Some of the mechnical and biological problems surrounding the use of fresh allograft inverted aortic valves as mitral valve substitutes are described. Certain aspects of the problem have been studied experimentally. In three sheep `fresh' aortic valve allografts were inserted, using cardiopulmonary bypass, into the main pulmonary artery, and were observed from 5 to 7 months after operation. The animals survived normally. Their normal pulmonary valves remained in situ. The technique is described. At subsequent necropsy, macroscopically the valves were found to be free from vegetation, and the cusps were pliable and apparently normal. Microscopically, the supporting allograft myocardium showed necrosis and early calcification. The valve cusp showed hyalinization of collagen, although beneath the endocardium this hyalinized collagen contained moderate numbers of fibroblasts with no evidence of proliferation. The endocardium and arterial intima of the allograft showed evidence of ingrowth from adjacent normal host endocardial tissues. The allograft itself was invested in a loose layer of fibro-fatty tissue, which, in view of the necrotic state of the graft myocardium, could well have been a reparative reaction rather than a homograft reaction. It is concluded that, although the cusps could function normally, the necrosis of the myocardium might in time lead to late failure of the graft. Further studies with the valve inserted at mitral level are indicated.
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