A xenogenic heart valve, or xenograft, is a replacement heart valve derived from the tissue of a non-human animal species. This type of bioprosthesis is used to treat valvular heart disease, a condition where one or more of the heart’s four native valves become damaged or diseased and can no longer function correctly. When a valve fails to open completely (stenosis) or close properly (regurgitation), it impairs blood flow and requires intervention to prevent complications like heart failure. Valve replacement surgery is the established treatment for severe valvular dysfunction, and xenografts represent a widely used option for restoring normal blood circulation through the heart.
Origin and Preparation of Xenografts
Xenografts are constructed using biological tissues harvested primarily from pigs or cows. Porcine (pig) aortic valves are often used whole, while bovine (cow) pericardium (the sac surrounding the heart) is commonly employed to create leaflets that are sewn onto a supportive frame. The use of animal tissue necessitates a rigorous preparation process to ensure the graft is safe for implantation and not immediately rejected by the human body.
The most common technique involves treating the harvested tissue with a chemical fixative, typically glutaraldehyde. This chemical bath sterilizes the tissue and cross-links the collagen fibers within the extracellular matrix. This cross-linking significantly reduces the tissue’s immunogenicity, transforming the pliable animal tissue into a durable, non-viable scaffold.
This fixation stabilizes the tissue structure and helps preserve the physical shape and mechanical properties of the valve leaflets, allowing them to withstand the continuous stress of the heart’s pumping action. Despite this preparation, the valve remains susceptible to long-term deterioration driven by biological and mechanical forces within the host.
Xenografts Compared to Mechanical Valves
The decision to use a xenograft is based on a fundamental trade-off compared to the primary alternative, the mechanical heart valve. Mechanical valves are constructed from synthetic materials like pyrolytic carbon and offer nearly permanent durability, often lasting for the patient’s lifetime. However, their synthetic surfaces pose a high risk of blood clot formation, necessitating that patients take lifelong anticoagulant medication, such as warfarin.
Xenografts, being made of biological tissue, are significantly less thrombogenic. This lower risk means patients typically do not need lifelong anticoagulant medication, greatly reducing their risk of serious bleeding complications, particularly from trauma or falls. This is an advantage for older patients or those for whom consistent anticoagulation management is difficult.
The main limitation of the xenograft is its finite lifespan due to structural valve degeneration. This vulnerability to wear and tear means the valve will likely require a re-operation over time. The choice of valve balances the risk of structural failure requiring surgery against the continuous risk of bleeding complications from anticoagulation. For this reason, guidelines often favor xenografts for older patients whose life expectancy aligns with the valve’s durability.
Expected Lifespan and Degradation
The functional lifespan of a xenogenic heart valve depends heavily on the age of the patient at the time of implantation. For patients over 70 years old, a xenograft may last between 15 and 20 years, often exceeding their remaining life expectancy. However, in younger patients, the valve’s lifespan is significantly shorter, sometimes lasting only 10 to 15 years, or even less in very young recipients.
This accelerated deterioration in younger patients is related to a more active calcium metabolism and a vigorous immune response. Younger bodies are biologically more dynamic, subjecting the xenograft tissue to greater mechanical stresses and biochemical activity that promotes faster breakdown. This gradual process results in structural valve degeneration, eventually impairing the valve’s ability to function correctly.
When the xenograft reaches the end of its functional life, it begins to fail either by becoming stiff and narrow (stenosis) or by leaking (regurgitation). This failure necessitates a re-intervention, which can be another open-heart surgery or a less invasive valve-in-valve procedure.
Specific Complications of Biological Valves
Structural valve degeneration in xenografts results from specific biological and mechanical processes acting on the preserved animal tissue. The most common cause of failure is calcification, involving the progressive build-up of calcium phosphate deposits on the valve leaflets. This mineral deposition causes the leaflets to lose flexibility and become rigid, leading to a narrowing of the valve opening and restricted blood flow.
Another form of structural failure involves the preserved tissue leaflets tearing or perforating due to fatigue from continuous opening and closing. When a leaflet tears, the valve can no longer close completely, resulting in significant leakage (regurgitation), which causes blood to flow backward. Both calcification and tearing lead to hemodynamic dysfunction, requiring the valve to be replaced.
Although the risk is lower than with mechanical valves, xenografts can also develop thrombosis (blood clot formation) on the valve surface. Evidence suggests that even small, subclinical blood clots can contribute to the inflammatory processes that drive structural degeneration. Furthermore, like any implanted device, xenografts are susceptible to infection, known as prosthetic valve endocarditis, which is a rare but serious complication.