Surgery of the Mitral Valve

Degenerative and myxomatous changes are the most common cause of mitral valve disease in North America and Western Europe today. These changes affect the leaflets and subvalvular apparatus, leading to mitral regurgitation. As the population ages, surgeons are seeing more patients with mitral insufficiency secondary to calcific mitral valve diseases.

Rheumatic fever continues to be the major cause of acquired valve disease worldwide. Rheumatic fever results in pancarditis, but the pathologic effects are noted predominantly on the endocardium and cardiac valves, particularly the mitral valve. During the acute phase of myocarditis, the left ventricle dilates, which causes stretching of the annulus of the mitral valve. The mitral insufficiency thus produced is temporary and disappears when the left ventricle regains its normal function. Rheumatic heart disease is a chronic and progressive condition. The earliest permanent change is the fusion of the commissures, followed by thickening and fibrosis of the valve leaflets. These pathologic events are responsible for the creation of the turbulent flow that, together with the continuing rheumatic process, further enhances the progression of the disease and eventual involvement of the subvalvular apparatus. The chords and papillary muscles become thickened, shortened, and fused to each other and to the mitral leaflets. A continuous cycle of progression of pathologic changes and increasingly disturbed flow is therefore created, eventually leading to severe mitral valve disease, notably mitral stenosis or mixed stenosis and insufficiency with or without calcification.

Functional mitral regurgitation may be caused by ischemic or nonischemic cardiomyopathies. The leaflets and subvavular structures are normal but leaflet coaptation is prevented by annular dilation, left ventricular wall motion abnormalities or generalized cavity dilation, and/or papillary muscle dysfunction. Ischemic heart disease and myocardial infarction may also lead to ischemic mitral valve prolapse due to papillary muscle or chordal injury.

Bacterial endocarditis can affect both normal and abnormal heart valve leaflets. The infection may burrow through and invade the mitral valve annulus. Infrequently, the endocarditis extends to the aortic valve and/or the subvalvular apparatus of the mitral valve. It can destroy the mitral valve leaflet configuration, resulting in gross mitral valve insufficiency.

SURGICAL ANATOMY OF THE MITRAL VALVE

The mitral valve forms the inlet of the left ventricle. It consists of two leaflets: the anterior (aortic) and posterior (mural) leaflets, which are attached directly to the mitral annulus and to the papillary muscles by primary and secondary chordae tendineae. A series of chordae tendineae originates from the fibrous tips of the papillary muscles and inserts into the free edges and the undersurfaces of the mitral leaflets, thereby preventing the prolapse of the leaflets into the left atrium during systole and contributing to the competency of the mitral valve. The attachments of the leaflets to the annulus meet at the anterolateral and posteromedial commissures. One-third of the mitral valve annulus provides attachment for the anterior leaflet, and the posterior leaflet arises from the remaining two-thirds of the annulus. Although from the strict anatomic point of view the mitral valve consists of two leaflets, there are multiple clefts within the posterior leaflet. These slits give rise to scallops of leaflet that may prolapse and give rise to valvular insufficiency. Most surgeons and echocardiographers have adopted the classification of Carpentier, which divides both the anterior and posterior leaflets into three functional segments (Fig. 6.1).

FIG. 6.1 Carpentier functional mitral valve components.

FIG. 6.2 Surgical anatomy of the mitral valve.

When the posterior annulus is studied from a strictly anatomic standpoint, it is attached to the left ventricular myocardium through the interposition of a narrow membrane and is therefore actually slightly elevated above the opening of the left ventricle. This subannular membrane extends underneath the posterior annulus to the region of both commissures and merges with the fibrous skeleton of the heart. The anterior leaflet is continuous with the adjoining halves of the left and noncoronary annuli of the aortic valve and also with the fibrous subaortic curtain located beneath the commissure between the left and noncoronary aortic sinuses (Fig. 6.2).

The annulus of the mitral valve is surrounded by many important and vital structures. The nearby left circumflex coronary artery traverses around the mitral annulus in the posterior atrioventricular groove. The coronary sinus also runs in the more medial segment of the same groove. The atrioventricular node and its artery, usually a branch of the right coronary artery, run a course parallel and close to the annulus of the anterior leaflet of the mitral valve near the posteromedial commissure. As mentioned earlier, the remainder of the anterior leaflet annulus is contiguous with the aortic valve. These relationships have significant clinical implications during mitral valve surgery (Fig. 6.3).

