Homograft Aortic Valve Replacement

Dateline:06/15/97

In last week's article I discussed the role of valve repair and mechanical prosthetic valve replacement for aortic stenosis. This week, the aortic homograft is the focus of attention.

A homograft (also called an allograft) is a organ or structure taken from a another member of the SAME species. For instance, an aortic valve may be removed from a human cadaver, rendered sterile by special methods, and then implanted in another human with a severely diseased valve.

Early History of the Homograft

Nature's wonder simply cannot be duplicated by Man.

As early as 1962, Donald Ross and Sir Brian Barratt-Boyes realized this when they attempted to replace diseased aortic valves with aortic homografts. Unfortunately, due to an incomplete knowledge of cell biology, and the rather crude preservation methods prevailing then, the long term durability of the valve was found to be inadequate, and the procedure abandoned.

It was in the mid-1970s that interest was again awakened in the aortic homograft, when the realization set in that artificial prosthetic valves had a considerable morbidity. Mark O'Brien pioneered the return of the homograft, with the new preservation technique called "cryopreservation".

Where are homografts taken from ?

An aortic homograft is harvested from a human heart. Since life is not possible without an aortic valve, a homograft can only be taken from two sources

• Human Cadavers
• Human Heart Transplant Recepients with otherwise healthy aortic valves

While in earlier days, these valves were harvested in the autopsy room, in recent times the removal of homograft tissue is done in "clean" conditions either in the operation theater or a sterile annexe to the autopsy room.

How are homografts preserved ?

The earliest method of sterilization of homografts was to place them in a solution containing a mixture of antibiotics. Repeated cultures of the solution were performed to confirm the absence of any infection, before using the valve for replacement in humans. This method is simple, but cannot be used to preserve valves for a long period. Cryopreservation is a more complex and expensive method using liquid nitrogen to freeze-dry the valve at temperatures as low as minus 42 degrees C. The advantage however is that the individual cells retain their viability allowing indefinitely long periods of preservation. This allows a center to develop a "valve bank" where valves of different sizes can be stored, allowing a choice of valves for surgeons.

The Operation

The homograft aortic valve replacement operation is technically more difficult than a prosthetic valve replacement. After excising the native diseased valve, the homograft is sutured in its position using any one of a variety of techniques. Numerous modifications of technique have been reported to suit the specific anatomy of each kind of patient. In the earlier days of homograft replacements, some situations like distorted aortic roots, large aortic valve rings and asymmetric enlargement of the aorta were considered unsuitable for homografting. Today however, there are very few contra-indications to using this operation (e.g. heavily calcified aortic root, gross distortion of the aortic ring).

What are the advantages of a homograft valve ?

As it replaces a native diseased valve with another natural human valve, the homograft operation uses the best replacement device ever - the one created by Nature. The homograft valve provides the best hemodynamic performance, with hardly any pressure gradient across it even during peak blood flow. There is no risk of blood clots forming on the valve, and so it does not need any anticoagulant (blood thinner) medication. The valve has been found to be durable even for ten years or more after implantation. It is highly resistant to infection and can be used even in the presence of infective endocarditis.

What are the drawbacks of a homograft ?

The operation itself is technically more difficult, and minor aortic valve leak may occur due to improper suturing of the valve. The results vary in different centers depending on the individual surgeon's experience with this technique.

When is a homograft the preferred choice ?

In any patient below the age of 45 years, the homograft valve must be considered as a choice to replace a diseased aortic valve, in view of the advantages mentioned above. In the presence of infective endocarditis of the native valve, the homograft is the valve of choice. When anticoagulant medication is contraindicated for other reasons (e.g. active peptic ulcer disease, severe high blood pressure, diabetic eye disease), the homograft is useful since there is no need for blood thinners.

In the next week's article, we will discuss the other option available for aortic stenosis - the Pulmonary Autograft (or Ross procedure).