TWENTY-FIVE YEARS OF HEART VALVE SUBSTITUTES: A REVIEW1

The replacement of malfunctioning heart valves with prosthetic counterparts has been an accepted routine clinical procedure for over twenty years. Hufnagel (1951) showed that manmade materials could be tolerated in the blood stream and his caged-ball prosthesis placed in the descending aorta was able to function in many patients for reasonably long periods. Total or partial valve replacement became a viable therapeutic option subsequent to the development of extracorporeal circulation, as described by Gibbon (1954). Since these early gays there have been many developments in the design and manufacture of artificial valves, and it is no exaggeration to say that several hundred different configurations have been considered. The vast majority of these designs have not gone beyond initial laboratory evaluation and, indeed, some have not even reached that stage. This may appear to suggest that much time and effort has been expended on valve development which has yielded a disproportionately small number of viable prostheses. Such an assessment is, however, quite unjustified since it neglects the immense wealth of knowledge that this research has created on a wide range of topics which are complementary to the ultimate development of an optimum valve design. For example, the need to test valves in the laboratory necessitated the development of sophisticated pulse duplicators which could reproduce the physiological and haemodynamic characteristics of the cardiac cycle. The design of such in vitro test systems has led to a greater understanding of normal valve performance in vivo. Furthermore, studies of the problems associated with the interaction of blood and foreign materials have yielded important information on anticoagulation techniques as well as encouraging the search for materials with improved properties of biocompatibility. This latter topic is of relevance to the whole field of prosthetic implants. The range of research projects which has been created as a result of prosthetic heart valve development is very extensive and, more importantly, has led to the involvement of nonmedical researchers from many different scientific backgrounds. This intimate collaboration of clinicians and scientists is an essential ingredient if optimum valve designs are to be obtained. The first valves to be used were almost entirely the result of surgical innovation. Whilst not wishing to detract from their initial success, it is clear that even today there is no ideal valvular replacement. To achieve the optimum design will require the continued collaboration of medical and non-medical scientists. In the sections which follow an account will be given of the development of various valve forms and the techniques used to test them in the laboratory. Since the authors are nonmedical scientists, there will be a tendency to.place more emphasis on the physical aspects of valve design. However, in the final analysis, what matters are the long-term follow-up results of clinical implantation. Such data, based on over 3000 cases, are available from the Multicentre Valve Trial set up by the Cardiac Surgical Research Club. Details of the more important results from this trial are also included, where relevant, in later sections of this paper.