FIG. 6.3 Vital structures surrounding the mitral annulus.

Functional mitral regurgitation occurs secondary to annular or left ventricular changes with anatomically normal leaflets and subvalvular structures. One etiology is simple annular dilation due to left ventricular enlargement. In this case, the leaflet motion is normal, but the leaflets are pulled apart, preventing normal coaptation. Localized left ventricular wall motion abnormalities result in displacement of the papillary muscles. This results in apical tethering of the leaflets with restricted mitral leaflet motion in systole. In some patients, both mechanisms contribute to functional mitral regurgitation.

Technical Considerations

Incision

A median sternotomy is the incision most commonly used. Standard aortic and bicaval cannulation is performed. A right anterior thoracotomy with femoral cannulation affords good access to the mitral valve and spares the midline sternotomy incision.

Myocardial Preservation

When satisfactory cardiopulmonary bypass has been established, the aorta is cross-clamped and cold blood cardioplegic solution is administered into the aortic root to bring about prompt diastolic cardiac arrest. Further administration of blood cardioplegic solution is often administered by the retrograde method (see Chapter 3).

Aortic Insufficiency

Satisfactory administration of cardioplegic solution into the aortic root can only be accomplished if the aortic valve is competent. Aortic insufficiency, if present, directs the cardioplegic solution into the left ventricular cavity, causing distension and stretch injury to the myocardium. This can be prevented by administering cardioplegic solution using the retrograde technique alone (see Chapter 3).

Exposure of the Mitral Valve

There are many different approaches for entering the left atrium to provide good exposure of the mitral valve.

Interatrial Groove Approach

The left atrium is opened with an incision just posterior to the interatrial groove (Fig. 6.4). The opening can be extended inferiorly onto the posterior wall of the left atrium.

Fatty Fragments

There is always a variable amount of loose fatty tissue in the interatrial groove. Fragments of fat and loose tissue may enter the left atrial cavity during the atriotomy. Similarly, when the atriotomy is being closed, fatty fragments may invaginate through the closure into the left atrium.

FIG. 6.4 Surgical approach to the mitral valve.

FIG. 6.5 Extension of the incision inferiorly to the back of the heart.

Extension of the Incision

Upward extension of the incision behind the superior vena cava should be avoided because subsequent closure may be difficult. Generous inferior extension to the back of the heart provides satisfactory exposure of the mitral valve in most cases (Fig. 6.5). Closure of this posterior extension of the incision is facilitated by suturing from inside the left atrial cavity under direct vision.

Drainage of Cardioplegic Solution

At least one of the caval snares must be loosened during administration of cardioplegic solution to allow the venous return from the coronary sinus to drain into the oxygenator. If both snares are down, cardioplegia may distend the right heart. If the right atrium is not opened at any time, the cavae do not necessarily require snares around them because venous drainage may be adequate.

Air Embolism

The aorta must be cross-clamped before opening the left atrium to avoid systemic air embolism.

Specially designed retractors are introduced into the left atrium. Optimal exposure is obtained when the retractor held by the assistant pulls the atrial wall at least 1 cm from the mitral annulus upward and slightly to the patient’s left. Many self-retaining retractors are available to improve exposure of the mitral valve. They may be particularly helpful if there is a shortage of assistants in the operating room.

Retractor Injury

Because the atrial wall may be somewhat friable, excessive pull on the retractor may produce a shearing tear of the atrial wall edges, thereby complicating closure. On many occasions, two smaller retractors provide better and safer exposure than a single large one because the assistant is able to divert the pulling force from one retractor to the other to accommodate the surgeon’s view (Fig. 6.6).

Transatrial Oblique Approach

If the left atrium is small, exposure of the mitral valve through the interatrial groove may be suboptimal. In reoperative procedures, dense adhesions may make dissection hazardous, particularly near the region of the interatrial groove. In such cases, an oblique transatrial approach provides excellent exposure of the mitral valve (Fig. 6.7). The aorta is cross-clamped, and cardioplegic solution is administered as before. After the aorta is clamped, an oblique incision is made on the right superior pulmonary vein with a long-handled no. 15 blade. Warm blood will gush out to decompress the left atrium. This will allow expeditious cooling and arrest of the heart.

