Chronic total occlusions (CTO) are present in up to 40% of patients with angiographically documented coronary artery disease and represent at least 10% of the target for coronary angioplasty attempts.1 However, despite improvements in angioplasty equipment — primarily guidewires — and operator skills, the success rate of CTO recanalization has reached the plateau of around 70%.1 Recent studies2,3 suggest that compared with unsuccessful CTO recanalization, those with successful procedures have a much better long-term outcome. In addition, the availability of drug-eluting stents would further improve the durable benefit of the successful procedure.4 A useful large animal model of CTO would facilitate the development of novel equipment and techniques to further enhance success rates. Until recently, animal models of CTO have relied on surgically placed ameroid constrictors, which cause gradual CTO by extrinsic compression5 and would not be appropriate for transluminal interventions. Copper is highly immunogenic and can cause inflammatory reactions in porcine coronary arteries.6 Thus, copper or other inflammatory agents coated on stents7 may provide a reliable means of gradual intraluminal CTO and proper evaluation of novel CTO angioplasty equipment. The objective of this study was to create a percutaneous porcine coronary model of CTO using copper-plated stents, hopefully similar to human coronary artery disease and more suitable for evaluation of novel CTO angioplasty equipment. Methods Study protocol. All procedures and care of animals were in accordance with institutional guidelines. A pilot study in 6 pigs evaluated the circumferential coating of 0.0005 inch thick copper (both luminal and abluminal surfaces of the stent) on the BX Velocity™ Coronary Stent (Cordis Corporation, Warren, New Jersey). These stents were implanted in the mid-left anterior descending (LAD) and mid-left circumflex (LCX) coronary arteries in 6 Yorkshire swine (specific pathogen-free, body weights: 30–35 kg.). Despite dual antiplatelet therapy and successful stent implantation without complications, there was a high mortality rate from stent thrombosis (4 of 6 within 1 week). Due to the high mortality rate during the pilot phase, two modifications were made to improve survival. The copper coating was limited only to the abluminal surface (electroplating of 0.0005 inch copper to the outer surface of the stent only) to minimize the risk of subacute thrombosis, and only one stent was implanted in the LCX in each pig to minimize the risk of mortality. Eighteen pigs were divided into 3 groups (n = 6/group), with the early group sacrificed at ~1 week after stent implantation, the intermediate group sacrificed at ~4 weeks, and the late group at ~8 weeks. There was 1 death in the intermediate group, and the remaining pigs survived until sacrifice. At the time of the follow-up angiogram prior to sacrifice, the degree of stenosis and grades of collateral vessels were assessed. The collateral vessels were graded according to the Rentrop classification8 as grade 0: none; grade 1: barely detectable filling of collateral channels without opacification of the occluded artery; grade 2: partial filling of the CTO artery; grade 3: brisk flow that fills the CTO artery completely. The initial and follow-up angiograms were analyzed quantitatively using QCA-CMS version 4.0 (Medical Imaging System, Leiden, Netherlands). The stent-to-artery ratio was calculated by dividing the stent diameter by the reference vessel size. At follow-up, the smallest diameter within the stent was measured (follow-up minimal lumen diameter). Percent diameter stenosis at follow-up was calculated by dividing the difference between the average of the proximal and distal reference segments (REF) and the follow-up minimal luminal diameter by the REF, and multiplying by 100. Stented segments were harvested and evaluated by the study pathologist as described in the literature.9 In addition to the stented segments, the corresponding myocardium was evaluated for signs of infarction and hibernation. Hibernating myocardium was identified by the depletion of contractile elements and inflammatory changes of the interstitium with fibrotic remodeling.10 Statistics. Angiographic and histologic data are presented as mean value ± SD. ANOVA was used to compare among the groups. A p-value Results Angiographic Result (Figure 1). One sudden death occurred within a day after stent implantation in the intermediate group. Otherwise, all pigs survived until sacrifice. The reference vessel size (2.53 mm) and stent-to-artery ratio (1.23) were similar among the groups. At follow-up, 3 of 6 stents were completely occluded in the early group, with the remaining patent stents having a mean 60% diameter stenosis. All of the stents in the intermediate and late groups were completely occluded. Most of the pigs with total occlusion (12 of 14) showed bridging collateral flows into the occluded artery, with a grade ? 