How a prosthesis in aortic valve replacement is chosen

Aortic valve replacement (AVR) is the most common heart valve operation performed today (1). Two types of valve prosthesis are used: biological and mechanical. Mechanical valves have evolved from the initial ball and cage design with tilting disc valves, to the current generation of bileaflet valves. Biological valves are of several types: bovine, pericardial or porcine valves, stented or stentless, homografts, pulmonary autografts, and more recently, sutureless biological valves and autologous pericardial valves (2).

1 - A description of the ideal valve substitute

The ideal valve substitute would be as durable as a native valve, it would be non-thrombogenic and wouldn’t require anticoagulation, it would have excellent haemodynamics with no gradient, be easily implantable, readily available, and it would have the ability to regenerate on its own (3). Such a valve, of course, does not exist. The pulmonary autograft does demonstrate many of these characteristics but it is not easily implantable and is dependent on a homograft in the pulmonary position. Commercially available valve prostheses have excellent haemodynamics, are easily implantable and readily available, but cannot regenerate, and either have limited durability (bioprosthetic valves) or are thrombogenic requiring anticoagulation (mechanical valves).

2 - Mechanical valves : disadvantages

Mechanical valves, made from pyrolitic carbon and titanium, are generally recommended in younger patients due to its increased durability, but have significant disadvantages. In one study, at 15 years, freedom from valve-related mortality and re-operation for endocarditis was only 80%. Freedom from bleeding and thromboembolic complications was only 42% (4). More recently, a prospective randomised controlled trial reported thromboembolic rates of 0.54%/patient-year for mechanical valves versus 0.24%/patient-year for bioprosthesis (p=0.3), bleeding rates of 1.47%/patient-year for mechanical valves and 0.72%/patient-year for biological valves (p=0.08) (5). The thromboembolic and bleeding rates were twice as high in mechanical valves compared to biological valves, although this was not significant, possibly due to the small number of patients in the study (n=280). In most patients, it would be reasonable to offer those younger than 65 years of age a mechanical valve, and those above 65 years of age, a biological valve unless they are already receiving anticoagulation (e.g. for an existing mechanical valve) (6,7).

3 - Relevance of age

The age of the patient is a key consideration in the choice of prosthesis, older patients generally being offered biological valves and younger patients mechanical valves.

4 - Other factors

However, the choice of prosthesis in younger patients may also depend on their lifestyle, profession, and personal preferences. Some younger active patients may prefer to have a biological valve and accept the need for likely re-operation in the future, as would those who are non-compliant to anticoagulation, have contraindications to it, or difficulties monitoring it. Women of child-bearing age planning a pregnancy may also prefer a biological valve to avoid anticoagulation and its teratogenic risk.

5 - Patients 60-65 years of age

Biological valves are increasingly being used in patients aged 60-65 years. In 2004, 30% of patients aged 60-65 years undergoing AVR in the U.K. received a biological valve; this had increased to 60% in 2008 (1). Younger patients with multiple co-morbidities may be offered a biological valve if their life expectancy is lower than the presumed durability of a biological valve (7). The durability of biological valves is dependent, amongst many factors, on the age of the patient. In patients above 70 years of age, freedom from re-operation due to structural valve deterioration is 90% at 15 years (8). In patients aged 50-70 years, freedom from re-operation is 80%, while in those less than 50 years, the freedom from re-operation is only 50% at 15 years.

6 - Stentless valves and homografts and pulmonary autografts

Most biological valves are mounted on a stent for ease of implantation. The absence of stents in stentless valves and homografts should theoretically have superior haemodynamics with lower gradients and larger effective orifice areas. However, the evidence for this is conflicting and it is uncertain whether stentless valves are advantageous over stented valves, except in cases of small aortic root, and in patients with poor left ventricular function, where the evidence in favour of stentless valves is consistent (9). This evidence must be balanced against the longer operating times for stentless valves particularly in patients with poor left ventricular function who already have a higher operative risk.
Homografts are not commonly used nowadays except in some cases of endocarditis, where the mitral flange can be of benefit in filling voids and infected spaces in the aortic root or the mitral valve. A recent randomised controlled trial reported a significantly higher rate of valve dysfunction at 8 years in homografts compared to a commercially available stentless valve (63% versus 15%, p<0.001), and a higher rate of re-operation (10% versus 0%, p=0.024) (10).
The pulmonary autograft is usually used in children and also in adults younger than 40 years of age due to its ability to grow and its longer durability with no need for anticoagulation. A recent randomised controlled trial showed it to have equivalent survival to a matched general population, and a re-operation rate of only 6% at 13 years (11). A recent meta-analysis reported an autograft deterioration rate of 0.78%/patient-year and right ventricular outflow tract deterioration rate of 0.55%/patient-year (12). However, it is a technically demanding procedure and its long term durability beyond 20 years in adults is uncertain.

Conclusion:

The choice regarding which valve prosthesis to use in AVR is dependent on the age, life expectancy, lifestyle and personal preference of the patient. Biological valves are generally used in those aged above 65 years although they are also increasingly being used in those aged 60-65 years. Stentless valves may have advantages in poor left ventricular function and small aortic roots but this condiseration must be balanced against longer operating times. Homografts are rarely used nowadays except in some cases of infective endocarditis. The pulmonary autograft may be advantageous in younger patients less than 40 years of age but is a technically challenging procedure and longer term results beyond 20 years are unknown.