Variations in incidence rates

the best valves currently in use. A linearized rate is calculated as the number of events divided by total patientyears, which is then multiplied by 100 to convert the rate to units of “events per 100 years” or “percent per year.” For mechanical valves, the OPC (percent/year) is estimated at 0.8 for valve thrombosis and 3.0 for thromboembolism. For biologic valves, the OPC (percent/year) is estimated at 0.2 for valve thrombosis and 2.5 for thromboembolism. Thus thromboembolism is a more common event than valve thrombosis. Other more recent sophisticated statistical analyses for determining OPC have included the use of Cox regression and multivariable parametric models; each of these techniques has a lack of standardization in the definitions and follow-up methods that it uses. Improved prosthetic valve design and anticoagulation methods have reduced thromboembolic rates after prosthetic cardiac valve implantation. With coumarin therapy, valve thrombosis and total thromboembolism in general occur in 0.2 and 1.8 per 100 patient-years, respectively.2 For the Starr-Edwards valves implanted after 1970, the total thromboembolism rate is 2.5% to 4.0% per patient-year3-5; for Bjork-Shiley tilting disk valves, the rate is about 4% per year in the mitral position and 2.5% per year in the aortic position5,6; with the Medtronic-Hall valves, the rate is 1% to 3%.7 The St Jude’s valve has the lowest reported incidence of thromboembolism: 2.5% per year in the mitral position and 1% per year in the aortic position.8-13 Studies consistently show that the majority of patients with mechanical valves who have thromboembolic complications have subtherapeutic International Normalized Ratios (INRs) at the time of the event.14,15