AORTIC VALVE REPLACEMENT OPTIONS
1.1. Pulmonary Autograft (The Ross Procedure)
This is the patients own pulmonary valve. The first pulmonary autotransplant was performed by Dr. Donald Ross at Guy’s Hospital on June 8, 1967, (thus the name the Ross Procedure) and the patient was still doing well 28 years later.
The Ross Procedure considers the patients own pulmonary valve to be the ideal replacement valve for several reasons:
- Normal aortic and pulmonic valves are both identical in size, shape and configuration. Both are trileaflet valves.
- Even when the aortic valve is deformed, the pulmonary valve is almost always created as nature intended. Thus the pulmonary valve provides the closest identical twin possible for the aortic valve.
- The pulmonary valve is usually within a millimeter or two of being the exact right size, is always sterile, and is comprised of living tissue.
- Since the autograft comes from the same individual, rejection is never a concern.
Results: The pulmonary autograft transplanted to the aortic position was not calcifying, leaking, or showing signs of degeneration. At 20 years, only 15% of patients required another valve operation, usually replacement of the homograft reconstruction of the right ventricle [Ross 1996]. Patient survival at 20 years is an impressive 80%.
1.2. Mechanical (prosthetic) Valves
A mechanical valve is made from artificial materials. Most commonly, the mechanical valves are made from a material called pyrolitic carbon. This is pure carbon heated and compressed in a trade secret process. The final smooth surface is obtained by layering carbon formed into a gas onto the surface of the compressed carbon. The resulting leaflets are very smooth and resistant to thrombus formation (i.e. clot). Some components of modern mechanical valves are made from titanium or a multi-metal compound called Elgiloy.
The first of these was the Starr-Edwards design, a functioning heart valve made from a steel cage enclosing a silicone rubber ball. Developed in 1961, it was first used to replace the human mitral valve, but had limited success when used as an Aortic valve replacement. One of the more commonly used valves today is the St. Jude bi-leaflet valve.
The major disadvantages of these valves are:
- Failure to grow in concert with the growing child.
- Lifelong risk of clotting on the valve, leading to stroke
- Lifelong risk of clots which freeze the mechanical leaflets
- Requirement for lifelong blood thinning agents (i.e. Coumadin)
- Complications of anticoagulation (i.e. spontaneous bleeding)
- Audible noise when the valve closes (clicking sound)
- Risk of late infection (prosthetic endocarditis)
- Some mechanical are not large enough for the recipient, resulting in stenosis.
Results: The patient implanted with a mechanical aortic valve continues to run some long term risks of either a stroke or major bleeding episode in the range of 4 to 8% (combined) per year. In addition, the long term survival is 60% at 15 years, with most deaths related to non-cardiac causes.
1.3. Animal Tissue (xenograft) Valves
These first appeared in 1972. The porcine (pig) aortic valve is the most widely used. The pig valves are made from animal heart valve tissue and thus are relatively free from the risk of clotting. This is an advantage in some patients who cannot take anticoagulation medication (pregnant women, patients with brain aneurysms, bleeding intestinal sources, etc.)
The porcine valve is tanned with a stringent chemical called glutaraldehyde, an agent similar to, but stronger than, formaldehyde. This processing toughens the leaflets and cross-links many of the proteins. In doing so, the valve is also rendered less likely to be rejected. However, the tanned leaflets are no longer living tissue.
Results: Calcification of the valve is a problem, and animal tissue valves usually begin to degenerate within 8 to 10 years after implantation. In patients less than 40 years of age, animal tissues valves may degenerate within 5 years of implantation.
1.4. Aortic Homografts
A homograft valve is a human cadaver valve (also called allograft). These valves historically have had problems with sizing and suturing of the valve, along with the expected problems of procurement and long term storage.
Although methods of preservation and long-term storage have improved, these valves have suffered from unique problems when used as an Aortic replacement, such as structural deterioration, difficult implant surgery, and occasional size mismatching.
Results: These valves will last an average of 15 years. Over time, the wall of the homograft aorta surrounding the valve becomes severely calcified, creating increased surgical difficulty at the time of subsequent replacement. Rejection of the aortic tissue may play a role in this form of degeneration.
2. GENERAL QUESTIONS ON THE ROSS PROCEDURE
2.1. What are the disadvantages of the Ross Procedure?
No matter how much improvement has been made in the surgical techniques for performing the pulmonary autotransplant procedure, there are still some drawbacks.
One disadvantage is the magnitude of the surgical procedure. It is a very technicaly demanding surgery.
Some critics are concerned that transferring of the pulmonary valve converts a patient with only aortic valve problems to a patient with 2 heart valves now in jeopardy. The potential for premature degeneration of the pulmonary substitute has also been voiced. Fortunately, the long term results now available tend to refute both of these concerns. Results have shown that premature failure of the homograft valve used to replace the pulmonary valve is much more common when human aortic tissue was used in the right side reconstruction, as opposed to human pulmonary tissue. There was only a 74% freedom from reoperation at 5 years when aortic homografts were used for the right ventricular reconstruction. However, there is a 94% freedom from reoperation at 5 years (and 83% at 20 years) when a pulmonary homograft is implanted into the right ventricular outflow tract. The reasons for this dramatic difference are not known.
In summary, the major disadvantages of this operation for the surgical treatment of aortic valve disease are outlined below.
- Lengthy operative times (over 2 and 1/2 hours to complete the repair)
- Technically more demanding on the surgeons skills.
- Cannot be used in every patient (such as those with Marfans syndrome or other connective tissue disorders)
- Cannot be effectively combined with other valve operations or coronary bypass (due to the lengthy operative times required).
- Limitations in availability of appropriate sized pulmonary homografts for replacing the donor site (particularly in children).
- Theoretically converts a single valve patient to a double valve patient.
2.2. What is the ideal candidate for the Ross Procedure?
- Who are less than 55 years of age (at the time of anticipated surgery)
- Who have a life expectancy of 20 years or more
- Who would have problems with being on anticoagulation medication, such as a history of bleeding. (regardless of age)
- Who are women of childbearing years
- Who have no other major cardiac lesion needing correction (multivessel coronary disease, mitral disease).
- Have isolated aortic valve pathology.
- Atheletes or the young for whom anticoagulation would carry a higher risk.
The accepted age range for patients having the pulmonary autograft procedure has expanded significantly. Many surgeons view the procedure as a solution for congenital aortic stenosis that can be performed early in life, and quite possibly eliminate the need for multiple aortic valve replacements. As increasing durability of the operation is evidenced, older patients with more active lifestyles become candidates for the procedure. The pulmonary autograft procedure is certainly indicated for young women of child-bearing potential, due to the problems associated with the use of blood thinning agents (Coumadin) during