The four valves within the human heart act as one-way doors, ensuring blood flows forward in one direction. When a valve becomes damaged, either by failing to open fully (stenosis) or failing to close completely (regurgitation), the heart must work significantly harder to move blood. This sustained strain can eventually lead to heart failure and other complications. When repair is not possible, a prosthetic valve is surgically implanted to restore unidirectional flow and normalize the heart’s function.
The Prosthetic Valve Defined
A prosthetic heart valve is a medical device designed to replace a diseased native valve, maintaining the passive, forward movement of blood through the heart chambers. All prosthetic valves share a similar structure designed to anchor the device and control flow. The device is secured to the heart tissue by a sewing ring, a cuff of fabric that allows the surgeon to stitch the valve into the native valve’s annulus.
The functional part of the valve is housed within a rigid or semi-rigid structure, often called the stent or housing. This structure supports the moving components, which are either rigid discs or flexible leaflets. These internal components are engineered to open and close in response to pressure changes, effectively replacing the function of the original valve tissue. The purpose is to provide an adequate opening for blood flow while preventing backflow into the preceding chamber.
Comparing Mechanical and Biological Valve Types
The choice of prosthetic valve falls into two main categories: mechanical or biological, each presenting a distinct trade-off in long-term patient management. Mechanical valves are constructed from durable synthetic materials, most commonly pyrolytic carbon, which is highly resistant to wear over decades of use. Their primary advantage is longevity, often lasting 20 to 30 years or more without structural deterioration.
The disadvantage of mechanical valves is their inherent thrombogenicity, meaning the artificial surfaces promote blood clot formation. This requires patients to take lifelong oral anticoagulant medication, such as Warfarin, to reduce the risk of stroke or valve obstruction.
In contrast, biological prostheses, or bioprosthetics, are crafted from animal tissue, usually porcine (pig) or bovine (cow) pericardium, which is treated and mounted on a frame. Biological valves are less likely to cause blood clots, often eliminating the need for long-term anticoagulation therapy after the initial post-operative period. However, this natural tissue is susceptible to structural deterioration, primarily through calcification and tearing. This limits their lifespan to an average of 10 to 15 years, often necessitating a re-replacement procedure later in life.
How They Mimic Natural Function
The physical function of any heart valve, native or prosthetic, is passive and governed by pressure gradients within the heart chambers. When the pressure in the upstream chamber, such as an atrium or ventricle, exceeds the pressure in the downstream chamber, the valve is pushed open, allowing blood to flow forward. The valve closes immediately when the pressure reverses, preventing blood from leaking backward.
Mechanical valves achieve this action using pivoting components, such as two semi-circular discs in the widely used bileaflet design. When the pressure gradient is favorable, these discs swing open, allowing blood flow through the valve orifice. Once the pressure gradient reverses, the discs rapidly pivot back into a closed position, creating a tight seal.
Biological valves mimic the natural tri-leaflet structure of the aortic valve using flexible tissue leaflets. These leaflets coapt, or come together, to form a seal when closed, offering a flow pattern closer to the native valve. The function is driven by pressure differences, ensuring the leaflets open and close with each heartbeat to maintain forward flow.
Living with a Prosthetic Valve
Adjusting to life with a prosthetic valve involves long-term management dictated by the type of valve implanted. Patients with a mechanical valve must adhere to a lifelong regimen of anticoagulation therapy, typically with a medication like Warfarin. This therapy requires regular blood monitoring, often through an international normalized ratio (INR) test, to ensure the blood is thin enough to prevent clots but not so thin as to cause excessive bleeding.
For those with a biological valve, the main consideration is structural valve deterioration over time. While they avoid the risks associated with lifelong blood thinners, patients must be aware that the bioprosthetic tissue will eventually wear out and may necessitate a second intervention. Regardless of the valve type, all patients must maintain awareness of infection prevention, as prosthetic valve endocarditis remains a risk.
Regular follow-up appointments, including periodic echocardiograms, are necessary to monitor the valve’s function and identify dysfunction, such as new stenosis or regurgitation. These imaging tests allow physicians to measure blood flow velocities and pressure gradients across the valve. Patient education regarding symptoms of valve failure and the need for prophylactic antibiotics before certain medical procedures is necessary.