The vena caval snares are secured. The opening in the right superior pulmonary vein is extended obliquely across the right atrial wall. By gently retracting the right atrial wall edges, the incision can now be extended across the interatrial septum and through the fossa ovalis just inferior to the limbus (Fig. 6.7B). At this time, a retrograde cardioplegic cannula can be introduced into the coronary sinus under direct vision. It can be secured with a fine purse-string suture of Prolene placed on the inside of the coronary sinus ostium, away from the conduction tissues (see Chapter 3). In this manner, retrograde infusion of cardioplegic solution can supplement the antegrade technique.

FIG. 6.6 Use of two small retractors to avoid tearing the atrial wall edges.

FIG. 6.7 A: Incision on the right superior pulmonary vein extends across the right atrium. B: Transatrial incision. Extension of the incision across the interatrial septum just to the limbus of the fossa ovalis. C: For additional exposure, the septal incision is extended along the fossa ovalis.

Overextended Septal Incision

Extension of the septal incision far beyond the anterior limbus of the fossa ovalis may divide the mitral valve annulus, making mitral valve replacement insecure. It could also create a passage outside the atrium into the transverse sinus. The septal incision should therefore terminate just distal to the anterior margin of the fossa ovalis. The septal incision can be extended inferiorly on the fossa ovalis if additional exposure is required (Fig. 6.7C).

The septal edges are retracted with two small retractors. This provides excellent exposure of the mitral valve without distorting it, an important advantage when mitral valve reconstruction is being contemplated (Fig. 6.8).

FIG. 6.8 Retraction of the septal edges to provide exposure of the mitral valve without distorting it.

Transatrial Longitudinal Septal Approach

When there are excessive adhesions from previous surgery, excellent exposure of the mitral valve can be obtained through a longitudinal septal approach. Depending on the size of the right atrium, an oblique or longitudinal incision is made on the right atrial wall. Excellent exposure of the right atrial cavity and interatrial septum is thus obtained. A longitudinal incision is made along the posterior margin of the fossa ovalis and extended both superiorly and inferiorly to provide good exposure of the mitral valve (Fig. 6.9A). The right atriotomy incision can be extended across the base of SVC, onto the roof of the left atrium (Fig. 6.9B). This way, the transatrial septal incision can also be extended into the roof of the left atrium (as far as the left atrial appendage base) to provide excellent exposure of the left atrium (especially when left atrium is not dilated (Fig. 6.9C).

Extension of right atriotomy beyond the base of SVC (transection of SVC) will likely divide the SA nodal artery. Post-operatively, many patients develop junctional rhythm for a few days until the sinus rhythm returns. Few patients may require a permanent pacemaker if there is not a return of sinus rhythm after 7 to 10 days.

Proximity to the Mitral Annulus

The annulus of the mitral valve is at the muscular septal wall most anterior to the fossa ovalis. Therefore, the longitudinal septal incision should be made posterior to the fossa ovalis, leaving a good margin of septal wall between
the opening and the mitral annulus. This segment of the septum is retracted to provide excellent exposure of the mitral valve.

FIG. 6.9 A: Transatrial longitudinal septal approach. B: Extension of right atriotomy across the base of SVC to the roof of the left atrium. C: Transatrial septal incision with extension to the roof of the left atrium (to the base of the left atrial appendage) provides excellent exposure of the mitral valve.

OPEN MITRAL COMMISSUROTOMY FOR MITRAL STENOSIS

Mitral stenosis secondary to longstanding rheumatic fever has continued to be the dominant mitral valve disease affecting large populations worldwide. The disease is now being seen with increasing frequency among immigrants coming to the United States and Western Europe, where rheumatic heart disease had become uncommon.

Mitral commissurotomy can be accomplished safely and precisely under direct vision. With the availability of cardiopulmonary bypass, the closed technique is rarely used today except in third world countries.

A median sternotomy is the incision of choice, although the mitral valve can be approached through either a right or left thoracotomy.

The left atrium is incised by one of the techniques described in the preceding text to expose the mitral valve. The mitral leaflets are identified and, by means of two fine Prolene traction sutures, gently pulled upward toward the left atrial cavity. At times, use of nerve hooks can also provide the same effect. Often this maneuver will stretch the valve leaflets apart and show the line of commissural fusion as a furrow extending between them. If visibility through the valve ostium is adequate, the chords and papillary muscles are examined for evidence of shortening and fusion to each other and, especially, fusion to the undersurfaces of the valvular leaflets.

FIG. 6.10 Technique for open mitral commissurotomy (see text).