2 (grade 2 = 4, grade 3 = 8). One pig in the early group and 1 pig in the late group showed no visible collateral channels despite total occlusion. Histologic results (Figure 2). Histology revealed mostly fresh red thrombus and intimal fibrin with prominent inflammation in the early group. In the intermediate group, there was organized thrombus with vascularized intima and some calcification around the stent struts. In the late group, the inflammatory changes and intimal smooth muscle cells were less, but more collagenous stroma and more organized calcification (more plate-like and crystalline than the amorphous granular form seen in the intermediate group) were observed. In addition, there were scanty revascularization channels in the late group. Interestingly, there were fibrotic components in the proximal and distal edges of the occlusions with softer, organized thrombus in the middle of the CTO in the late group, suggesting that the major areas of difficulty are at the entrance and exit segments of the CTO with percutaneous recanalization (Figure 2). There was also hibernating myocardium with scanty cytoplasmic fibrils and increased cytoplasmic glycogen in all groups, although the late group had the greatest number (1.23 ± 0.88 hibernating myocardial fibers/cm2 in the early group, 1.34 ± 1.12 in the intermediate group, and 3.36 ± 1.96 in the late group; p Discussion This study shows the feasibility and reproducibility of a new percutaneous porcine coronary chronic total occlusion model. By limiting the copper-plating to the abluminal side and implantation of the stent in the left circumflex coronary artery only, the majority of the pigs (94%) survived until sacrifice and had total occlusion by 1 month after stent implantation. The late (8-week survival) occlusions had both fibrosis and calcification, similar in composition to that of human CTOs. Previous studies. Ameroid constrictors can cause CTO in coronary arteries within a month.5 Although this model is excellent in producing chronic ischemia with mild infarction and is useful in studying therapies for chronic ischemia such as angiogenesis,11 it is not an appropriate model for percutaneous CTO evaluation. On the other hand, any inflammatory agents such as copper or polymers, with or without a stent platform, may provide a practical percutaneous model of CTO. Van der Giessen et al.7 applied polymers to approximately 90 degrees of a stent circumference and found marked inflammatory response to polymers with resultant severe stenosis at 1 month. Thus, in addition to copper or other metals, inflammatory polymers may be potential candidates for causing CTO, including a polymer cylinder.12 Need for a percutaneous CTO model. Conventional percutaneous approaches for CTO treatment have been associated with two major difficulties; (i) the initial failure of successful recanalization, usually by the inability to cross the occlusion with a guidewire; and (ii) subsequent low rate of long-term patency after successful recanalization.1 With the availability of drug-eluting stents,4 the former limitation is the most difficult issue, as long-term patency should improve with drug-eluting stents. To further enhance the initial success rate, novel CTO devices or methods need to be designed and tested. For these reasons, there is an urgent need for a large animal percutaneous CTO model. Observations from the current CTO model and potential contributions to CTO revascularization. This study shows that in the porcine model, CTO could be reliably created by one month following copper-plated stent implantation, and that the histology is similar to that of CTOs in humans.13 Katsuragawa et al.13 showed that in CTOs with higher recanalization rates, namely those with tapered occlusion, there is fibrous tissue with small vascular channels, similar to the intermediate CTO in our study. On the other hand, in the CTOs with low recanalization rates, such as the abrupt occlusions, there was no recanalization by vascular channels, and loose fibrous tissue was dispersed in the occluded segment, similar to the late occlusions in our study. Thus, our model with different ages of occlusions, may provide an opportunity to study recanalization strategies for different types of clinical CTOs. For instance, the older occlusions with calcification may require rotational atherectomy for improved success. Another advantage of the animal model is that we could create CTOs of varying anatomy, such as occlusion at the site of a side branch that could be used to recanalize more challenging anatomy.
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