When a patient’s natural heart valve becomes diseased or damaged, replacement surgery is necessary. This involves removing the impaired valve and implanting a fully functioning replacement. One common solution uses biological tissue sourced from non-human species. These biological replacements are known as xenografts, meaning tissue transplanted from one species to another.
Primary Animal Sources for Heart Valve Replacements
The two species primarily used to create these biological valve replacements are pigs and cows. Tissue from pigs results in a porcine valve, while tissue from cows is used for a bovine valve. These animals are chosen because their heart tissue exhibits structural properties and mechanical compatibility similar to human valves.
A porcine valve is fabricated using the entire aortic valve harvested from a pig’s heart. A bovine valve is constructed from a sheet of tissue called the pericardium, the sac surrounding a cow’s heart. This pericardial tissue is shaped and assembled onto a frame to create a functional valve structure. Both tissues have shown comparable long-term results, and the choice often depends on the specific valve design and the surgeon’s preference.
Understanding Bioprosthetic versus Mechanical Valves
Replacement heart valves fall into two major categories: bioprosthetic and mechanical. Bioprosthetic valves are derived from animal tissue and function by opening and closing like a natural valve. The main advantage of a bioprosthetic valve is that it generally does not require the patient to take long-term blood-thinning medication, known as anticoagulants.
Mechanical valves are constructed from man-made materials, typically a special form of carbon. These devices are exceptionally durable and and can last a patient’s entire lifetime without wearing out. However, the synthetic materials of a mechanical valve increase the risk of blood clots forming on the valve surface. To mitigate this risk, patients with a mechanical valve must adhere to a regimen of lifelong anticoagulation therapy.
The decision between the two types involves assessing patient factors and lifestyle. Bioprosthetic valves are recommended for older patients (over 65) because they reduce the risk of outliving the valve’s lifespan. Mechanical valves are chosen for younger patients due to their superior durability. However, patients must manage the increased risk associated with lifelong blood thinners.
Making Animal Valves Safe for Human Implantation
Before an animal-derived valve can be implanted, the tissue must undergo a treatment process to ensure its safety and performance within the human body. The fundamental step in this preparation is chemical fixation, most commonly achieved using a solution of glutaraldehyde. This chemical bath alters the tissue at a molecular level.
Glutaraldehyde fixation works by cross-linking the proteins within the animal tissue. This stabilizes the tissue’s structure, making it more robust and less likely to break down rapidly after implantation. The process significantly reduces the tissue’s immunogenicity, which is the ability to provoke an immune response. Reducing immune rejection is necessary for the xenograft to be successful.
The fixation also sterilizes the tissue, ensuring it is free of pathogens before implantation. Despite this preparation, a primary limitation of bioprosthetic valves is the potential for calcification. To counter this, manufacturers incorporate decalcification treatments, sometimes involving alcohol, to shield the tissue from mineral deposits that can lead to premature failure.
Expected Durability and Need for Replacement
The primary trade-off for avoiding lifelong blood thinners is the limited durability of the bioprosthetic valve. Animal tissue naturally begins to wear out over time, and the main cause of failure is structural valve deterioration (SVD). This deterioration is largely driven by tissue calcification, where calcium deposits accumulate and stiffen the valve leaflets, preventing them from opening and closing correctly.
The typical lifespan for a bioprosthetic valve is estimated to be between 10 and 20 years. However, this longevity is highly dependent on the patient’s age at the time of surgery. Younger patients often experience a faster rate of structural deterioration, sometimes requiring reoperation within a decade. For patients over 70, the tissue valve may last for the remainder of their lives.
When the valve fails, reoperation is necessary to replace the worn-out prosthesis. While mechanical valves rarely require this, the possibility of reoperation is an accepted part of the treatment plan for patients who opt for a bioprosthetic valve to avoid long-term anticoagulation risks. Modern advancements, such as transcatheter valve-in-valve procedures, now offer less invasive options for replacing a failing bioprosthetic valve in many cases.