Fig. 12.1
Schematic of approach to the mitral valve through the Sondergaard’s groove. The Sondergaards groove is developed to separate the right atrium from the left atrium (a). The left atrium is entered 4–6 cm in front of the pulmonary veins, close to the mitral valve (b–d). IVC inferior vena cava, LA left atrium, RA right atrium, RSPV right superior vena cava, SPV superior pulmonary cava, SVC superior vena cava (From Cohn [34])
Alternatively, in some cases of previous aortic valve replacement, or in some reoperative mitral valve operations with significant adhesions between the right atrium and the pericardium, a trans-septal approach can be utilized. This involves making a longitudinal right atriotomy and entering the left atrium by incising the fossa ovalis. After the prosthetic valve is implanted, the interatrial septum is closed using a running 3-0 prolene stuture or with a pericardial patch.
In cases where the exposure of mitral valve is difficult, additional maneuvers may be necessary for better visualization. Circumferential dissection can be carried out around the SVC and the IVC to extend the left atrial incision both cephalad and caudad and allow better retraction of the left atrium. The SVC can also be divided circumferentially to enhance left atrial retraction and expose the mitral valve. SVC is then re-anastomosed in an end to end fashion using a running 5-0 prolene suture. The mitral valve exposure can also be enhanced by making an additional incision perpendicular to the left atrial incision and extending into the right atrium and the septum through the fossa ovalis.
Mitral Valve Implantation
Mitral valve replacement should be a total chordal sparing replacement; non-chordal sparing mitral replacement should be completely abandoned. Several studies comparing no chordal sparing techniques, partial or complete chordal sparing mitral valve replacement have demonstrated that left ventricular volume and function are much better preserved with complete chordal sparing techniques [11–13].
Several techniques have been described for total chordal preservation during MVR for functional MR, including anterior flip-over, in which a C-shaped curved incision is placed in the anterior leaflet 2–3 mm away from the anterior annulus and extended from anterolateral to posteromedial commissures and the entire anterior leaflet apparatus is moved posteriorly (Fig. 12.2a, b). A second method of achieving chordal preservation is to separate the anterior leaflet from the annulus as described above, but then resect the center of the anterior leaflet and affix the remaining portions of the anterior leaflet to the anterior and posterior commissures using a prolene suture (Fig. 12.2c, d). The sutures for MVR are placed around the posterior annulus, and incorporating the posterior leaflet, thus placing both the chordal apparatus and leaflets posterior to the mitral valve prosthesis. This is critically important for mechanical valves because any loose native leaflet tissue may impede opening of prosthetic valve leaflets.
Fig. 12.2
Techniques of chordal preservation for mitral valve replacement. A flap is cut from the central portion of the anterior leaflet and is flipped to the posterior annulus (a, b). Alternatively, the central portion of the anterior leaflet is resected (Shaded area in c). The remnants are anchored to the anterior and posterior commissure and incorporated in the annular stitches (c, d). PL posterior leaflet (Figures 12.2a, b Courtesy of Dr. Steven Bolling, University of Michigan, Ann Arbor, Michigan)
MV prosthesis can be affixed to the mitral annulus using either a running or an interrupted technique. We prefer the interrupted technique where pledgeted 0-Ticron stitches are placed circumferentially in a horizontal mattress configuration. The pledgets are typically placed on the atrial side and the suture is passed from the atrium to the ventricle then passed either around or through the leading leaflet edge to allow for an intra-annular placement of the MV prosthesis. First suture is placed in the midportion of the posterior annulus; the strings of this suture can then be used to retract and expose the annulus to facilitate placement of the next stitch. Sutures are, placed in this fashion, first towards the posterior trigone, then towards the anterior trigone, and finally around the anterior annulus. Several important structures lie in close proximity to the mitral valve and their location must be kept in mind as sutures are being placed (Fig. 12.3). The circumflex artery lies adjacent to the posterior annulus and comes closest to annulus in the region of the anterior commissure of the mitral valve. The coronary sinus lies close to the posteromedial aspect of the mitral annulus. The left- non coronary commissure of the aortic valve lies behind the midpoint of the anterior mitral valve leaflet. The conduction system lies close to the posteromedial trigone. These structures can be damaged if excessively deep sutures are placed during mitral valve replacement.
