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Objective:The optimalorientationof a bileaflet mechanical valvefor tricuspidvalvereplacement (TVR) hasnot yetbeen determined. Theaim of this study was to use fiberoptic cardioscopy to evaluate the effect of orientation of a mechanical valve implanted in the tricuspid position on bileaflet mechanical valve behavior. Methods: Twelve pigs (50—59 kg) underwent TVR with a St. Jude Mechanical Heart Valve (25 mm standard cuff model) after cardioplegic arrest. The mechanical valve was implanted horizontally in six pigs (Group H), and vertically in another six pigs (Group V). The heart was perfused with pellucid Krebs—Henseleit solution in situ and the mechanical valve behavior was observed with a fiberoptic endoscope during different heart rates (HRs) induced by ventricular pacing (60, 90, 120, 150 min 1 ). All images were recorded on a high-speed videosystem every 4 ms. The closingtime lag (CTL) betweenthe valve leaflets was calculated and compared betweenthe two groups. Results: In Group H, the lower valve leaflet tended to open incompletely and close earlier than the upper leaflet. The calculated CTL was 303 60 ms, 65 48 ms, 40 9 ms, and 40 26 ms at pacing HRs of 60, 90, 120, and 150 min 1 , respectively. In contrast to Group H, there was little difference in CTL between the right and left leaflets in Group V. The calculated CTL was 9 12 ms, 11 10 ms, 1 3 ms, and 6 7m s at pacing HRs of 60, 90, 120, and 150 min 1 , respectively. There were significant differences in CLT between the two groups at each ventricular pacingrate(P < 0.01).Conclusions:Orientationofan implanted bileaflet valveinthetricuspidpositionsignificantlyinfluencedleafletmotion.In a horizontal orientation, the lower valve leaflet opened incompletely and closed earlier than the upper leaflet. These results suggest that the gravity might affect leaflet motion and that bileaflet mechanical valves should be implanted vertically in TVR to prevent abnormal leaflet motion and thrombus formation.