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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
AJNR Am J Neuroradiol. Author manuscript; available in PMC 2010 August 13.
Published in final edited form as:
PMCID: PMC2921447
NIHMSID: NIHMS215159

Five-Year Follow-Up in Elastase-induced Aneurysms in Rabbits

Abstract

Background and Purpose

To report aneurysm patency rates up to 5 years in the elastase-induced aneurysm model in rabbits.

Materials and Methods

Twenty five elastase-induced aneurysms were created in New Zealand White rabbits and followed for up to five years. Thirteen (52%) rabbits died during followup for reasons unrelated to the aneurysms. Digital subtraction angiography (DSA) was performed at one month and at two and five years in the 12 surviving subjects. Aneurysm patency and dimensions, including neck diameter, aneurysm width and height, were evaluated at each time point in relation to external sizing devices. Differences of aneurysm sizes (neck width and aneurysm width and height) among time points were compared using the Student’s t test.

Results

Eleven (92%) of the 12 aneurysms in the subjects that survived for five years remained fully patent throughout follow up. A single, narrow necked aneurysm showed partial thrombosis at the 2 and 5 years time points.

Conclusion

Experimental, elastase-induced aneurysms in rabbits demonstrate high rates of patency up to five years following creation. When planning for very long term studies, investigators should plan for relatively high rates of mortality unrelated to aneurysm pathology.

Introduction

Long-term patency of experimental aneurysms represents an important attribute for testing of aneurysm occlusion devices. Aneurysm models that suffer from spontaneous thrombosis are considered of limited value in the evaluation of new, endovascular occlusion devices. Notwithstanding the wide variety of experimental aneurysms described in numerous species, reports regarding long term patency rates remain uncommon. (1-4). Patency is poor in side-wall aneurysms in swine (1). Side-wall canine aneurysms appear to offer better patency rates than those of swine aneurysms (2-3).

The rabbit elastase-induced aneurysm model has been widely used for testing endovascular devices (5-17). Previous reports have noted excellent patency of untreated elastase induced aneurysms up to 2 years after creation (18). In this study, we report the patency rate of elastase-induced aneurysms in rabbits at five years.

Methods

Aneurysm Creation

Elastase-induced, saccular aneurysms were created in 25 New Zealand White rabbits (body weight 3–4 kg) using the rabbit elastase model. Some of these same subjects were reported in previous papers detailing the use of intravenous digital subtraction angiography (IVDSA) (19) as well as the patency rate of aneurysms up to two years (18). The Institutional Animal Care and Use Committee at our institution approved all procedures. Detailed procedures for aneurysm creation have been described (5). Briefly, anesthesia was induced with an intramuscular injection of ketamine, xylazine and acepromazine (75 mg/kg, 5 mg/kg, and 1 mg/kg, respectively). Using sterile technique, the right common carotid artery (RCCA) was exposed and ligated distally. A 1-2 mm bevelled arteriotomy was made and a 5F vascular sheath (Cordis Endovascular, Miami Lakes, FL) was advanced retrogradely in the RCCA to a point approximately 3 cm cephalad to the origin of RCCA. Fluoroscopy was performed by injection of contrast through the sheath retrograde in the RCCA, in order to identify the junction between the RCCA and the subclavian and brachiocephalic arteries (Advantx; General Electric Company; Milwaukee, Wisconsin). A 3F Fogarty balloon (Baxter Healthcare Corporation, Irvine, CA) was advanced through the sheath to the level of the origin of the RCCA with fluoroscopic guidance and was inflated with iodinated contrast material. Porcine elastase (5.23 μ/mgP, 40.1 mgP/ml, approximately 200 U/mL; Worthington Biochemical Corporation, Lakewood, NJ) was incubated within the lumen of the common carotid above the inflated balloon for 20 minutes, after which the catheter, balloon and sheath were removed, the RCCA was ligated below the sheath entry site.

Digital Subtraction Angiography follow-up

Follow-up angiography was performed at each time point (one month, two years, and five years after creation). Twelve rabbits (48%) of 25 rabbits survived for the full duration of the study, in which follow-up angiographic images at each time point were available in 11 aneurysms; the other aneurysm only had images at 2 and 5 year time points. Thirteen rabbits (52%) died from causes unrelated to the aneurysm itself; five from stroke, two from cancer, four from chronic liver failure, and two from anesthesia-related complications before five years after aneurysm creation. For the one month and five year time points intra-arterial digital subtraction angiography (IADSA) was performed. IVDSA was performed at two years. Details regarding the technique of IADSA and IVDSA have been previously reported (19). Aneurysm dimensions, including neck width and dome width and height were measured and calculated from these DSA images in reference to external, radio-opaque sizing markers. DSA images also were used to evaluate interval changes in aneurysm dimension over time.

