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Off-Pump Mitral Valved Stent Implantation: A Histological Evaluation
Sinje Haasler, Saskia Pokorny, Irma Haben, Katharina Huenges, Jochen Cremer, Georg Lutter.
University Hospital Schleswig-Holstein, Department of Cardiovascular Surgery, Kiel, Germany.

OBJECTIVE: The aim of this study was the histological evaluation of mitral valved stents in an experimental in vivo porcine model four to twelve weeks after implantation with a focus on inflammatory reactions.
METHODS: The self-expanding mitral valved stents were implanted in the healthy porcine model via transapical approach in the beating heart. In this study, a histological evaluation of five animals is presented. The stents were implanted in vivo for a period of four weeks or longer. Posthumously, samples of the native mitral annulus and the atrial stent’s bed were taken and histologically analyzed (H&E, Masson's trichrome, Elastica van Gieson, Movat’s Pentachrome and von Kossa). Furthermore, CD45+ leukocytes, CD20+ B-cells, CD3+ T-cells and CD68+ macrophages were detected by specific immunohistochemically staining.
RESULTS: The tissue’s architecture showed largely physiological regular structure, but also presented local areas in which the integrity of the tissue was damaged to different extents. In four of five animals the formation of fibrin was detected. In all but one animal partly mild (n=2) or no (n=2) accumulations of calcium carbonate were shown.
Some inflammation in the surrounding tissue was detectable. In three of five animals a strong, in one animal a locally restricted and in one animal only a sporadic presence of leucocytes was found. A moderate (n=4) or strong (n=1) distribution of macrophages was detectable. Only low numbers of T-cells and B-cells were found in all animals but one. These results indicate an innate immune response but only low adaptive immune response after up to 12 weeks post-implantation. No differences between annulus and the atrial stent’s bed were visible.
CONCLUSIONS: The histological evaluation of five mitral valved stents showed widely a good integration into the native heart`s tissue. The stent surrounding tissue was classified and characterized for the first time. However, signs for inflammation and slight presence of calcification show potential risks. This has to be evaluated in more detail in a future study including a larger number of cases.

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