Before a patient undergoes heart valve replacement surgery, the surgeon will ask the patient and family for their opinion on what type of valve to use. Patients and families themselves lack knowledge in this area and are generally unsure of how to decide when asked this question. The type of valve chosen has a definite impact on the patient’s quality of life after surgery. Ideally, the prosthetic valve should be durable, last as long as possible, and not cause any other problems for the patient. However, no current prosthetic valve can meet this ideal requirement.
Human blood, as it flows through the blood vessels and heart, comes into contact with the endothelial cells of the blood vessels, and the blood does not clot. Once the blood comes into contact with almost any foreign body other than the endothelial cells of the blood vessels, the blood clotting process is activated and clots are produced. Prosthetic valves, as the name implies, are artificial, and when implanted in the heart they are certainly foreign bodies that must activate the clotting process.
After the clot is generated, it may block the activity of the artificial valve leaflet, leading to mechanical failure of the valve, causing stenosis or incomplete closure of the artificial valve, or it may be washed away from the valve by the blood flow and blocked in the arteries of other parts of the body with the blood flow (medically called arterial thromboembolism, mainly manifested as cerebral thrombosis, limb artery thromboembolism, etc.). The solution to this problem is to take anticoagulant drugs (most commonly warfarin) that reduce the clotting ability of the blood.
So far, the prosthetic valves officially approved for clinical treatment are divided into two types according to their manufacturing materials, mechanical and biological valves.
The mechanical valve is manufactured from carbon materials, metals and artificial fabrics. It has several advantages: first, it is robust, that is, it is durable and will not be damaged by wear and tear. In simulated work on a test bench, mechanical valves can withstand more than 100 years of wear; the second is the small-bore (19 mm and below) mechanical valve with a relatively large geometric orifice area, especially in the new generation, whose application is not easily replaced in certain patients with small aortic roots; and the third is the low height of the valve, which is suitable for certain special conditions. Clinical data have demonstrated that the incidence of valve thrombosis is lower with bileaflet valves than with tilting disc-type mechanical valves.
However, mechanical valves require patients to take oral warfarin for lifelong anticoagulation therapy and cannot be used in patients with contraindications to anticoagulation therapy. Anticoagulation is associated with a risk of fatal complications-bleeding and embolism. The rate of bleeding is higher in the Chinese Han population. In addition, patients on oral warfarin anticoagulation after implantation of a mechanical valve have some trouble with pregnancy and undergoing other procedures and require staged heparin replacement therapy, but many local hospitals currently have no experience with the management of such conditions.
Female patients may also have problems with increased menstruation. Although physicians emphasize the importance of standardized anticoagulation therapy and patients do what they are told to do, overall, a certain percentage of patients still die or become disabled due to complications from anticoagulation therapy. Another trouble is non-valvular structural prosthetic valve dysfunction, which is simply a mechanical disorder of the prosthetic valve. This is due to excessive tissue proliferation around the implanted prosthetic valve, which interferes with the normal activity of the valve leaflets, causing them to open and close abnormally. Prosthetic mechanical valve dysfunction is also one of the main reasons why valve patients undergo reoperation.
Another type of prosthetic valve is the bioprosthetic valve, which is made from the pericardium or aortic valve of another animal with some artificial stents and fabric that are finely processed. Among them, there are sub-stent bioprosthetic valves and stentless bioprosthetic valves. This biological valve is not biologically active, which means that it is not metabolically dead in the body and does not renew, repair, or grow on its own.
To put it more colloquially, the biologic flap is in the body like a superior, fine leather product that works in a good environment. After the biologic flap is implanted in the body for a long time (usually about 3-6 months), its surface is covered with deposited tissue such as fibrin and vascular endothelium due to the nature of the material used to manufacture it, as if a coat of paint had been applied to it. In this way, its surface does not come into contact with the blood, avoiding the activation of the blood clotting reaction and eliminating the need for oral warfarin anticoagulation.
