Valve substitutes in cardiac surgery.

Since 1955, with the progress of safer methods of extra corporeal circularion, open heart surgery started to go through great advancements. As far as the treatment of vaive disease is concerned those improvements made it possible to think about repla cing the diseased heart valves with prostheses. Even before the era of open heart surgery, correction of aortic insufficiency had been clinically experimented placing in the descending thoracic aorta either a rigid prosthesis — Hufnagel valve — or one homologous graft (Lam and Murray). Some of the details of the Hufnagel valve were actually adapted to the project of the first rigid prosthesis piaced in subcoronary position, to replace a diseased aortic valve, and Dwight Harken was responsible for this work in 1960. This was very similar to the pioneer work done, aimost simultaneously, by Albert Starr. Since that period mulciple types of mechanical valve prosthesis have been described, and this shows a constant effort oriented to find a valve substitute thar would function as nearly as possible to rhe normal valve. The characteristics, mentioned by Harken, which the ideal substitute should follow, have been used in the background af ali those works and experiments and they were the following: durability, adequate haemodynamic performance — with acceptabie transvalve gradients and non significaht regurgitation — and, finaliy, without haemol ysis and without the risk of thromboembolism. With the finality of producing a valve that could really foilow the above cited principies, various concepts and ideas were described. One of the first, which was later recognized as mistaken, was that the artificial valves should be anatomically as similar as possible to the normal human valves. Valves of flexibie synthetic material (like Tefion), described by Bahnson, Kay, Morrow and Mc Goon, were very eariy discarded as the risks were too high of deterioration, disinsertion, infection and thrombosis. A totally different concept, initially received with skepticism, but readily accepted as really valid, was rhereafter introduced by Harken and Starr: the concept of a prosthe sis using a poppet as occluder.