance with the guidelines of the Canadian Council of Animal Care. Stent-grafts were implanted in a group of 18 dogs with aneurysms created in the abdominal aorta. Type I endoleak was created in four aneurysms; type II endoleak, in 13 aneurysms; and no endoleak, in one aneurysm. Doppler ultrasonography and NIVE examinations were performed at baseline and at 1-week, 1-month, 3-month, and 6-month follow-up. Angiography, CT, and macroscopic tissue examination were performed at sacrifice. Strain values were computed by using the Lagrangian speckle model estimator. Areas of endoleak, solid organized thrombus, and fresh thrombus were identified and segmented by comparing the results of CT and macroscopic tissue examination. Strain values were compared by using the Wilcoxon rank-sum and Kruskal-Wallis tests.

RESULTS

All stent-grafts were successfully deployed, and endoleaks were clearly depicted in the last follow-up elastography examinations. Maximal axial strains over consecutive heart cycles in endoleak, organized thrombus, and fresh thrombus areas were 0.78% ± 0.22, 0.23% ± 0.02, 0.10% ± 0.04, respectively. Strain values were significantly different between endoleak and organized or fresh thrombus areas (P < .000) and between organized and fresh thrombus areas (P < .0002). No correlation was found between strain values and type of endoleak, sac pressure, endoleak size, and aneurysm size.

CONCLUSION

NIVE may be able to help characterize endoleak and thrombus organization, regardless of the size, pressure, and type of endoleak.

Models of abdominal aortic aneurysm: characterization and clinical applications

Abdominal aortic aneurysms (AAAs) are responsible for considerable morbidity, mortality, and cost to society. The pathogenesis of AAA formation, however, remains poorly understood. Animal models have been used in a range of experiments designed to provide further objective scientific assessment of the pathogenesis as well as the treatment of AAA. The purpose of this manuscript is to review the current models of AAA and their potential clinical implications.

Assessment of endoleak significance after endovascular repair of abdominal aortic aneurysms: A lumped parameter model

The outcome of endovascular repair of abdominal aortic aneurysms (AAAs) is greatly compromised by the possible occurrence of endoleak. Previously, the causes and effects of endoleak on a patient-specific basis have mainly been investigated in experimental studies. In order to both reconcile and physically substantiate the various experimental findings, a lumped parameter model of an incompletely excluded AAA was developed. After experimental validation, the model was applied to study the effects on the intrasac pressure of the degree of endoleak, the degree of stent-graft compliance, and the resistance of a possible outflow tract formed by a branching vessel. It is concluded that the presence of endoleak leads to elevated intrasac pressure, the mean of which is mainly governed by the outflow tract resistance, while the pulse pressure is governed by both the endoleak resistance and the stent-graft compliance. Based on the agreement of the current results with previous findings, it is further concluded that the lumped parameter modelling method provides a useful numerical tool for validating experimental endoleak studies.

Aneurysm Sac Pressure after EVAR: The Role of Endoleak

OBJECTIVE

The relation between endoleak and aneurysm sac pressure is not completely clear. This review evaluates the effect of endoleaks on aneurysm sac pressure and summarizes the present knowledge regarding aneurysm sac pressure after EVAR.

METHODS

A systematic search of literature was carried out using MEDLINE, EMBASE and Web of Science. Studies were included if aneurysm sac pressure measurements as well as systemic pressure measurements were performed during or after EVAR. Mean pressure indices (MPI), ratio mean aneurysm sac pressure to mean systemic pressure), in the absence of endoleaks and in the presence of different type of endoleaks were compared.

RESULTS

Stent-graft deployment does not seem to result in immediate reduction of aneurysm sac in the absence of an endoleak. Aneurysm sac pressure is elevated in the presence of an endoleak. However, the MPIs differ widely between studies both in the absence and presence of an endoleak.

CONCLUSION

MPI is not specific to the type of endoleak. This implies that the same type of endoleak does not necessarily pose the same MPI and by this the same hazard of aneurysm rupture, because the aneurysm sac pressure is directly related to the aneurysm wall stress.

Evaluation of pressure transmission and intra-aneurysmal contents after endovascular repair using the Trivascular Enovus expanded polytetrafluoroethylene stent graft in a canine model of abdominal aortic aneurysm

OBJECTIVE

Endotension has been defined as persistently increased pressure within the excluded sac of an abdominal aortic aneurysm (AAA) resulting in increasing aneurysm size after endovascular repair in the absence of endoleak. Devices that use expanded polytetrafluoroethylene (ePTFE) have been associated with the development of endotension and continued AAA enlargement. In this study, intra-aneurysmal pressure and aneurysm content were evaluated after endovascular repair with the Enovus ePTFE stent graft in a canine model.

