Pathology of Substitute Heart Valves: New Concepts and Developments

All types of contemporary cardiac valve substitutes suffer deficiencies and complications that limit their success. Mechanical and bioprosthetic valves are intrinsically obstructive, especially in small sizes. Mechanical valves are associated with thromboembollc problems; the chronic anticoagulation used in virtually all mechanlcal valve recipients causes hemorrhage in some. Calcification limits the success of porcine and pericardial bio‐prostheses, allograft valves, and the yet experimental trileaflet polymeric prostheses. The predominant mechanism of calcification in porcine, pericardial, and allogratt valves is cell mediated, being nucleated at the membranes and in organelles of the transplanted cells. In polymeric leaflet valves, calcification is both extrinsic (in adherent thrombus) and Intrinsic (subsurface and acellular in the solid elastomer). Nevertheless, except for a few notable exceptions, contemporary mechanical valves are durable. Other Important potential complications of prosthetic and bioprosthetic valves include paravalvular leak, endocarditis, or extrinsic interference with function. Moreover, aortic valvular allografts undergo progresshe noncalclfic degeneration, tearing, sagging, and/or retraction. Studies of retrieved long‐term cryopreserved allograft explants demonstrate severe degeneration, with distortion of normal architectural detail, loss of endothelial and deep connective tissue cells, and variable inflammatory cellularity. Thus, they are morphologically nonviable valves, whose structural basis for function seems primarily related to the largely preserved collagen, and they are unlikely to have the capacity to grow, remodel, or exhibit active metabolic functions. Since calcification intrinsic to the cusps is the major pathologic process necessitating bioprosthetic valve reoperations, efforts to prevent formation of mineral deposits are active. The most promising anticalcification techniques are preimplantation treatments of glutaraldehyde fixed tissue with either detergents such as sodium dodecyl sulfate, 2‐amino oleic acid, cations such as Fe3+ or Al3+, or dlphosphonate drugs such as ethane hydroxydiphosphonate, or tissue preservation alternatives to aldehydes, such as polyepoxide compounds or dye mediated photooxidation treatment. Promising innovative concepts also under Investigation include modifications of bioprosthetic valve stent design and tissue mounting techniques to reduce cuspal stresses, tissue treatments to enhance endothelialization, nonstented porclne valves, and minimally cross‐linked autologous pericardial valves. (J Card Surg 1994; 9[Suppl]:222–227)