Available Options for Heart Valve Replacement
General Description of Available Products
Mechanical heart valve prostheses today have two leaflets (opening and closing parts of the valve) housed inside a hollow tube called the orifice with a sewing cuff on the outside to provide a means of attachment to the patient’s heart tissue (Figure 4). The two leaflets are held inside the orifice by leaflet tabs inserted in “hinge” (pivot) areas of the valve.
|Figure 4. Components of a mechanical heart valve and unique features of the On-X Prosthetic Heart Valve manufactured by On-X Life Technologies, Inc. in Austin, Texas.
Click to enlarge figure.
Valve parts are made of different materials including pyrolytic carbon for the valve orifice and leaflets, woven or knitted material (polyester, polytetrafluoroethylene) for the sewing cuff and radio-opaque metal rings (titanium) used to attach the cuff to the orifice. Also, small particles of tungsten within the leaflets make them visible during X-ray (radio-opaque). Pyrolytic carbon is a special carbon coating on the valve parts that contact blood. Older valve designs use pyrolytic carbon with silicon carbide inclusions in the structure. Benefit of latest design: For the On-X Prosthetic Heart Valve, the components are fit together in a design that mimics as closely as possible the function of the natural valve (Figure 5).
|Figure 5. Natural design of the On-X Prosthetic Heart Valve|
The On-X heart valve is unique because it is the latest and most advanced bileaflet mechanical design by Jack Bokros, PhD (Figure 6), the originator of carbon technology for producing implantable carbon devices. Dr. Bokros' work with carbon began in the early 1960's at General Atomic Company during research to develop special coatings for nuclear fuel particles. One of the coatings, pyrolytic carbon, had very beneficial blood contact properties and was destined to revolutionize the mechanical heart valve replacement field. Using this technology in 1977, his group at General Atomic produced the carbon components for the first viable bileaflet heart valve replacement, the St. Jude Medical device. In the late 1970's, the Bokros group established CarboMedics, Inc. In the mid-80’s, Dr. Bokros patented two valve concepts that were used to produce the CarboMedics and ATS Medical valve replacements.
|Figure 6. Jack Bokros, PhD, pioneer of implantable carbon technology with the On-X Prosthetic Heart Valve and founder of Medical Carbon Research Institute, LLC (MCRI).|
Using the experience extending from 1963, in 1994 Dr. Bokros founded Medical Carbon Research Institute to utilize a new pure form of isotrophic carbon, On-X Carbon, to develop a mechanical valve with more natural qualities – the On-X Prosthetic Heart Valve. On-X Carbon does not contain the silicon carbide of older valve models – silicon carbide causes more blood clots (thrombogenesis) than pure carbon. It is also stronger than other pyrolytic carbons and is necessary for the unique structure of the On-X valve. In October 2005, Dr. Bokros received the Excellence in Surface Sciences Award from the Surfaces in Biomaterials Foundation. Features of the On-X valve (Figure 4) not included in other mechanical designs include:
|Feature||Benefit to Patient|
|On-X Carbon||Pure pyrolytic carbon–less thrombogenic than silicon carbide|
|Leaflets that can open to 90°||Less turbulence, less blood damage, fewer complications|
|Near natural orifice length||Less turbulence, less blood damage, fewer complications|
|Inlet flare||Less turbulence, less blood damage, fewer complications|
|Two point leaflet contact||Less noise, less turbulence, less blood damage|
|Greater pivot washing||Less chance of clotting|
|Actuated pivot||Leaflets operate efficiently even under low flow4|
|Shorter leaflet closing angle||Less turbulence, less blood damage, fewer complications|
|Leaflet guards||Less scar tissue ingrowth (pannus) or leaflet impingement problems, lower complications|
This unique On-X design reduces internal turbulence that is characteristic of older bileaflet valves (Figure 4 and 5).5 Why is this important? Turbulence can damage red blood cells and other blood components; damaged cells can activate the clotting mechanism of the blood and clots can result. Reduction of turbulence in the On-X design is accomplished by a flared inlet (opening), leaflets that can open to 90° and a more natural length of the valve wall (orifice). Therefore, the On-X heart valve produces fewer complications than other valves. A comparison of complication rates is available in the latter part of this guide.
Tissue prostheses (valves) (Figure 3 and Table 2) are manufactured using several different methods. The materials most widely used are chemically-treated pig (porcine) heart valves or cow (bovine) pericardium (the pericardium is a sac of tissue that surrounds the heart). Human heart valves (homografts) that have been preserved by freezing are also available but in very limited numbers. Each type of tissue valve has its advantages and disadvantages. Below is a description of the general types of tissue valves. You should discuss these options thoroughly with your surgeon and cardiologist. The pro’s and con’s of each type of valve are discussed in the following section of this guide. Current brand names of tissue valves are reviewed in Table 1.
- Stented tissue heart valves are cow or pig tissue that has been chemically treated and sewn to a stent (frame usually made of metal wire or plastic) that allows the valve leaflets to mimic the natural valve.
- Stentless tissue heart valves are pig aortic roots (aortic valve and piece of aorta) that have been chemically treated and reinforced with a pliable exterior cuff.
- Homograft heart valves are human aortic roots that have been harvested from donors and deep frozen (cryopreservation).
Ross procedure (pulmonary autograft) is the use of the patient’s own tissue to replace his/her aortic valve and a segment of the aorta.6 The circulation on the right side of the heart includes a pulmonary valve and pulmonary artery that are structured almost identically to the aortic valve and aorta (left side of your heart). The surgeon removes this pulmonary root and transplants it to the aortic root. He/she will replace the pulmonary root with a homograft or possibly a stentless valve.7 Not all surgeons are comfortable with this procedure but can refer you to surgeons who perform them frequently. A potential disadvantage to the Ross procedure may be an extended length of surgery and increased risk of operative complications.6 The On-X Prosthetic Heart Valve offers an advanced mechanical choice that incorporates the best of both mechanical and tissue options. Whatever valve you choose, heart valve replacement surgery can safely extend life and improve quality of life for those who need it (Table 2). To find out more, please read this entire guide. Table 2. Options for Heart Valve Replacement*
|Two leaflets (silicon-alloyed carbon) that do not open to 90°||Leaflets from cow or pig tissue treated with a preservation agent||Two leaflets (On-X carbon) that open to 90°|
|Short cylindrical carbon orifice||Plastic or metal frame to mount tissue||Near natural length/shape carbon orifice|
|Sewing cuff of synthetic fabric||Sewing cuff of synthetic fabric||Sewing cuff of synthetic fabric|
|______||OR, aortic root of pig or human donor that may be reinforced with synthetic fabric||Advanced features not included in other mechanical designs|
*References for these summary tables are found in the following text.