Abstract: AIM: To explore the relationship between the size of the occluder and the incomplete endothelialization after atrial septal defect(ASD) closure.METHODS: Equal-sized atrial septal defect models were established by homemade atrial septal perforators in 18 adult and healthy canines.Canines were then divided randomly into three groups with six canines in each group.Eight-mm occluders with diameter 2 mm larger than the ASD were implanted in group A,12-mm occluders with diameter 6 mm larger than the ASD were implanted in group B,and 16-mm occluders with diameter 10 mm larger than the ASD were implanted in group C.After 3 months and 6 months,morphological changes of the occluders and endothelialization were observed.RESULTS: ASD models were successfully established in all 18 canines and the average ASD diameter was(6.0±0.2) mm measured by TTE.Morphology of the occluders was flat in group A and the average thickness was(10.0±0.3) mm.In group B the morphology was conchoid and the average thickness was(15.4±1.7) mm,significantly thicker than in group A(P<0.05).The morphology of the occluders was dumbbell-shaped in group C with an average thickness of(24.3±2.5) mm,significantly thicker than in group B and group A(P<0.05;P<0.01).Three months after operation,the intima in group A was almost completely covered.Incomplete intimal coverage was observed in group B and the intima was "island-like" 3 months after operation,and the coverage area increased 6 months after operation.In group C,most of the occluders were exposed at 3 months as well as 6 months after operation.CONCLUSION: Oversized occluders in transcatheter therapy of atrial septal defects may result in incomplete endothelialization.