A Novel Method of Thrombus Preparation for Use in a Swine Model for Evaluation of Thrombectomy Devices

Animal Care

All animal experiments were conducted in accordance with policies set by the Chancellor's Animal Research Committee of the University of California, Los Angeles. A total of 2 Yorkshire swine were used in this study.

Preparation of New Experimental Thrombus

Twenty milliliters of whole blood was obtained from a 7F sheath, which was placed in the right femoral artery of a swine. The content was then mixed with 2 g of barium sulfate powder to obtain radiopacity. To minimize the barium's anticoagulant effect, which is generally explained by absorption of the K-dependent coagulation factor,⁴ we paid special attention to the rotational motion when the content was mixed. The syringe was slowly rotated for 10–30 seconds so that the blood component was partially mixed with the barium layer formed at the bottom. Vigorous mixing was avoided. The mixture in the syringe was incubated at room temperature for 120 minutes until it showed a multiply-layered structure of blood constituent and barium. After the sedimentation, a solid component was separated from the serum component. The solid component showed 2 layers, a fibrin-rich layer and an erythrocyte-rich layer.

Using a number 11 surgical scalpel, we carefully resected a small piece of clot, measuring approximately 5 mm in diameter and 20 mm in length, from the aforementioned solid component with both fibrin-rich and erythrocyte-rich layers. Each prepared thrombus was then inserted into a silicone tube (MX 451 FL; Medex, Dubin, Ohio) with saline and prepared for injection.

Preparation of Conventional Experimental Thrombus (Thrombin-Induced Thrombus)

As a control, a conventional experimental thrombus (thrombin-induced thrombus) was also prepared. The detailed preparation process is described elsewhere.² Briefly, 10 mL of swine whole blood was mixed with 25 IU of bovine thrombin (Dade Behring, Newark, Delaware). One gram of barium sulfate was mixed with the blood component for 10 seconds, and the mixture was kept in a silicone tube for 60 minutes at room temperature. After the incubation period, the created clot was removed from the silicone tube and cut to a length of 20 mm.

Assessment of Mechanical Properties (Manual Elongation Test)

Ten samples were selected from each group, and a qualitative assessment of manual elongation was performed by using forceps.

Thromboembolization of External Carotid Arteries with the New Experimental Thrombus

With the swine under general anesthesia, we advanced a 6F guiding catheter (Envoy; Cordis, Miami Lakes, Florida) through the femoral sheath and placed it in the right lingual artery. After connecting the silicone tube containing a prepared clot to the guiding catheter, we injected 2 mL of saline into the tube to push the clot into the guiding catheter. Next, 3 mL of saline was injected into the guiding catheter under fluoroscopy to inject the clot into the target artery. The same procedure was repeated in the ipsilateral internal maxillary artery and the ascending pharyngeal artery.

Angiographic Evaluation

Postprocedural angiography was performed after the thromboembolization to evaluate the following: 1) the radiopacity of each injected thrombus, 2) fragmentation of injected thrombus, 3) total/partial occlusion of each target artery, and 4) induced vasospasm.

Histologic Analysis

We analyzed 10 samples of the following types of thrombus: 1) new experimental thrombi, 2) thrombin-induced thrombi, and 3) thromboemboli retrieved from patients with stroke. Each sample was fixed in 10% buffered formalin. Samples from the patients were thromboemboli retrieved from the middle cerebral arteries of patients with acute ischemic stroke by using the Merci Retrieval System (Concentric Medical, Mountain View, California). The specimens were embedded in paraffin, cut at 5-μm thickness, and processed with hematoxylin-eosin staining. Histologic examination was performed by a neuropathologist (H.V.V).