Minimally Invasive Mitral Valve Surgery: Partial Sternotomy Approach
Keywords : minimally invasive cardiac surgery, minimally invasive mitral valve replacement
· Mitral valve dysfunction is a common pathologic process. The process may involve any component of the valve or subvalvular structures, including the valve leaflets, the annulus, the papillary muscles, the chordae tendineae, and the left ventricular wall.
· The anatomic description of the mitral valve is best visualized using a three-dimensional approach to its location in the heart. The anterior portion of the mitral valve annulus is positioned posterior to the aortic annulus and is bordered by the left and right fibrous trigones. The atrioventricular (AV) node and the bundle of His are adjacent to the right trigone. The circumflex artery runs along the posterior annulus of the mitral valve and may be at risk during mitral valve repair or replacement (Fig. 20.1).
· Chordae tendineae extend from the anterior and posterior papillary muscles to both leaflets. Primary chordae attach to the free margin of the leaflet, whereas secondary chordae attach to the middle and posterior aspects of the leaflets closer to the annulus.
Step 1. Preoperative Considerations
· Most common indications for mitral valve replacement are rheumatic mitral stenosis and infective endocarditis. Replacement is less commonly performed for degenerative disease and functional mitral regurgitation.
· Mitral valve repair is indicated in those with severe myxomatous disease with the presence of gross redundancy of both anterior and posterior leaflets, especially in the younger population. In older adults, mitral regurgitation usually is a result of fibroelastic deficiency disease.
· Long-standing, severe mitral stenosis results in pulmonary hypertension, right ventricular dysfunction, and a variable degree of tricuspid valve regurgitation. If severe, this can result in secondary hepatic and renal dysfunction, with a resultant increase in operative risk.
· Mitral annular calcifications are frequently present in older adults, especially in cases of rheumatic mitral stenosis. Calcifications typically involve the posterior aspect of the mitral annulus and can extend to the base of the posterior leaflet and the base of the left ventricle. Severe calcification of the mitral annulus appears as a horseshoe sign on the preoperative chest radiograph or coronary angiogram (Fig. 20.2).
· Standard preoperative assessment of mitral valve disease is performed by transthoracic or transesophageal echocardiography. Transesophageal echocardiography allows more precise assessment of the anatomy and function of the mitral valve and represents the gold standard in preoperative assessment and planning of the operation. It is an essential tool intraoperatively for assessment of the valve repair or replacement after weaning from cardiopulmonary bypass.
· Mechanical prostheses are indicated for patients younger than 65 years; biologic valves are used more commonly in older adults. This paradigm may shift as the valves evolve in quality and durability, along with the possibility of the use of catheter-based prostheses.
· Selection of the surgical approach depends on the cause of the mitral valve disease, the presence of concomitant coronary or valvular disease, body habitus, and anatomic chest wall deformities.
· Most patients who require isolated mitral valve surgery are candidates for a minimally invasive approach. Relative contraindications to a minimally invasive approach include morbid obesity and extensive mitral annular calcifications.
· The standard incision for minimally invasive mitral valve repair or replacement is a 6- to 8-cm skin incision and a partial upper sternotomy, extending to the left fourth intercostal space. This approach is described in detail in this chapter (Fig. 20.3).
· A small sternal retractor with removable blades (Baxter Healthcare, Deerfield, IL) is used to retract the sternum before proceeding with the dissection. The thymic remnants are divided and ligated with nonabsorbable sutures, and the upper pericardium is divided along the midline.
· The retractor is then removed and a pericardial sac is formed by placing stay sutures in the skin using 2-0 silk sutures. Transient hypotension may occur when the edges of the pericardium are pulled up and toward the skin owing to the displacement of the superior mediastinum and an associated decrease in venous return to the right atrium.
· The sternal retractor is reinserted to expose the great vessels and the right atrium. Cardiopulmonary bypass is initiated by cannulation of the ascending aorta and the superior and inferior venae cavae. The ascending aorta is cannulated by a flexible aortic cannula (21 F), whereas bicaval cannulation is accomplished by placing flexible venous cannulae (24 F) into the distal superior vena cava (SVC) and through the right atrial appendage into the inferior vena cava (IVC). Vacuum-assisted venous drainage is used in all cases. Finally, an antegrade cardioplegia cannula is placed in the proximal ascending aorta (Fig. 20.4).
· After initiation of bypass, the right atrium is isolated by encircling both the SVC and IVC using flexible vessel loops. To place these loops, the pericardial reflections around the vessels must be dissected. The isolation of the IVC is facilitated by the use of a semicircular Favaloro clamp (Fig. 20.5).
· Cross-clamping of the aorta is performed with a Cosgrove Flex Clamp (Edwards Lifesciences, Irvine, CA) so that there is minimal obstruction of the surgical field. A modified Buckberg solution is administered as the main component of cold, antegrade blood cardioplegia.
· Then, 2-0 silk sutures are placed into the right atrial appendage medial to the insertion site of the IVC cannula to ensure retraction of the atrial wall edges. A right atriotomy is performed, with the incision extending between the SVC and aorta superiorly and the base of the right atrium inferiorly. A cannula for retrograde cardioplegia can be inserted directly into the coronary sinus at this point (Fig. 20.6).
· A transseptal incision is performed through the midportion of the fossa ovalis. It is extended superiorly across the roof of the left atrium between the SVC and the aortic root.
· Exposure of the mitral valve is achieved by placing two or three pledgeted 3-0 polypropylene stay sutures into the medial aspect of the incised septum. Further retraction of the interatrial septum is provided by two low-profile, handheld retractors (Fig. 20.7).