A right-angled clamp is introduced through the mitral valve opening and placed directly below the fused commissures. It is then opened gently beneath the leaflets to facilitate incision with a no. 15 blade onto the commissures without severing the chordal attachments (Fig. 6.10). Occasionally, the papillary muscles are fused to the undersurface of the leaflet, making commissurotomy hazardous. With the opened right-angled clamp in place, the commissure is first incised near the annulus; this incision is extended inward over the clamp, cutting vertically into the papillary muscle and the thickened, fused chords for a short distance.

Injury to Papillary Muscle

Care must be taken to divide the head of the papillary muscle fused to the undersurface of mitral leaflets straight along its long axis. Oblique division may weaken or even result in a partial division of the papillary muscle, necessitating its repair or reimplantation or even requiring mitral valve replacement.

Overextension of Commissurotomy

The extent of commissurotomy must be as complete as possible without producing valvular incompetence. If the incision is extended too far toward the annulus, annuloplasty may become necessary (see Mitral Valve Reconstruction section).

CLOSED MITRAL COMMISSUROTOMY FOR MITRAL STENOSIS

Closed mitral commissurotomy is now rarely performed in most Western countries. Consequently, only a few of the current generation of cardiac surgeons have had adequate experience with this technique. Nevertheless, closed mitral valvotomy remains a good operation in selected subgroups of patients, and the long-term results have been consistently satisfactory. In third world countries, closed valvotomy continues to be the preferred form of therapy because of its simplicity and low cost compared with openheart procedures.

FIG. 6.11 Technique for closed mitral commissurotomy.

Technique

A left posterolateral or anterolateral thoracotomy is made through the bed of the fifth rib. The lung is retracted posteroinferiorly, and a long incision is made anterior and parallel to the left phrenic nerve. The pericardium is then suspended with traction sutures. The left atrial appendage is identified and excluded with a side-biting clamp. A purse-string suture of 2-0 Prolene is placed around the left appendage. Another purse-string suture, reinforced with pledgets, is then placed into the apex of the left ventricle. The left atrial appendage is incised within the purse-string suture, and the surgeon’s right index finger is introduced into the left atrium. The mitral valve is palpated to detect calcification, the degree of mitral stenosis, or the presence of insufficiency (Fig. 6.11).

Tear in the Atrial Appendage

The index finger should be introduced gently, without undue pressure. If the atrial appendage tears, it will result in brisk bleeding.

Blood Clot

Preoperative echocardiography is always performed to study the mitral valve pathology and to detect the presence of a blood clot in the left atrial appendage. Nevertheless, before applying clamps to the appendage or introducing a finger into the atrial cavity, the left atrial appendage should be palpated carefully to detect a blood clot. If thrombus is suspected, it should be excluded by clamping the base of
the appendage and then removed. If this is not possible, the closed procedure should be abandoned, and the operation converted to an open valvotomy with the use of extracorporeal circulation.

Mitral Orifice Occlusion

The index finger should not occlude the mitral orifice for more than two or three cardiac cycles to avoid precipitating dysrhythmia and possible cardiac arrest.

When the right index finger is in the left atrium, the heart is elevated with the remaining three fingers and palm of the right hand to bring the left ventricular apex into view. With a no. 11 blade held in the left hand, the surgeon makes a small ventriculotomy within the previously placed apical purse-string suture. This can also be performed by the surgeon’s assistant if desired. This opening is now enlarged with a series of Hegar dilators until it accommodates the diameter of the Tubb valvulotome. The Tubb dilator is then introduced into the left ventricle with the surgeon’s left hand and advanced through the mitral valve into the left atrium. It is then opened quickly to the preset limiting extent of 3.5 to 4.5 cm, closed, and removed. The surgeon’s finger is then removed, and the purse-string suture on the left ventricular apex is snugged down and tied over pledgets.

Premature Opening of the Dilator

It is most important not to open the dilator until the surgeon can feel its tip with the right index finger in the left atrial cavity. Premature opening of the dilator may injure or tear the subvalvular structures and result in mitral insufficiency (Fig. 6.12).

Inadequate Dilator Closure

After completion of the dilation, the dilator must be closed completely before removal. Inadequate closure of the dilator will cause tearing of the left ventricular opening during withdrawal.

FIG. 6.12 Premature opening of the dilator, which may injure or tear the subvalvular structures and result in mitral insufficiency.

Valvotomy Adequacy

Adequacy of the valvotomy and any evidence of a mitral insufficiency jet must be ascertained with the surgeon’s index finger while it is still in the left atrium.

