Distal extension of the allograft is seldom required, usually only in cases with nonconfluent central pulmonary arteries or when a long LV-PA conduit is required. The use of proximal extension in all cases of RVOT reconstruction incorporating a valved allograft conduit has been based on the following principles: 1. Placement of the allograft valve cephalad to the native position to reduce sternal compression. 2. Use of a complete cylinder of prosthetic material at the base of the allograft to limit root dilatation. 3. The use of a knitted, collagen-impregnated Dacron tubular prosthesis as the extension. The need for a competent valve to protect the right ventricle from volume or pressure overload is a fundamental reason for implanting an allograft valve conduit. Insufficiency of the allograft valve can occur early or late postoperatively and is related to either distortion by the sternum, dilatation of the valve root after implantation or leaflet degeneration. Valve obstruction may also result from sternal compression or leaflet degeneration in the early or late postoperative period. Children requiring a valved allograft will have significant right or biventricular hypertrophy that narrows or obliterates the retrosternal space, most easily recognized on a lateral chest film.When the allograft is sutured directly into the right ventricular outflow tract, the bulk of the muscular base of the allograft and the enlarged heart displace the valve anteriorly, resulting in compression of the allograft by the sternum at the time of chest closure, particularly in infants and small children. The distortion of the natural circular valve structure alters leaflet coaptation, producing insufficiency or reduces the effective cross-sectional area, producing stenosis. These changes can be demonstrated by color Doppler flow imaging. We have observed that sternal compression may produce insufficiency without sufficient compression to cause restriction to antegrade flow. By incorporation a short cylindrical segment of Dacron, the allograft valve will sit cephalad and posterior to the upper end of the right ventriculotomy and will also be angled more toward the distal anastomosis to the native pulmonary artery. The allograft valve will be less susceptible to sternal compression in this position. Additionally, the complete ring of Dacron sewn to the muscular base of the allograft will fix the diameter of the root and reduce early or late dilatation and valve insufficiency. The length of the complete cylinder of Dacron varies, depending on the size of the patient, from 6 to 20mm. The use of the long obliquely cut Dacron extension below the cylindrical portion for anastomosis in the RVOT also removes the trifurcated suture lie and potential sites of troublesome bleeding where the ventricular wall, the base of the allograft and the head meet using the conventional technique.1 Early experience with woven Dacron conduit containing a biologic valve was unsatisfactory because of the development of a thick, fibrous, obstructing pseudo-intima immediately proximal to the valve.2,3 It was inferred that the use of Dacron extensions proximal to an allograft valve conduit would meet a similar fate. Haveric and co-workers in Europe demonstrated that the use of high porosity knitted Dacron allowed fibrous ingrowth and anchoring as well as vascularization of the pseudo-intima,4 markedly reducing the risk of obstruction. During our early experience in Canada and the United Kingdom, bleeding from the porous knitted Dacron conduit was managed with a commercial form of fibrin glue applied to the conduit immediately prior to use.5 Unfortunately, the commercial fibrin sealant was unavailable in the United States, and less satisfactory sealant techniques were employed. Since 1990, a collagenimpregnated knitted Dacron vascular graft (Hemashield, Meadox Medicals, Inc., Oakland, NJ) has been used widely with excellent handling characteristics and hemostasis.2,6 This conduit is now used exclusively in our practice for this procedure. the valve commissures, leaving adequate allograft tissue to augment the PA confluence as necessary. 6. With the distal end of the allograft held against the native PA confluence, the posterior wall of the cylindrical portion of the Dacron extension is then marked where it meets the upper end of the defect in the RVOT. The Dacron extension is tailored obliquely from this point to a length 5-8 mm longer than the right ventriculotomy. 7. The distal anastomosis between pulmonary artery confluence and allograft conduit is completed with running 4-0 polypropylene suture. The proximal anastomosis between the hooded Dacron extension and right ventricle is then completed with a running 4-0 polypropylene suture and appropriate tailoring of Dacron to fit the defect.
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