Statistical Analysis

Differences of aneurysm sizes (neck width and aneurysm width and height) among time points were compared using the Student’s t test.

Results

Eleven (11/12, 92%) aneurysms remained completely patent angiographically throughout the five years following creation (Figures 1, ,2).2). At two years a single, narrow necked aneurysm (neck size 1.4 mm, width 4.5mm, ratio of width/neck 3.2) showed partial thrombosis (Figure 3), which remained stable at five years.

Figure 1
Serial digital subtraction angiography (DSA) in an elastase-induced aneurysm. A. Anteroposterior (AP), intra-arterial (IA) DSA performed one month after creation demonstrates an aneurysm cavity along the brachiocephalic artery, at the origin of the ligated ...
Figure 2
Serial digital subtraction angiography (DSA) in another elastase-induced aneurysm. A. Anteroposterior (AP), intra-arterial (IA) DSA performed one month after creation demonstrates an aneurysm cavity along the brachiocephalic artery, at the origin of the ...
Figure 3
Serial digital subtraction angiography (DSA) in an elastase-induced aneurysm undergoing partial thrombosis. A. Anteroposterior (AP) intraarterial (IA) DSA one month after creation showing patent aneurysm cavity. B. AP intravenous (IV) DSA image at two ...

Aneurysm Neck Dimensions

Mean neck widths at one month, two years, and five years after creation were 3.2 ± 1.1 mm, 3.0 ± 0.9 mm, and 3.0 ± 1.0 mm, respectively. There were no significant differences observed among these neck widths (p = .79).

Aneurysm Width

Mean aneurysm widths at one month, two years, and five years after creation were 3.9 ± 1.2 mm, 3.9 ± 1.2 mm, and 4.1 ± 1.0 mm, respectively. There were no significant differences among these aneurysm widths (p = .64).

Aneurysm Height

Mean aneurysm heights at one month, two and five years after creation were 8.9 ± 2.4 mm, 8.4 ± 2.3 mm, and 8.4 ± 2.1 mm, respectively. There were no significant differences among these aneurysm heights (p = .60).

Discussion

This study confirms excellent long-term patency of the elastase-induced aneurysm model in rabbits. In this relatively small study of long term patency, only a single, narrow necked aneurysm showed partial spontaneous thrombosis over time. Untreated human aneurysms also show low rates of spontaneous thrombosis over (21-24). Indeed, observation of spontaneous aneurysm thrombosis in humans is the subject of case reports (22-24). As such, our observed, long term patency rate of the elastase induced aneurysms offers further support for the application of this aneurysm model not only for evaluation of endovascular devices but also of aneurysm hemodynamics and biology (25, 26).

The short- and long term patency of elastase induced aneurysm model has previously been reported. The model demonstrates early growth, up to 3 weeks following creation, after which aneurysm dimensions stabilize (17). This early growth may reflect either ongoing injury to elastic lamina or ongoing interplay between aneurysm hemodynamics and mechanical integrity. Previous studies have demonstrated ongoing patency of this model up to two years (18). The current study confirms ongoing patency in untreated aneurysms up to five years after creation.

Long term patency rates of most types of experimental aneurysm models are rarely reported. One study reports the patency of 326 vein patch aneurysms in 310 canines over a six year period. Of these, 102 were sidewall (lateral) and 224 were bifurcation aneurysms. Spontaneous occlusion occurred in 9 (9/102, 9%) of the sidewall aneurysms and in only 1 (1/224, 0.4%) of the bifurcation aneurysms (2). Future model development would ideally offer such long term patency reports.

This study suffered several limitations. The cohort was small, limited by the substantial financial requirements for maintaining experimental subjects for up to five years. Thus, the impact of aneurysm neck size or aneurysm dimension cannot be determined. Second, a relatively high proportion of the rabbits in our study expired from age-related pathologies, including chronic liver failure. One might argue that, basing our patency rate on the initial cohort instead of the surviving subset, our actual patency rate would have been 11 (44%) of 25 subjects. In any event, the high age-related mortality rate should be carefully considered by investigators planning very long term rabbit aneurysm studies, with extra subject added at the outset as needed.

Conclusion

Experimental, elastase-induced aneurysms in rabbits demonstrate high rates of patency up to five years following creation. When planning for very long term studies, investigators should plan for relatively high rates of mortality unrelated to aneurysm pathology.

Acknowledgments

Supported by NIH R01 NS46246

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