Transapical Mitral Valve Implantation—Macroscopic and Histologic Findings after Up to 8 Weeks Follow-up

Objectives: To evaluate macroscopic and microscopic findings at up to 8 weeks follow-up after off-pump transapical transcatheter mitral valve implantation (taTMVI) in the pig.

Methods: Fifteen self-expanding TMVI devices were implanted into the mitral annulus of pigs via a transapical approach in an off-pump fashion. After a follow-up of 28 ± 15 days, necropsy was performed and samples from the tissue around the mitral annulus as well as from the newly formed tissue on the stent’s polytetrafluoroethylene (PTFE) membrane were taken. The samples were stained with Movat’s pentachrome, Elastica van Gieson, and von Kossa. All sections were examined by light microscopy.

Results: Stent positioning was successful in all 15 cases; 60% (9 of 15) of the animals survived for the desired period of at least 4 weeks and were euthanized 28 to 58 days after stent implantation. Cause of death in the remaining six animals was cardiac failure (4 of 6), pneumonia (1 of 6), and anesthetic incident during follow-up diagnostics (1 of 6).

Necropsy showed no signs of extensive myocardial infarction around the apical access and fixation mechanism of the stent. Proper stent positioning in the mitral annulus was maintained in 93.3% (14 of 15) with one stent migrated atrially without loss of function and hemodynamic impairment; 66.7% showed fractures in the Nitinol-struts or ruptures of a fixation tether. Thrombi attached to the valved stent were found in 13.3% (2 of 15).

Histological examination showed no tissue reaction to the Nitninol-struts but well preserved overall structures around the mitral annulus in 86.7% (13 of 15). Endocarditis was found in one case and one case showed a localized necrotic area of the myocardium underneath the disc-like atrial part of the stent as the apical fixation force was relatively high.

The microscopical examination of the tissue on the atrial part of the stent showed a fibrinous sheath and calcium deposits as sign of a foreign body-type reaction to the PTFE membrane, predominately in the sections of animals with shorter follow-up. In sections of animals with follow-up of 4 weeks or more increasingly more collagenous tissue and ingrowing elastic fibers could be found.

Conclusion: Procedural success and short-term outcome after TMVI procedures in pigs is satisfactory. Adverse macroscopic findings are due to the factor of self-designed and manufactured experimental devices. Histological examination shows encouraging results as progressive healing and neointimalization could be found over time.

No conflict of interest has been declared by the author(s).