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Mechanical valve thrombosis in a chronic animal model: differences between monoleaflet and bileaflet valves.

BACKGROUND AND AIM OF THE STUDY
The occurrence of mechanical valve thrombosis was studied in a chronic sheep model of valve implantation in the pulmonary position.


METHODS
Six monoleaflet and six bileaflet mechanical valves were implanted in young sheep. Anticoagulation was stopped at one week postoperatively, after which weekly fluoroscopic evaluation was performed. Animals were sacrificed at the moment of any abnormality on the fluoroscopic image or, if no abnormality occurred, at fixed time intervals of two and three months. After explantation, valves were examined macroscopically, by stereomicroscopy, histology and scanning electron microscopy. Lung biopsies were taken.


RESULTS
All monoleaflet valves remained functionally intact for up to three months without anticoagulation, but explantation after two or three months revealed thrombi attached to struts, cuspal surface and valve ring. All bileaflet valves, except one, showed obvious dysfunction on fluoroscopy (one or both cusps fixed) within a time frame of three to eight weeks after implantation. Histology and scanning electron microscopy showed primary thrombotic material in one or both hinges, obstructing further cusp movement. Lung embolism was detected in only one animal among biopsy specimens.


CONCLUSION
The low-pressure environment of the pulmonary position caused primary thrombotic changes in all implanted mechanical valves. The thrombosis caused severe mechanical dysfunction only in the bileaflet valves, probably due to their delicate hinge mechanism. These findings may be used in the further development of an animal model of mechanical valve thrombosis.