Development of a symmetric canine abdominal aortic aneurysm model with clinical relevance for endovascular graft studies

Abdominal aortic aneurysm model in swine with bovine pericardium patch

Background

Aneurysm repair using endovascular techniques has grown in importance as materials have improved. Studies of endovascular prostheses require experimental models that reproduce anatomic and pathophysiological characteristics of human aneurysms.

Objectives

To describe a porcine model of abdominal aortic aneurysm.

Methods

This prospective cohort study used eleven Large White female pigs with a mean age of 12 weeks in two study phases. In phase I, the aneurysm was produced with a bovine pericardium patch by retroperitoneal surgery conducted under general anesthesia. In phase II, 15 days later, the animals underwent arteriography and were then euthanized before specimens were removed for histological analysis.

Results

Formation of parietal thrombus was observed in all animals. Microscopic analysis showed calcifications around thrombus in 82% of the animals. There was lymphoplasmacytic infiltration in the graft and adjacent area, with fibrosis in nine animals. Three pigs had substantial myointimal thickening, and eight had microcalcifications. Mortality was zero, and there were no ruptures, ischemia, or surgery site infections.

Conclusions

This is a unique model, using inexpensive, biocompatible material. Bovine pericardium is easy for the surgeon to handle and has very similar characteristics to autologous tissue in terms of integration with the cell wall.

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.

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.

A new model of abdominal aortic aneurysm with gastric serosa patch: surgical technique and short-term evaluation

The purpose of this work was to develop an abdominal aortic aneurysm (AAA) model that resembles human aneurysms with potential for further growth, patent collateral vessels, and a predictable tendency to rupture, and that can be used in the development of new endoprostheses and implant training. An infrarenal AAA model was created in five domestic swine using an autologous gastric serosal patch. Pre- and postsurgical digital subtraction aortograms (DSA) were obtained to document the appearance and dimensions of the aneurysm. Animals were followed up with DSA and ultrasonography on days 7, 14, 30, 45, 60, and 90 after model creation. Aneurysmal diameters were measured with both techniques in all examinations. On day 90, animals were euthanized, target arteries were harvested, and pathological evaluation was performed. The nonparametric Wilcoxon test was used to assess any differences in measured diameters. All the animals survived the surgical procedure. The aneurysmal diameters increased from 8.14+/- 2.15 to 13.28+/- 1.18 mm immediately after surgery (p < .05), but no subsequent significant growth of the aneurysmal sac was seen during follow-up. In this experimental setting, measurements obtained with DSA were slightly larger than those obtained with ultrasound. Two animals died of AAA rupture on days 6 and 10 (40% rupture rate). Pathological examination showed lack of elastic laminae and increased collagen content in the aortic patch. Thus, model showed a tendency to rupture, but no significant potential for further aneurysmal growth. It might be useful for training in endovascular therapies, but its usefulness for preclinical endovascular device testing is limited by its lack of growth potential.

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

OBJECTIVE

The clinical significance of retrograde collateral arterial perfusion of abdominal aortic aneurysms after endovascular repair (type II endoleak) has not been completely characterized. In this study a canine model was used to analyze intra-aneurysmal pressure, thrombus histologic characteristics, endoleak patency, and radiographic appearance of type II endoleaks originating from single and multiple aneurysm side branches.

METHODS

Prosthetic aneurysms with an intraluminal solid-state strain-gauge pressure transducer were created in the infrarenal aorta of 14 mongrel dogs. A single collateral side branch was reimplanted in 4 animals, multiple side branches were reimplanted in 6 animals, and no side branches were reimplanted in 4 control animals. Intra-aneurysmal and systemic pressure was measured for 60 to 90 days after creation of the type II endoleak. Endoleak patency and flow were assessed with duplex ultrasound scanning and cine-magnetic resonance angiography. Histologic analysis of the intra-aneurysmal thrombus was also performed.

RESULTS

Stent-graft exclusion reduced intra-aneurysmal pressure significantly in all animals, as compared with systemic pressure (P < .001). All intra-aneurysmal pressure values are indexed to the systemic pressure, and are represented as a percentage of the simultaneously obtained systemic pressure, which has a value of 1.0. Type II endoleaks originating from multiple side branches exhibited significantly increased intra-aneurysmal systolic pressure, mean pressure, and pulse pressure, as compared with endoleaks derived from either a single side branch (systolic pressure: multiple, 0.70 +/- 0.28 vs single, 0.50 +/- 0.19; P < .001; mean pressure: multiple, 0.78 +/- 0.23 vs single, 0.59 +/- 0.22, P < .001; pulse pressure: multiple, 0.41 +/- 0.25 vs single, 0.17 +/- 0.15, P < .001) or excluded control aneurysms that had no side branches and no endoleak (systolic pressure, 0.17 +/- 0.09; mean pressure, 0.14 +/- 0.10; pulse pressure, 0.098 +/- 0.08; P < .001). Cine-magnetic resonance angiograms and duplex ultrasound scans documented persistent patency of multiple branch endoleaks up to the time of euthanasia. In contrast, single side branch endoleaks thrombosed within 3 days (P < .001). Thrombus in the aneurysm sac in close proximity to the endoleak contained intact red blood cells and limited fibrin. Thrombus distant from the endoleak demonstrated extensive fibrin deposition and degraded red blood cells.

CONCLUSION

The canine model may be used to reliably measure intra-aneurysmal pressure in the presence of patent and thrombosed type II endoleaks. In this model 2 or more side branches are necessary to maintain persistent patency of type II endoleaks. These endoleaks are associated with significantly elevated intra-aneurysmal pressure, that is, 70% to 80% of systemic pressure. These results suggest that persistent type II endoleaks have clinical significance.

CLINICAL RELEVANCE

Endoleaks originating from retrograde flow in the side branch vessels of the aneurysm generate significant levels of intra-aneurysmal pressure, that is, 70% to 80% of systemic pressure. At least 2 patent side branch vessels appear to be necessary to cause persistent patency of type II endoleak in the canine model. Further studies will be necessary to enable more complete characterization of retrograde endoleaks and to extend these findings to allow clinical application. However, these results suggest that persistently patent type II endoleaks are clinical significance and may require more intensive follow-up intervention.

Endovascular Aortic Aneurysm Repair with Stent-Grafts: Experimental Models Can Reproduce Endoleaks

PURPOSE

To develop canine aneurysm models that can reproduce type II endoleaks after endovascular aneurysm repair (EVAR) with stent-grafts.

MATERIALS AND METHODS

A fusiform infrarenal abdominal aortic aneurysm model (AAA) was surgically created with a jugular vein patch, while preserving collateral vessels (n = 3). To allow comparative studies within the same animal, a bilateral iliac aneurysm model was also constructed with venous patches and surgical re-implantation of the sacroiliac trunk (n = 3). Stent-grafts were implanted by femoral approach at least 2 months later in both aortic and iliac models. Follow-up imaging was performed by Doppler ultrasound (US) and angiography until animals were killed 3 months after EVAR.

RESULTS

Angiography revealed immediate type II leaks in all cases. Leaks were still present at autopsy 3 months after EVAR in all cases, and were revealed at pre-death angiography in all but one case. At autopsy, leaks were characterized by the presence of large endothelialized channels that formed within the thrombus between the stent-graft and the aneurysmal wall.

CONCLUSION

As shown in this pilot study, persistent type II leaks after EVAR can be reproduced in aortic and iliac animal models. The iliac model can be created bilaterally in the same animal, thus allowing for comparative evaluation of different therapies. These models could be used to better understand the mechanisms of endoleak, and to assess future developments aimed to improve the outcomes after EVAR.