Haemodynamics and durability of mitral bioprostheses--an in vitro study.

The haemodynamic characteristics and durability of mitral bioprostheses were evaluated and compared in vitro in a pulse duplicator and in a fatigue test system. Porcine xenografts were shown to be the most stenotic of all bioprostheses, studied at rates simulating both rest and exercise; on the other hand the new generation of biological valves showed a clear improvement in haemodynamics compared with the standard porcine and pericardial prostheses so far used clinically. Durability tests performed at a rate of 1600 to 1800 beats per minute and with a closing pressure ranging from 80 to 100 mmHg showed that the Ionescu-Shiley and Edwards pericardial xenografts last significantly longer than the Hancock and Carpentier Edwards porcine valves and Hancock pericardial bioprosthesis. Correlation between the mode of failure of pericardial and porcine valves in vitro and in vivo clearly demonstrates that a major role in valve failure is played by mechanical stress. This can be either a consequence of the continuous trauma sustained by the tissue hitting against the bare dacron cloth of the sewing ring during the movements of the cusps. or a result of fatigue occurring and the bending points of the leaflets. The results of this study underline the importance of a continued in vitro evaluation of both haemodynamics and durability of bioprostheses, since some of the pericardial valves which had the best haemodynamic performance showed the worst resistance to fatigue-induced failure. A careful selection of the tissue, modification of the stent design and avoidance of a dacron covered frame are suggested to improve long-term durability of biological prostheses.