Air Embolism

Every precaution should be taken to prevent air from entering the left atrium or left ventricle during the procedure.

Conversion of a Closed Mitral Valvotomy to the Open Technique

In young adults, the mitral lesion may be fibrotic but elastic and without calcification. The surgeon may find it possible to stretch the orifice maximally with a Tubb dilator only to note that the orifice resumes its previous stenotic size on removal of the dilator. Such patients must be treated with open mitral commissurotomy.

Standby Cardiopulmonary Bypass

It is always a prudent precaution to perform this procedure with a heart-lung machine available on standby so that the surgeon will have the option of using cardiopulmonary bypass if it becomes necessary. Venous drainage can be accomplished through a cannula placed in the main pulmonary artery. Arterial return may be through a cannula either in the descending aorta or femoral artery. A left atriotomy will give excellent exposure of the mitral valve (Fig. 6.13).

FIG. 6.13 Left atriotomy for exposure of the mitral valve if cardiopulmonary bypass becomes necessary.

MITRAL VALVE RECONSTRUCTION

The mitral apparatus includes the leaflets, annulus, chordae tendinae, papillary muscles, and the left ventricle. Mitral valve incompetence can be the result of annular dilation, leaflet abnormalities, chordal elongation or rupture, papillary muscle injury or displacement, and/or alterations of left ventricular size, shape, or wall motion. For this reason, it is necessary to examine and evaluate every aspect of the mitral valve complex in detail so that efforts at valvular reconstruction will be fruitful. The shape and size of the annulus is noted. Nerve hooks or forceps are used to determine the pliability and motion of the leaflets. The leaflet motion is classified as normal (type I), prolapsed (type II), or restricted (type III). The chords and papillary muscles are then assessed. Mitral valve reconstructive strategies may address any of the components that maintain the valvular competence (annulus, leaflets, chordae, papillary muscles, or the ventricle). Most contemporary techniques have focused on the annulus, the leaflets and the chordae.

All techniques described previously for approaching the mitral valve provide excellent exposure. The transseptal approach, however, has the added advantage of allowing the valve to be evaluated in its normal anatomic configuration without being distorted by excessive retraction. This is a point of importance when contemplating reconstructive procedures (Fig. 6.8).

Reconstruction of the Mitral Valve Leaflets

Mild asymptomatic mitral valve prolapse may progress to clinically significant mitral insufficiency. It is most often the result of myxomatous and degenerative changes of the chords with varying degrees of leaflet abnormality. The P2 segment is most commonly involved, and is managed surgically with a quadrangular resection and annuloplasty. Some surgeons use a triangular resection of the involved leaflet segment to avoid annular reapproximation and to simplify the procedure. Alternatively, the prolapsing segment may be supported with neochordae.

Quadrangular or Triangular Resection

A quadrangular or triangular portion of the posterior leaflet that encompasses the prolapsed segment of leaflet is resected. The posterior annulus in the area of the quadrangular resection is reduced with two to three interrupted sutures of 2-0 Ticron. This step may be unnecessary in triangular resection technique. The leaflets edges are reapproximated with 5-0 Prolene sutures tied on the ventricular or atrial aspect of the leaflets (Fig. 6.14). The posterior annulus is always reinforced with one of the annuloplasty support systems (see below).

FIG. 6.14 Leaflet resection with an annuloplasty

Excessive Excision of Leaflet Tissue

Good judgment is needed when excising redundant prolapsed leaflet tissue. Too large an excision may compromise adequate repair. In cases in which a relatively large segment has been removed, leaflet approximation may be facilitated by sliding the remaining leaflet segments toward each other. This can be accomplished by detaching the remaining posterior leaflet segments from the annulus, commissure to commissure, and reattaching them back to the annulus after reduction annuloplasty. The posterior annulus must be reinforced with one of the annuloplasty systems to avoid any tension on the repair (see below) (Fig. 6.15).

Mitral Insufficiency from Improper Leaflet Apposition

Taking too wide a bite in the posterior mitral leaflet for reapproximation may decrease the surface area and produce mitral insufficiency by preventing proper coaptation of the leaflets.

Suture Untying

Fine Prolene sutures may come untied if they have not been tied securely. This can result in disruption of the repair and significant valvular regurgitation. In this context, it should be borne in mind that fine Gore-Tex sutures have a greater tendency to untie, and therefore should be avoided in leaflet repair.