Fig. 12.3
Structures at risk of injury during placement of annular stitches for mitral valve replacement. AC anterior commissure, BH bundle of his, CS coronary sinus, LC left coronary sinus of the aortic valve, LCx left circumflex artery, NC non coronary sinus of the aortic valve, PC posterior commissure
Valve sizing for IMR should be prudent and not “oversized”. With modern mitral valve prostheses there should not be a worry of stenosis. Overly large bulky valves may impair LV dynamics and interventricular conduction, in these already poor LVs. Also, if concomitant aortic valve replacement is planned, oversizing of the mitral prosthesis may necessitate undersizing of the aortic prosthesis and result in patient prosthesis mismatch. Once appropriate valve is chosen, individual sutures are passed through the sewing ring while maintaining proper suture orientation. Attention must be paid to orienting the valve properly to avoid LV outflow tract obstruction, especially by the strut posts of the bioprosthetic valves. This can be avoided by aligning one of the valve posts with the native anterior commissure and allow the valve posts to straddle the LVOT. This is not an issue with mechanical valves due to their lower profile as compared to bioprosthetic valves. The exact orientation of a mechanical valve is not important as long as the leaflets open and close without obstruction. Once all the sutures are passed through the valve sowing ring, the valve is lowered to the annulus and sutures are tied. During this step, it is crucial to ensure that none of the suture strings are looped around the valve posts, as this will result in improper seating of the valve on to the annulus and significant paravalvular MR.
Presence of mitral annular calcification (MAC) can make the procedure very challenging and even perilous. Surgical approach must be modified to avoid catastrophic complications. In presence of severe MAC, we place the sutures in a non-everting fashion from the ventricular to the atrial side, thus reducing the stress on the annulus as the sutures are tied down. Aggressive debridement of annular calcification can result in injury to the circumflex artery or even disruption of the atrioventricular groove. We debride the calcium only to the minimum extent necessary to allow sutures to be placed around the annulus. Ultrasonic tissue ablation can be used to facilitate annular debridement in a controlled fashion.
After the sutures are tied, the prosthetic valve is checked for proper function. For bioprosthetic valves, this can be done by instillation of saline into the left ventricle and checking proper leaflet apposition. For mechanical valves, the leaflets are held in the open position and inspection of the subvalvular structures is conducted circumferentially. Any tissue that may restrict leaflet opening must be resected. A metal hook is used to check the suture line on the sowing ring to detect any gaps. If any gaps are found, extra stitches can be placed to reef up atrial tissue to prevent paravalvular leaks.
Completion of Procedure and Weaning from Bypass
After satisfactory valve inspection, left atrium is closed. To achieve this, pledgeted 3-0 prolene sutures are placed at either end of the atriotomy incision and run towards the middle. Once in the middle, they are snared and a left ventricular suction catheter is placed through the prosthetic valve under direct vision, and brought out through the atriotomy incision. The heart is de-aired, and aortic cross clamp is removed. Temporary atrial and ventricular epicardial pacing wires are placed. The LV vent is removed once adequate LV ejection is achieved. TEE is performed to ensure complete de-airing and proper valve functioning. Once heart is de-aired, and adequate cardiac function is observed, patient is weaned from bypass, and protamine is administered. All cannulae are removed, hemostasis is ensured, and chest is closed after placement of drainage catheters. In patients with depressed cardiac function and concomitant coronary artery disease, intra-aortic balloon pump placement may be necessary.
