Prosthetic Valves
The main difference between mechanical and bioprostheses can be summed up by a simple comparison: the need for life-long anticoagulation with warfarin (
Coumadin) vs the need for a repeat operation. In other words,
mechanical valves last forever but are associated with an inherent risk for thrombus formation and embolization (thromboembolic complications), and therefore
necessitate life-long anticoagulation therapy, which is accompanied by its inherent risk of bleeding (hemorrhagic complications). This poses a
great dilemma for young patients who do not want to take warfarin because an active lifestyle will likely increase the risk of injury and hemorrhage. In addition, warfarin use must be carefully monitored as measured by the INR, and noncompliant patients as well as those who live in remote areas may have only limited access to such monitoring.
Under these circumstances, the risk of a repeat operation outweighs the risk of taking warfarin.
Another important factor to consider, especially with mechanical valves, is their
track record as far as design malfunction is concerned. Therefore, the
number of years a certain valve design has been in use without any mechanical malfunctions is of paramount importance in deciding which valve to use. The implications for such failures are huge. Not only can they put a patient's life at risk, but also the financial and litigation burdens could be enormous.
It is not surprising that the pace of progress in the introduction of new prosthetic devices into clinical practice is very slow. It takes, on average, between 8 and 10 years for a new device to be approved by the United States Food and Drug Administration (FDA), and the process is similarly long for regulatory agencies in other countries. It is not unusual for new valve devices to be subjected to clinical studies when the previous models are just approved and introduced into clinical practice.
Mechanical Valves
Forty-five years ago, the ball-and-cage valve was the first commercial mechanical valve introduced to clinical practice. It was a high-profile valve and required intense anticoagulation. The valve subsequently underwent modification with changes made in the ball structure (silastic, stainless steel). Concerns about increased thrombogenicity and the less than perfect hemodynamics led to the next generation of valves -- tilting disc valves (
Bjork-Shiley, Medtronic Hall, and
Omniscience valve designs). One modification to the
Bjork-Shiley valve, in which it was reshaped to form a convexo-concave valve, faced a serious problem with strut fractures and led to a significant decline in the use of these valves.
Constituting the third generation of mechanical valves, the bileaflet valve was first introduced into clinical practice in 1977 by St. Jude Medical (Minneapolis, Minnesota) and is in widespread use today. The device is a low-profile valve that has excellent hemodynamics, requires less intense anticoagulation, and has had no reported structural failures. Other bileaflet valves approved by the FDA are manufactured by CarboMedics, Inc. (Austin, Texas), ATS Medical, Inc. (Minneapolis, Minnesota), Baxter Edwards CVS (Irvine, California), Sorin Biomedica (Saluggia, Italy), Medtronic (Minneapolis, Minnesota), and Medical Carbon Research Institute (Austin, Texas).
Candidates for mechanical valves are usually young patients who have no contraindications to warfarin therapy. Women of childbearing age who want to have children should not take warfarin because of its association with congenital defects. Patients with any bleeding tendencies or those likely to be exposed to repeated injuries should also avoid taking the drug. The rationale for using a mechanical valve in younger patients is related primarily to the long-term durability of the mechanical prosthesis with no concerns about structural valve degeneration or the need for repeat surgery. The only exception to this rule is the development of prosthetic valve infective endocarditis, especially with the virulent staphylococcal infections. Under these circumstances, annular abscess formation can lead to perivalvular leak and the need for premature repeat valve replacement.