One of the hardest working muscles in the body is heart muscle: In a normal lifetime, the human heart beats about 3.5 billion times, give or take a few million. So it shouldn’t be a surprise that some parts of the heart wear out, and in particular, the heart valves take a pounding — from disease, structural defects, calcification, and normal wear and tear.
Each year, about 300,000 people need new valves. Valvular heart disease can be divided into two main types: stenosis, which means the outflow track diameter is smaller and makes the pumping of blood more difficult for the heart, and regurgitation, which is when the valve does not close tightly and allows for the backflow of blood during diastole.
Many people with valvular disease require surgery, either to repair or to replace the diseased valve once stenosis or regurgitation begins to be a threat to life. The ultimate treatment is a heart valve replacement, first performed in 1952.
There are two broad categories of replacement heart valves: mechanical and bio-prosthetic. Mechanical valves, also called artificial valves, are made of metal and other materials. They are also subject to clotting complications that require long-term anticoagulation treatment.
Bioprosthetic valves also come in two types: human valves and modified valves from animal donors. These valves are less likely to cause blood clotting than are mechanical replacements, and also are designed to function like natural heart valves. Their greatest limiting factor is that they tend to wear out and need to be replaced about every 10 years.
In 1998 the American Heart Association issued guidelines that recommended that most patients age 65 and older be given a bio-prosthetic valve because the risk that clotting carries (strokes and pulmonary emboli) is greater than the risk from having a second valve replacement.
Despite this recommendation, a study using Medicare data found that overall, only about 50 percent of patients undergoing a heart valve replacement received a bioprosthetic valve. Further analysis showed that the top high volume valve replacement facilities were 2.3 times as likely to implant a bioprosthetic valve than were the lowest volume hospitals.
Calcification is a risk associated with bioprosthetic valves, but advances in medical technology are leading to improvements.
In early December 2007, the United States Food and Drug Administration approved the sale of a new heart valve that fights calcification. The St. Jude Medical Epic heart valve combines an anticalcification technology with a valve design that has been around for more than 15 years.
The Epic boasts a triple composite construction devoid of the septal muscle bar. Lacking the septum allows this heart valve to maximize blood flow and removes a potential calcification site. It also has a low-profile design that facilitates implantation and enhances blood flow in both aortic and mitral positions.
First of all, glutaraldehyde treatment fixes the bioprosthetic valve by performing three tasks: It sterilizes the valve tissue, renders the tissue bioacceptable by destroying antigenicity, and stabilizes the collagen crosslinks for durability. Bioprosthetic valves are typically stented, meaning the animal tissue is supported by a polyester-covered plastic frame. A sewing cuff is attached to the valve and enables the surgeon to sew the valve in place.
But the patented new technology, termed Linx technology, that is applied is the key breakthrough. After the animal bioprosthetic valve has been fixed with glutaraldehyde, an ethanol treatment is applied. This reduces a number of risks.
It reduces glutaraldehyde toxicity, a known calcification risk. Ethanol also removes 99 percent of cholesterol and 94 percent of the phospholipids that are potential binding sites for tissue calcification. It also results in a more stable collagen helix.
In sheep models, the use of the Linx technology demonstrated excellent resistance to leaflet calcification compared to glutaraldehyde controls. The Linx technology competes with a technology named ThermaFix, which is used by a major competitor of SJM, Edwards Lifesciences. ThermaFix is a proprietary thermal technology that also aims to reduce calcification through a two-step process to reduce glutaraldehyde and phospholipids. There is no head-to-head study to compare the two processes available at this time.
The Epic valve will be sold in the United States this year. Complete data for more comprehensive evaluation are unavailable at this time, but given the FDA’s recent reluctance to approve advances in technology without compelling evidence, it is likely that this new approval has acceptable clinical support.
Obviously, as with most implants, managed care organizations seldom become involved with reviewing the actual type of implant. For this reason, this valve is unlikely to be evaluated by most plans. But the SJM Epic valve demonstrates the march of medical technology and the remarkable amount of competition in medical science. It also ensures that each of us will benefit from Tomorrow’s Medicine. -
