When considering a valve substitute you must consider the original design. The trileaflet aortic valve with coronary sinuses allow maximum valve opening with minimal turbulence on the leaflets and distribute the closing energy evenly across the leaflet. When this design is altered even in our own valves such as a bicuspid valve, early leaflet degeneration and calcification occur. Therefore, any substitute must take these design characteristics into consideration. What we are going to discuss today is the use of stentless biologic valves, both human and animal, in the pursuit of the perfect aortic valve replacement.

One of the earliest attempts to alleviate the stenosis created by replacing the aortic valve was the aortic homograft. This was pioneered by Dr. Barrett-Boyes and Dr. Mark O’Brien in New Zealand and Australia respectively. The problems with homografts in the 70s were related to difficulties with procurement and longterm storage. The sterilization techniques commonly damaged the valve and epithelium and resulted in early valve failure. Many of the initial valves were done as scalloped subcoronary implants and due to the flexibility of the valve and flimsiness of the tissue it was difficult to orient the valve properly resulting in a high rate of insufficiency. Methods of preservation with antibiotic solutions and long-term storage with cryo techniques have greatly improved the longevity of the valve. At this point aortic homografts are expected to last 15-18 years, 80 percent of the time. They still, however, suffer some unique problems. The surgical implantation is somewhat complex and requires a fair amount of experience. Also, finding adequate sized homografts is often difficult since they have to be donated as with any human organ. Over time the wall of the homograft aorta surrounding the valve becomes severely calcified creating difficulties at the time of subsequent replacement and in effect stenting the valve leading to failure of the valve leaflets.

In 1967, Sir Donald Ross at Guys Hospital performed the first operation where a pulmonary valve was removed and used to replace the diseased aortic valve. The pulmonary valve was then replaced with a pulmonary homograft valve conduit. Mr. Ross felt that the pulmonary valve would be an ideal replacement for the aortic valve since they are both very close in size, shape and configuration. The pulmonary valve is a perfect trileaflet valve and in utero actually carries more pressure-load than the aortic valve. The pulmonary tissue is the same as the patient’s tissue and has been shown to remain alive after reimplantation. Dr. Ronald Elkins in Oklahoma has shown growth in the pulmonary autograft after implantation in children. The first implant done in 1967 by Mr. Ross was still living in 1995, 28 years later, with excellent valve function. Mr. Ross’ original series reported in 1996 at 20 years showed only 15 percent of the patients requiring another valve operation and patients’ survival at 20 years was 80 percent. Unfortunately, there are some disadvantages with the procedure. Due to the magnitude of the operation it requires a relatively long operative time and is a technically demanding procedure requiring precise suturing techniques to avoid complications. This discourages other complex procedures from being performed simultaneously such as coronary artery bypass or mitral valve surgery. Double valve pathology is also created since the pulmonary valve has to be replaced with a pulmonary homograft. The pulmonary homograft has a higher rate of failure than the pulmonary autograft and most replacements are as a result of failure of the pulmonary homograft. It also cannot be used in patients with a connective tissue disorder such as Marfan’s syndrome.

More recent developments in the use of stentless technology have centered around utilizing porcine xenografts. There are three valves of this type that receive the most attention. The CryoLife 300 is a glutaraldehyde fixed porcine xenograft that consists of three separate noncoronary cusps sutured together to form a trileaflet valve. This valve has been primarily evaluated by Dr. Mark O’Brien in Australia and has had good results in his hands. However, it has not yet received wide use and has not been approved in the United States for implantation. Two other porcine xenografts have, however, been approved in the United States for implantation and are being used world-wide. These include the St. Jude Toronto SPV (stentless porcine valve) that was developed by Dr. Tirone David in Toronto, Canada. It is a glutaraldehyde, low-pressure fixed porcine valve that has been denuded of most of its aortic wall and covered with a thin Dacron sheath. It is inserted as a subcoronary scalloped valve with a proximal and distal suture line. The other porcine xenograft is the Medtronic free-style valve which is a porcine xenograft valve conduit. The leaflets have zero pressure fixation and have been treated with Alpha-aminooleic acid to prevent calcification, however, the wall of the aorta is not effectively treated with this substance. It can be used as a mini-root replacement or scalloped and used as a subcoronary implant. Insertion consists of a proximal suture line with a distal suture line either as a root replacement or as a scalloped subcoronary inclusion cylinder. All of these valves have shown reproducible implantation with low complication rates and no increase in mortality over standard aortic valve replacement. They are showing superior hemodynamic performance, particularly in the small aortic root. Unfortunately, they have been in use for only around eight years and their longevity is unknown. Due to the maintenance of the normal aortic physiology and flexibility of the aortic root it is felt that these valves could last 15 years or longer. There have been some implants in the younger age group (less than 65 years old), and so far at 7-8 years there is no evidence of increased calcification.

The perfect device for aortic valve replacement should provide the following: normal function, minimal changes in lifestyle, normalization of left ventricular function, normal longevity and low operative risk. Stentless technology, although not perfect, is allowing us to come closer to achieving these results. These valves allow us to get a much larger effective valve orifice especially in patients with small aortic annuli. They have normal flow characteristics and very low gradients at normal activity levels. The flow is laminar in nature since the valves have no stent to obstruct flow and also maintain normal aortic architecture. Since there is only tissue in the bloodstream with a small amount of suture material, the only anticoagulation recommended is one aspirin per day. Recent data shows that due to the low pressure gradients and normal function there is significant left ventricular mass reduction which occurs continuously over the first year. This is much more prominent than with standard valve replacement. The only major question remains longevity; however, it is felt that they should do substantially better than the stented valves due to the decrease in stress placed on the leaflets by removing the stent. Operative risks have continued to be low. In particular with the subcoronary implant it is essentially the same as with any standard stented aortic valve.

Stentless technology is continuing to change. Future advances will include better anti-mineralization techniques that will allow a xenograft root replacement that will have both the valve leaflets and the wall of the aorta treated such that calcification should be decreased substantially. Silver impregnation with a Dacron coating and suture material is being looked at to decrease the risk of postoperative SBE.

At the present time the indication for various valves are as follows. Mechanical valves are primarily used in patients under-65 years of age in which anticoagulation is not a contraindication. It should also be used in any patient with renal failure since the calcium metabolism is increased and causes early calcification of biologic valves. In addition, any patient over the age of 65 who is going to require anticoagulation should be considered. Stented bioprosthetic valves are primarily reserved for patients in the over-65 age group due to early calcification in younger patients. Although they do not require chronic anticoagulation as long as they are in sinus rhythm, most surgeons do anticoagulate them with Coumadin for the first 6-8 weeks and then switch to one aspirin per day. Pulmonary autografts (Ross procedure) are primarily used in patients in the under-55 age group although can be used in older patients. They require no anticoagulation other than one aspirin per day. At present this is the biologic valve of choice in patients under 45 years of age. Aortic homografts are primarily used in patients over the age of 45 since they still have a 15-18 year longevity and have increased calcification in younger patients. They are particularly useful in patients with Marfan’s disorder, aortic dissections, small aortic root, and SBE. They require no anticoagulation other than one aspirin per day. Nonstented bioprosthetic porcine xenografts are being used in patients in the over-45 year age group. They require no anticoagulation and are particularly useful in small aortic roots. At present the long-term results are not available; however, eight year data shows very satisfactory performance and no evidence of early calcification in patients under 65 years old. In a few cases where these valves have required removal, their replacement has been relatively easy in that the valve itself is not difficult to remove.

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