METHODS

Prosthetic ePTFE aneurysms, each containing a solid-state, strain-gauge pressure transducer, were implanted in the infrarenal aorta of 13 mongrel dogs (25-35 kg). A second pressure transducer was inserted into the native aorta for systemic arterial pressure measurement. The stent graft was then deployed to exclude the aneurysm via distal aortic access. Comparison was made among three distinct stent grafts: the Trivascular Enovus (nonporous ePTFE; four animals), the original Gore Excluder (porous ePTFE; five animals), and the Medtronic AneuRx (Dacron; four animals). Daily systemic and intra-AAA pressures were measured for 4 weeks. Intra-aneurysmal pressures were indexed to simultaneously measured systemic pressures. After 4 weeks, the aorta, the prosthetic aneurysm, and its contents were harvested, photographed, and processed for histologic investigation with hematoxylin and eosin and Masson trichrome staining.

RESULTS

Within 24 hours after exclusion, the mean arterial pressure and pulse pressure within the AAA sac tapered to less than 20% of systemic pressure for all three stent graft types. Throughout the postoperative period, significantly lower indexed intra-aneurysmal pressures were present in the Enovus- and AneuRx-treated aneurysms as compared with those treated with the original Excluder stent graft (0.05 +/- 0.04, 0.16 +/- 0.06, and 0.06 +/- 0.03 for the Enovus, Excluder, and AneuRx, respectively). Histologic analysis of the Enovus-treated aneurysms demonstrated intraluminal content characterized almost entirely by erythrocytes and infrequent white blood cells without the fibrin organization-characteristics of acute or chronic thrombus. This contrasted with the content of the Excluder-treated aneurysms, which contained poorly organized fibrin deposition suggestive of acute thrombus, and of the AneuRx-treated aneurysms, which demonstrated mature, well-organized collagenous connective tissue.

CONCLUSIONS

Exclusion of the AAA with the Enovus stent graft resulted in nearly complete elimination of intra-aneurysmal pressure in this model. Histologic analysis of the aneurysm content further suggested complete exclusion, including elimination of circulating clotting factors and fibroblasts responsible for thrombus formation and reorganization. Ultimately, clinical evaluation will be necessary to demonstrate the effectiveness of this stent graft in preventing the development of endotension.

Assessing the effectiveness of endografts: Clinical and experimental perspectives

The increasing use of endografts to treat abdominal aortic aneurysms has prompted the need for improved postoperative imaging and surveillance. Although patients benefit from decreased morbidity with endovascular repair compared with open abdominal aortic aneurysm repair, the long-term outcome of stent repair has yet to be fully determined. The persistence of endoleaks highlights the need for close follow-up, particularly because this may lead to aneurysm rupture, even after endograft repair. The current mainstay of assessing the healing of endografts is obtaining serial helical computed tomography angiography (CTA) to identify endoleaks, graft migration, thrombosis, and structural failure. CTA is not completely effective at identifying endoleaks and predicting aneurysm rupture, however. Other modalities have been studied to improve on current imaging methods, including three-dimensional CTA with volumetric analysis, contrast-enhanced duplex ultrasound imaging, cine magnetic resonance angiography, and explant analysis. In vitro and large-animal models of abdominal aortic aneurysm have also been developed to study the pathophysiology and treatment response of aneurysm exclusion. Thus, clinical and experimental models of endograft healing are attempting to define the optimal method of postoperative surveillance of endovascular repair.

Type I and Collateral Flow in Experimental Aneurysm Models Treated with Stent-Grafts

PURPOSE

To evaluate the influence of a collateral branch on the evolution of type I leaks after endovascular aneurysm repair in an experimental model.

MATERIALS AND METHODS

Bilateral aneurysms were constructed in the common iliac arteries of 12 dogs by using venous patches. A collateral branch was added on one aneurysm on one side by implantation of the sacroiliac trunk on the sac. Balloon-expandable stent-grafts were implanted 2 months later. In six dogs, type I endoleaks were created on the side with the collateral branch by using plastic deformation of the stent-graft (group 1). Stent-grafts were fully expanded on the contralateral side in the same dogs (group 2, control group). In the remaining six dogs, type I endoleaks were created on the side without the collateral branch (group 3), and stent-grafts were fully expanded on the side with the collateral branch, creating a type II endoleak (group 4). Follow-up imaging was performed with Doppler ultrasonography and angiography until the animals were sacrificed at 3 months. Leaks were classified as major, moderate, or absent with use of findings at imaging and pathologic examinations.

