The long-term clinical assessment of heart valve substitutes

Mechanical and other problems of artificial valves

This chapter has discussed the design, development, laboratory testing and clinical performance of artificial heart valve replacements. The published material on this subject is extensive and clearly this present chapter represents only a limited selection of the many topics and researchers associated with the production of clinically implantable valves. Any omissions should not be regarded as a criticism, but simply the result of economy of space. The field is still developing and the situation regarding the advantages and disadvantages of the various types of valves as illustrated in Table 3 still obtains notwithstanding the many improvements which have been made with current designs. Valve development is, and will continue to be, closely allied to advances in our understanding of the behaviour of the materials of construction. Ultimately, improved haemodynamic performances of the various valve configurations will result from designs based on data from computerised fluid dynamic analysis combined with finite element stress analysis according to recognised engineering design principles.

Valve data collection: problems and pitfalls

Since 1981, the Department of Medical Physics and Clinical Engineering at the University of Sheffield has been responsible for the organization, management and data collection associated with the largest multicentre heart valve implant patient follow-up study in the Western world. At the present time, the database comprises information on over 16,000 valve implants, which have been provided by 57 surgeons working at 22 centres in the UK. All this data is available for in-depth statistical analysis. Over 30 individual valve models presently are included in the Study and these can be categorized into five main types: ball, disc, porcine, pericardial and homograft. Analysis includes descriptive statistics as well as valuable information on the various performances of the different valves. Survival and event-free survival graphs are obtained by actuarial methods and individual valve types can be studied in depth in terms of freedom from thromboembolic complications and valve dysfunction. Whilst this approach provides interesting and valuable survival data, it does not take account of the wide variation in prognostic factors which occur within large groups of patients. This latter problem can be addressed by the use of proportional hazards analysis and this paper provides details of this approach and typical results obtained from the use of this method. These include the comparative performances of the major types of valves currently in use in terms of the event-free survival of the patients.