Obstruction of mechanical mitral prostheses: analysis of pathologic findings.

BACKGROUND: The pathologic and echocardiographic findings observed in 87 patients with mitral valve obstruction were reviewed to ascertain the incidence of pannus formation versus that of thrombosis, the relationship between the two, and the time to the occurrence of pannus versus the time to thrombosis.
METHODS: Pannus morphology (concentric or eccentric), its location on the valve (atrial, ventricular, atrioventricular), and the presence and relationship of associated thrombi (atrial, ventricular, atrioventricular) were analyzed. The times between valve replacement and the occurrence of obstruction were also compared.
RESULTS: There were 10 caged-ball valves, 65 tilting-disc valves, and 12 bileaflet valves. Seventy-two patients underwent prosthetic replacement, and 15 underwent thrombolysis. Pannus alone was found in 27, pannus and thrombus in 39, and thrombus alone in 21. Primary thrombosis occurred earlier than pannus formation (p = 0.04); this was true for patients with bileaflet valves (p = 0.006) and those with tilting-disc valves (p = 0.04). Pannus was atrial in 19.7% (13/66), ventricular in 21.2% (14/66), and atrioventricular in 59.1% (39/66). Pannus morphology was concentric in 22.7% (15/66) and eccentric in 77.3% (51/66). Atrial secondary thrombi occur more often in patients with atrioventricular pannus (p = 0.04). Eight patients had reobstruction; this was caused by pannus formation in 5 and by thrombosis in 3. Five underwent reoperation, and 3 underwent thrombolysis. Reobstruction occurred earlier than the first event.
CONCLUSIONS: The frequency of pannus formation is much higher than that of thrombus formation, but thrombosis is of earlier onset than pannus formation. Thrombosis is due to the deposition of clots on the prosthesis, and a pannus occurs as the result of an inflammatory reaction developing on both valve surfaces.