RESULTS

No endoleaks were observed in group 2 (control group). Endoleaks were persistent in 83% of aneurysms in groups 1 (5/6 type I), 3 (5/6 type I), and 4 (5/6 type II). Type I leaks were major in three of six cases when associated with a collateral branch (group 1) and moderate when they were not (group 3; P < .05). Before sacrifice, larger aneurysmal diameters were observed only in group 1 (110% +/- 18) as compared with control group 2 (85% +/- 12) (P < .05).

CONCLUSION

More prominent leaks and larger aneurysms are observed when a collateral branch is associated with a type I endoleak.

Evaluation of the accuracy of a wireless pressure sensor in a canine model of retrograde-collateral (type II) endoleak and correlation with histologic analysis

BACKGROUND

The utility of intra-aneurysmal pressure determination is dependent on the ability to measure pressure in the presence of endoleak and thrombosis. In this study, the accuracy of a CardioMEMS wireless pressure sensor (CardioMEMS, Atlanta, Ga) transducer in the presence of thrombus associated with type II endoleak was measured.

METHODS

Type II endoleaks were created in four mongrel dogs by implanting four collateral arterial side branches (lumbar and caudal mesenteric arteries) as a Carrel patch onto a 3-cm prosthetic polytetrafluoroethylene abdominal aortic aneurysm (AAA). The aneurysm was excluded 2 weeks later from antegrade perfusion by a stent graft. The wireless pressure sensor was positioned in the AAA external to the stent graft. A Konigsberg intraluminal solid-state strain-gauge pressure transducer (Konigsberg Instruments, Pasadena, Calif) that is accurate in the presence of thrombus served as the control to determine AAA pressure. Both of the transducers were implanted on the luminal surface of the aneurysm, 180 degrees opposite from the Carrel patch and endoleak channel. Intra-aneurysmal pressure resulting from the type II endoleak was measured twice daily for 4 weeks using both transducers. A total of 56 pre-exclusion and 224 post-exclusion distinct pressure determinations were made. Intra-aneurysmal pressure was indexed to the systemic pressure that was simultaneously measured by a strain-gauge pressure transducer implanted in the native aorta. Histologic analysis of the aneurysm contents was performed with hematoxylin and eosin.

RESULTS

The intra-aneurysmal systolic, mean, and pulse pressures produced by the type II endoleak were significantly lower than systemic pressure in all animals and were < 60% of systemic pressure (P < .001). Close correlation between the wireless transducer and the control strain-gauge transducer was observed (R = 0.83, P < .001). Arteriography and Doppler ultrasound documented retrograde flow through the aneurysm side branches and persistent endoleak patency up to the time of euthanasia. Pathologic analysis demonstrated the endoleak channel to be patent and separated from the transducers by thrombus, which surrounded both transducers.

CONCLUSIONS

Intra-aneurysmal pressure generated by type II endoleaks may be accurately measured through thrombus using a wireless pressure sensor in the canine model. The wireless sensor has the potential for clinical applicability in diagnosing and characterizing type II endoleaks.

An animal model for type II endoleaks with use of a tsuzumi drum-shaped stent-graft

PURPOSE

To create an animal model of type II endoleaks after endoluminal deployment of a specially designed stent-graft (SG).

MATERIALS AND METHODS

Five swine were used. A tsuzumi drum-shaped SG consisting of a covered Z stent with its diameter narrowed at the center was deployed in the midthoracic aorta. In this way, a residual space (RS) was created between the aortic wall and the graft to simulate an aneurysm sac. A 5-F catheter was placed into the RS, and then aortography, RS angiography, and pressure measurements were performed. Follow-up was performed at 3 and 10 days after the procedure. Mean pressure indexes (MPIs) were calculated as the ratio of the mean RS pressure to the aortic pressure. Histologic examination was also performed.

RESULTS

RSs with two or three pairs of intercostal arteries were successfully created in all cases. Aortography showed two type II endoleaks in five swine just after SG deployment and four type II endoleaks at 10 days. RS angiography showed circulation between the RS and the intercostal arteries in all cases. The mean MPI was 69.4% +/- 10.4% just after SG deployment and increased to 87.8% +/- 5.2% at 10 days. By gross examination, RS patency was retained.

CONCLUSIONS

A swine model of type II endoleaks was successfully created endoluminally. This model does not require direct surgery to the aorta and its side branches and promises to be useful to study the mechanism of and therapy for type II endoleaks.