Prosthesis heart valves; choosing the right one for your patient
It is more than 50 years since Starr and Edwards’ description of successful prosthetic valve replacement in 1961.Although, Mitral valvotomy for mitral stenosis and other techniques for repair of the diseased mitral valve, particularly mitral valve prolapse, have been developed and refined, valve operation remains as a promising approach and continue to rise in last decades. Each year, approximately 90 000 valve substitutes are now implanted in the United States and 280 000 worldwide.
Types of heart valve prosthesis
- Ball valves: The original Starr-Edwards prosthesis comprised a silastic ball which seated in the sewing ring when closed and moved forward into the cage when open.
- Disc valves: The Bjork-Shiley prosthesis is comprised of a single graphite disc coated with pyrolite carbon which tilts between two struts of the housing which is made of stainless steel or titanium.
- Bileaflet valves: Bileaflet valves have two semicircular leaflets which open and close creating one central and two peripheral orifices. It is now the most commonly implanted type of mechanical prosthesis in the world.
All mechanical prosthesis has an absolute requirement for anticoagulant treatment. The potential advantage of avoiding the hazards of anticoagulation has led to the search for a valve replacement of suitable biological material which would not require long term anticoagulant treatment. A number of different approaches to the problem of finding a suitable biological valve have been made. An autologous or autogeneous valve is fashioned from the patient’s own tissue such as fascia lata or pericardium. An autograft valve is one translocated from one position to another—for example, when the patient’s own pulmonary valve is used to replace a diseased aortic valve. A homograft (or allograft) valve is one transplanted from a human donor. A heterograft (or xenograft) valve is one transplanted from another species such as a pig, or manufactured from tissue such as bovine pericardium. Porcine valves are treated with glutaraldehyde which both sterilises the valve tissue and renders it biologically acceptable to the recipient.
Most bioprosthesis are mounted on stents attached to a sewing ring, but stentless valves which are sewn in free hand have become available. Stentless valves have a greater effective orifice area compared with stented valves, but are technically more difficult to implant.
Bovine pericardial valves are fashioned from bovine pericardium mounted on a stented frame.
Biological prosthesis with stents
Biological prosthesis without stents
Studies comparing different types of mechanical prosthesis
Thromboembolism has been reported as occurring at a higher rate following Starr-Edwards replacement than Bjork-Shiley. Bileaflet prosthesis appears to have the lowest risk of thromboembolism. Rates of thromboembolism are higher following mitral valve replacement than following aortic valve replacement. The criteria in favor of using a mechanical valve include the following:
- the informed patient wants a mechanical valve and has no contraindication for long-term anticoagulation;
- the patient is already on anticoagulation (mechanical prosthesis in another position or at high risk for thromboembolism);
- the patient is at risk of accelerated bioprosthesis structural deterioration (young age, hyperparathyroidism, renal insufficiency); and
- the patients is younger than 65 years of age and has a long life expectancy.
Studies evaluating different types of biological prosthesis
Porcine valve failure is happen seven or more years after implantation, particularly in younger patients. One study compared results with stentless porcine prosthesis with stented prosthesis in the aortic position showed apparently enhanced durability of the stentless prosthesis. Advocates of the stentless prosthesis point to its superior haemodynamics with an effective valve area some 10% larger than stented prosthesis of equivalent size. A bioprosthesis may be preferred rather than mechanical prosthesis in the following situations:
- the informed patient wants a bioprosthesis;
- good-quality anticoagulation is unavailable (contraindication or high risk, compliance problems, lifestyle);
- the patient is older than 65 years of age and/or has limited life expectancy; and
- the patient is a woman of childbearing age. Bioprosthesis degenerate more rapidly in young patients and during pregnancy.
. P. Pibarot, J.G. Dumesnil, “Prosthetic heart valves: selection of the optimal prosthesis and long-term management”, Circulation, 2009; 119: 1034-1048.
. P. Bloomfield, “Choise of heart valve prosthesis”, Heart 2002; 87: 583-589.