· Once the mitral valve is exposed, the anterior leaflet is incised, leaving a residual rim of approximately 5 mm; two small areas of the leaflet that contain chordae from the anterior leaflet and posterior papillary muscles are retained. Gentle traction on the partly detached anterior leaflet allows for excellent visualization of the anterior portion of the mitral annulus and secure placement of the everted pledgeted sutures (2-0 Ethibond sutures; Fig. 20.8).
· The retained areas of the anterior leaflet containing the chordae are attached to the lateral and medial annulus with pledgeted 2-0 Ethibond sutures (Fig. 20.9A). This placement preserves the chordae and ensures that they will not obstruct the left ventricular outflow tract or cause uneven heaping of tissue on the posterior leaflet. An alternative approach to preserve the chordae is simply to incise a portion of the anterior leaflet and fold the rest of the leaflet onto the posterior aspect of the annulus (see Fig. 20.9B).
· Next, pledgeted sutures are placed along the posterior annulus by running a needle through the annulus and then into the body of the leaflet, around any annular calcification (see Fig. 20.9C).
· Calcification of the posterior leaflet should be débrided. In patients for whom extensive decalcification is needed, the annulus is reconstructed with a strip of autologous pericardium (Fig. 20.10).
· Depending on the age of the patient, a biologic or mechanical prosthesis is used for valve replacement. The sutures are placed into the valve sewing ring, and the valve is lowered into place and secured (Fig. 20.11). Technical considerations should include orienting the valve to keep the struts from obstructing the left ventricular outflow tract in the case of a biologic prosthesis being used. If a mechanical valve is used, attention needs to be directed toward the mobility of the leaflets, without obstruction from the subvalvular apparatus.
· Repair of the mitral valve is feasible through the same exposure. Depending on the pathology of the mitral valve causing the mitral valve regurgitation, leaflet repair is conducted. The repair is completed with a partial annuloplasty band (Fig. 20.12).
· Air must be displaced from the left ventricle to the left atrium before the aortic cross-clamp is removed, either by filling the left ventricle with saline or by administering antegrade cardioplegia. Running 4-0 polypropylene sutures are then used to close the interatrial septum and the right atrium sequentially.
· Mediastinal and right pleural tubes are placed. Pacing wires are placed into the anterior surface of the right ventricle. The patient is weaned from cardiopulmonary bypass after adequate de-airing.
· Simple stainless steel wires are used for the sternal closure in the usual fashion.
· Patients who have undergone uncomplicated, minimally invasive mitral valve repair or replacement can be extubated in the operating room. Early extubation allows for faster recovery and a shorter intensive care unit stay.
· Patients with long-standing mitral stenosis and severe pulmonary hypertension with right ventricular dysfunction often benefit from inotropic support with phosphodiesterase inhibitors (e.g., milrinone) and pulmonary vasodilators (e.g., nitric oxide, sildenafil).
· Long-term warfarin anticoagulation with an ideal international ratio of 2.5 to 3.5 is indicated for patients receiving mechanical prostheses, with consideration given to the administration of low-dose aspirin in addition to warfarin.
· Patients with atrial fibrillation who have undergone mitral valve repair or replacement with a biologic prosthesis maybe maintained on anticoagulation for the first 6 to 12 weeks or as long as they remain in atrial fibrillation and are not contraindicated for anticoagulation.
· Emergency cardioversion is seldom successful in patients with chronic atrial fibrillation who have undergone mitral valve surgery.
◆ A partial sternotomy allows excellent access to the aortic and tricuspid valves, allowing for multivalve minimally invasive surgery.
◆ This approach provides an excellent cosmetic result in both men and women, with a very low incidence of sternal nonunion, because most of the sternum is left intact.
· A transseptal approach provides excellent exposure of the mitral valve but may cause transient postoperative dysfunction of the sinus node due to transection of the nodal artery. However, the incidence of permanent postoperative sinus node dysfunction is no different from that in patients operated on through a conventional sternotomy and left atrial approach.
· The stay sutures on the pericardium should be attached to the drapes, which allows for traction and lifting of the mediastinum toward the sternum. This approach helps expose the anatomic structures, better especially in a deep chest.
· The operative field should be flooded with continuous CO2 to reduce intracardiac air and peripheral embolization. CO2 dissolves more readily in blood, hence displacing other components of air.
· The partial sternotomy and its extension into the fourth intercostal space via a J incision should be carried out with care to avoid injury to the left internal thoracic artery and vein. An oscillating saw with a narrow 1-cm blade may be used to make J the incision; its use is more controlled, instead of curving the regular saw to the left side in one step.
· The superior part of the transseptal incision should be at least 1 cm lateral to the base of the aortic root to avoid injury to the aortic valve during closure of the incision. A lesser margin may distort the aortic root, leading to aortic insufficiency due to traction caused by external sutures.
· Sutures along the lateral aspect of the mitral annulus should be placed with care to avoid injury to the circumflex artery, especially in a left dominant system with the artery running through the AV groove.
· Inspection before decannulation should include the dome of the left atrium, with care taken to allow for meticulous hemostasis of the suture line. This area is very difficult to expose in a minimally invasive approach once the heart is full and cardiopulmonary bypass has been withdrawn.
· Mediastinal chest tubes should be placed while the heart is still decompressed on cardiopulmonary bypass. This allows for excellent visualization of the operative field. Pacing wires are placed in the same fashion.
· Meticulous closure with specific attention to hemostasis should be performed. There should be a higher index of suspicion for tamponade with increased chest tube output because the pericardium is not opened in its entirety.