Comparison of Mechanical and Tissue Heart Valve Performance
Mechanical heart valves have been available since the 1950’s. Over time, material and design technology has greatly improved. The dominant design and material combination today is bileaflet composed of pyrolytic carbon. The strength of mechanical heart valves is that the pyrolytic carbon parts do not wear out (structural failure) from the normal wear and tear inside the heart. Multiple studies have shown that patients survive longer with mechanical heart valves (Tables 3a22-46, 3b22-46).
|Table 3a. Aortic survival rates of On-X, mechanical and tissue heart valves. Values may be estimated from graphic representations.22-46|
|Table 3b. Mitral survival rates of On-X, mechanical and tissue heart valves. Values may be estimated from graphic representations.22-46|
Tissue heart valves have been available since the 1960’s. The first generation of tissue valves deteriorated and failed rather quickly (a few years) after implant. Newer preservation methods developed in the 1980’s and 1990’s have helped extend the life of tissue heart valve prostheses. The advantage of tissue valves is that in approximately 50% of patients, anticoagulant medication is not needed on a long-term basis.47 Anticoagulant treatment will be discussed throughout this guide. Small stented tissue prostheses have sub-optimal efficiency [see next section, Valve Efficiency (Hemodynamic) Comparison]. Both tissue and conventional mechanical valves produce similar thrombembolic complication rates (Table 4a).47 Patients implanted with tissue valves have been found by one surgeon to be five times more likely to need reoperation than patients implanted with mechanical valves.47 There will be more bleeding, sometimes with serious results, for patients who must take anticoagulant medication. Design improvements of the On-X Prosthetic Heart Valve that lower turbulence have created lower complication rates than those for other valve options (Table 4b24,29,33,36,39,45,49,50). Survival rates for mechanical valves are superior to those of tissue valves (Tables 3a and 3b).
|Table 4a. Comparison of complication rates of older mechanical and tissue heart valves 47|
|Events||Bioprostheses % per pt.yr.||Mechanical % per pt.yr.|
|Table 4b. Comparison of complication rates of On-X and other valves. 24,29,33,36,39,45,49,50
Click to enlarge.
Reoperation rates for mechanical valves are superior to those for tissue valves (Table 523,24,26,27,29,34,35, 37-41,45,51,52) and
reoperation risks increase with age (Table 653, 54).
|Table 5. Comparison of reoperation rates of On-X, mechanical and tissue heart valves. Values may be estimated from graphic representations.23,24,26,27,29,34,35,37-39, 40,41,45,51,52|
Reoperation to replace a failing tissue valve or a malfunctioning mechanical heart valve carries risk for death which increases drastically as age increases. If a patient receives a tissue valve in his/her fifties or sixties, they will require reoperation in their sixties or seventies when risk of death will be much higher (11%-17%) than a reoperation at an earlier age. In a patient’s eighties, risk of death from reoperation can be as high as 32% (Table 6).
Table 6. Reoperation risk increases with increasing age.53, 54
|60-70 years||> 70 years||> 80 years|
|Figure 8. Tissue heart valves start to fail before 10 years postoperatively.8|
Valve Efficiency (Hemodynamic) Comparison
Stented tissue valves and older mechanical valves have exhibited poor efficiency especially in small sizes.37,39,44 Efficiency is measured by hemodynamic performance which is related to restriction of blood flow caused by the implanted valve. Hemodynamic evaluation of implanted heart valves postoperatively is made with echocardiography (sound waves). Two common measurements of hemodynamics are gradient and effective orifice area (EOA). Gradient is the pressure difference from one side of the valve to the other and is better when it is low (under 10 millimeters of mercury at rest for implanted valves). EOA is the area of the valve that actually allows forward flow of blood and is better when it is larger (acceptable EOA in implanted valves is at least 1.5 square centimeters for normal sized adults). On-X heart valve design reduces turbulence that is characteristic of older valve designs. By doing so, efficiency is increased. A comparison of On-X hemodynamics to tissue valves is found in Table 7. Favorable gradients are low and favorable EOA’s are high.
Table 7. Comparison of On-X and tissue heart valve hemodynamics33,43,55,56
|On-X Valve33||Stented Tissue55 Carpentier Edwards Pericardial||Stentless Tissue43 Medtronic Freestyle||Stentless Tissue56 Toronto SPV|
|Valve Size||Gradient mmHg||EOA cm2||Gradient mmHg||EOA cm2||Gradient mmHg||EOA cm2||Gradient mmHg||EOA cm2|
|19||8.7||1.5||22||0.82||12||1.0||This size not manufactured|