Pathological substrates of thrombus formation after valve replacement with the Hancock bioprosthesis

Twelve patients who had undergone single or multiple heart valve replacement with the Hancock bioprosthesis have been proved at postmortem examination to have had atrial or cusp-related thrombosis. Four, who died within the first postoperative month ('early' group), had isolated mitral valve replacement (MRV); the thrombus was located on the sewing ring in two, in the left atrial body in one, and in the left auricle in one. One of these patients experienced postoperative cerebral thromboembolism. Risk factors in this group were considered to be the dilated left atrium, chronic atrial fibrillation, and the absence of anticoagulant treatment, which were noted in all, and low-output syndrome, which was observed in three. In eight patients thrombosis was found at necropsy or reoperation 26 to 78 months after heart valve replacement ('late' group). In five patients thrombus filled one or more cusp sinuses; only one exhibited severe degeneration of cusp tissue. Thromboembolic episodes were recorded in one patient. Three had atrial thrombosis due to severe prosthetic valve stenosis caused either by calcifications of the leaflets or fibrous tissue overgrowth on the device. Our findings suggest the following: (1) Early thrombosis after MVR involves the host tissue-valve interface or the left atrial cavity; the risk factors are the presence of a dilated and fibrillating left atrium, low-output syndrome, and the lack of anticoagulant treatment. (2) Late thrombosis is frequently a prosthetic cusp thrombosis; since severe cusp degeneration was rarely observed, primary thrombogenicity of the prosthetic tissue might be considered. (3) Severe prosthetic valve stenosis may otherwise predispose to late atrial thrombosis.