Concomitant Procedures
If concomitant CABG is planned, the distal anastomoses are performed prior to mitral valve replacement. Adequate hemostasis must be achieved at distal anastomotic sites as they may not be accessible after mitral valve implantation because aggressive lifting or manipulation of the heart is avoided after MVR as it may result in ventricular perforation by a valve strut, especially in the setting of calcified annuli. This order also allows delivery of cardioplegia through the vein grafts.
If patient also needs AVR at the time of mitral surgery, this is generally performed after the MVR. Many surgeons open the aorta, resect leaflets and debride the aortic annulus in preparation for AVR, before directing attention to the mitral valve, and then return to implant the aortic valve after the MVR has been completed. In absence of significant aortic regurgitation, this approach can sometimes be modified to leave the aortic valve in situ till after the MVR to give a robust dose of antegrade cardioplegia after MVR before opening the aorta. Tricuspid valve repairs are performed after the mitral and the aortic valves have been replaced.
Postoperative Care
Inotropic support is generally necessary to maintain adequate perfusion, but should be weaned as feasible. Once the bleeding has subsided, anticoagulation is initiated. Anticoagulation is recommended for both bioprosthetic (short-term) and mechanical prosthetic valves. We generally bridge with intravenous heparin while awaiting for therapeutic INR level (2.5–3.5).
Results
Operative mortality for mitral valve replacement has decreased significantly in recent years. A recent study by Acker et al. [1] reported risk of mortality and stroke after MVR for ischemic mitral regurgitation to be 4 % and 3.2 % respectively at 30 days and 14.3 % and 4.0 % respectively at 1 year. There was reduction in heart failure symptoms in 61.2 % of patients, and improvement in physical health in 18.4 % of patients at 1 year. A subset of patients remain in heart failure due to presence of depressed LV function prior to the operation. Chikwe et al. [9] studied survival and outcomes after bioprosthetic and mechanical mitral valve replacement in patients 50–69 years of age and reported 4–5 % risk of mortality, 2 % risk of stroke, 4–6 % risk of bleeding, 10–13 % incidence of atrial fibrillation, 4 % risk of acute kidney injury and 16–21 % risk of respiratory failure within 30 days of MVR. Actuarial 15 year survival was 57.5 % in the patients who received mechanical prosthetic and 59.9 % in patients who received a bioprosthetic valve. While risk of reoperation was lower in patients who received a mechanical prosthetic (5 % vs 11.1 %), risk of bleeding (14.9 % vs 9 %) and risk of stroke (14 % vs. 6.8 %) was higher as compared to patients who received a bioprosthetic valve. Similar to their results, Ribeiro et al [14] found no difference in long term survival between valve prosthesis types. These authors reported a 10 and 20 year survival of 74.2 % and 69.3 % after replacement with mechanical prosthetic and 71 % and 56.6 % with biologic valve prosthesis respectively. The probabilities of remaining free of reoperation at 10 years after surgery was higher using a mechanical substitute (92.7 %) than after surgery with a bioprosthesis (86.4 %). However, in this series the probability of remaining free of bleeding events at 10 years after surgery was similar when using a mechanical substitute (91.0 %) or a bioprosthesis (94.0 %, p = 0.267). Based on these observations these authors have suggested that implantation of bioprosthetic valves may be reasonable even in younger patients.
The incidence of thromboembolism is about 1.5–3 % per patient year after MVR and is thought to be similar between currently used mechanical and bioprosthetic valves. Patients with large left atrium with intra-atrial clot or chronic atrial fibrillation are at a higher risk of thromboembolic complications. As long as effective anticoagulation is maintained, thrombosis of mechanical valves is relatively uncommon. Patients who present with mechanical valve thrombosis may need surgery if other methods fail or if patient is in shock because of MV obstruction. Generally, this can be treated with removing the clot from the valve, and does not require re-replacement of the valve as long as there is no concern for infection and the leaflets are functioning normally.
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