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Mitral valve replacement


Mitral valve replacement is performed when the mitral leaflet tissues are thickened or calcified to the point that repair is impossible, likely to result in inadequate hemodynamic performance of the valve, or unlikely to be long lasting.

Morphology

Valves that do not qualify for repair and must be replaced have often been damaged by the rheumatic process. This manifests as marked thickening and scarring of leaflet tissue, with reduction of leaflet mobility; thickening and shortening of supporting chordae tendineae, fixing the leaflets to the papillary muscles and further reducing leaflet mobility; and calcification of leaflets. Some extreme cases of myxomatous degenerative disease may result in marked deformity of the valve leaflets and thinning and lengthening of chordae tendineae, making repair unpredictable and even undesirable.

Figure 33-1, A Operative specimen of rheumatic-type mitral valve stenosis and regurgitation. The leaflets are thick, and the commissures are fused. There is inadequate leaflet tissue to close the mitral orifice. B Operative specimen of rheumatic-type mitral valve stenosis and regurgitation, viewed from the ventricular aspect to demonstrate leaflet supporting structures. Chordae tendineae are markedly thickened and shortened. C Operative specimen of mitral valve regurgitation due to severe myxomatous degeneration. The specimen shows marked “hooding” of leaflet tissue and inadequate leaflet tissue to close the mitral orifice. D Operative specimen of mitral valve regurgitation due to severe myxomatous degeneration, viewed from the ventricular aspect to demonstrate leaflet support. Chordae tendineae are thinned and degenerated, with no uniformity. Such chordae would not likely support a valve repair for very long. E Operative photograph of rheumatic-type mitral valve stenosis and regurgitation. The mitral leaflets are scarred, thickened, and calcified; the commissures are fused. Repair of this type of valve would not restore leaflet flexibility and mobility to provide adequate hemodynamic performance.

Incisions

Mitral valve replacement operations are performed with the patient on cardiopulmonary bypass under cold cardioplegic conditions. Usually a single (two-stage) cannula suffices for venous drainage. When the need for greater exposure is anticipated or when the operation is being performed along with another procedure on the tricuspid valve or other right heart structure, two venous cannulae are employed. The use of suction to remove pulmonary venous blood from the left atrium is also required. A short, pediatric left atrial drainage cannula with multiple holes passed through the right superior pulmonary vein, across the posterior wall of the left atrium, to the left inferior pulmonary vein works well for left atrial drainage. There are three types of incisions in the left atrium to expose the mitral valve: inferior, superior, and transseptal.

Figure 33-2, A Inferior approach. This incision is the most common method of exposing the mitral valve and can be considered standard. An incision is made in the left atrium just behind and parallel to the interatrial groove on the right side. The pericardial reflection over the inferior vena cava is opened to allow access to the posterior wall of the left atrium. The incision is extended posteriorly behind the inferior vena cava. This incision provides adequate access to the mitral valve in most situations. B Superior approach. The superior vena cava is mobilized to provide access to the roof of the left atrium. An incision is made in the left atrium just behind the interatrial groove over the pulmonary veins. The incision is extended into the superior aspect of the left atrium, behind the superior vena cava and aorta. The incision may extend to the base of the left atrial appendage with retraction of the superior vena cava and aorta. Excellent exposure of the mitral valve is achieved with this approach. The incision will not enter the atrioventricular groove if it is directed high on the roof of the left atrium toward the right pulmonary artery. It should be recognized, however, that there is strong evidence that the incidence of supraventricular arrhythmia is higher with this approach than with inferior incisions. C Transseptal approach. Two venous cannulae and vena cava tourniquets are required. An incision is made in the right atrium parallel to the atrioventricular groove. The incision passes medial to the right atrial appendage and extends along the medial aspect of the right atrium to the junction with the left atrium. It should be recognized that this incision is likely to divide the artery that supplies blood to the sinoatrial node. Normal sinus rhythm may not be present after the operation, so this incision is best reserved for patients with atrial fibrillation that is not being treated concomitantly. D The atrial septum is opened through the foramen ovale. The septal incision is extended superiorly to join the right atrial incision. The incision is then continued over the roof of the left atrium to provide a wide opening. The incision may be carried to the base of the left atrial appendage and should be directed toward the right pulmonary artery to keep it high on the roof of the left atrium and away from the atrioventricular groove. E A self-retaining retractor is placed in the tricuspid orifice to pull the right ventricle anteriorly. Retraction stitches are placed on the atrial septum rather than using mechanical retraction of the septum; this avoids pressure on the atrioventricular node and the bundle of His. This incision provides the best possible exposure of the mitral valve. It is especially useful in mitral valve reoperations and for procedures combined with tricuspid valve operations.

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