For these reasons, the advantage of the biologic flap is that it requires only 3 to 6 months of anticoagulation after surgery and no continuous anticoagulation therapy thereafter. A special point to note here is that patients with persistent atrial fibrillation after bioprosthetic valve implantation must also continue anticoagulation therapy. The American Heart Association guidelines recommend warfarin systemic anticoagulation for patients without valve disease who simply have persistent atrial fibrillation.
This is true for pure atrial fibrillation, let alone if you have a prosthetic valve. Of course, the obvious disadvantage of a bioprosthetic valve is that it is not very durable and can be damaged over time, which is known medically as disfigurement. Stented bioprosthetic valve destruction occurs in the fifth year for the mitral valve, the eighth year for the aortic valve, and after the tenth year, the rate of destruction increases rapidly enough to have an impact on survival rates.
The longevity of a bioprosthetic valve is related to several factors. The first is the amount of force applied to the valve. The higher the pressure and the larger the valve, the greater the total force on the valve and the sooner it will be destroyed. This is why biological valves break down sooner in the mitral position than in the aortic position. The second is the rapidity of the heart rate. This is well understood: when the total number of operations is constant, the more valve operations per unit time, the less total operation time. The third is the blood calcium metabolism. Either a high blood calcium metabolism or an abnormal blood calcium metabolism may accelerate calcification of the biological valve. In children, the bones are in the growth phase and the blood calcium metabolism is active, so the valve is prone to damage when the bioprosthetic valve is used in children and adolescents.
Patients with chronic renal failure have abnormal blood calcium and phosphorus metabolism, and the bioprosthetic valve is also prone to damage. In diabetic patients, the probability of valve destruction is higher if insulin is applied, fasting blood glucose or glycated hemoglobin is high. Currently, authoritative clinical data demonstrate that pregnancy does not accelerate the destruction of biologic valves. The inherent inhomogeneity of the biomaterial is also a factor that affects valve durability and is difficult to completely avoid. In the case of the bovine pericardium, for example, differences in the quality of the pericardium between individual animals can affect the durability of the final valve if this difference is not reduced during the selection process. The uniformity of artificial materials is generally much better than that of biologic materials.
Biological valves can only be replaced with new ones if they break down. The risk during revalve replacement surgery is also a factor that must be considered. When any one of the valves is damaged after the use of a biologic valve (not in the early postoperative period), the entire biologic valve in the heart must generally be replaced at the same time, unless there is a very clear technical reason for the surgery. The surgical risks associated with reoperative replacement of two or three prosthetic valves, whether in this country or abroad, are undoubtedly significant.
Therefore, this factor is worth considering when a patient anticipates the need for two or three prosthetic valves if his or her life expectancy is still long.
Smaller diameter stented bioprosthetic valves, which have a smaller effective flap area than newer mechanical valves of the same diameter, do not have good hemodynamic results after implantation, so only some manufacturers produce 19 mm diameter bioprosthetic valves, and the smallest prosthetic valve diameter in most manufacturers’ products is 21 mm. The advantage of the stentless bioprosthetic valve is that the effective flap area of the valve is relatively large.
It has four problems.
First, the surgical technique is complex.
Second, it is too expensive.
Third, the current commercial stentless bioprosthetic valve can only be used in the aortic valve position.
Fourth, it is difficult and risky to operate again after valve destruction. This valve should be used mainly in elderly patients with small aortic roots.
In summary, physicians generally tend to advise patients to choose a bioprosthetic valve when there is an older patient (especially over 65 years of age), sinus rhythm, single valve disease, good financial situation, contraindications to anticoagulation therapy, and no combination of chronic renal failure. Bioprosthetic valves may also be considered in female patients who wish to have children after surgery. Doctors are more supportive of the mechanical valve option if the patient is young, has persistent preoperative atrial fibrillation, has multiple valve lesions, and has a small aortic root.