Despite the absence of randomised comparison, it is widely accepted that, when feasible, valve repair is the optimal surgical treatment in patients with severe degenerative mitral regurgitation (MR) due to the well documented advantages of such an approach over valve replacement in terms of perioperative mortality, preservation of postoperative left ventricular (LV) function and long-term survival in this setting.1,2 Conversely, in patients with secondary (functional) MR, the role of mitral repair is less well established and is still object of debate. In this review we will mainly focus on the basic principles and on the current results of mitral repair in both degenerative and functional MR.
Degenerative Mitral Regurgitation
Degenerative MR represents a rather common pathology, affecting 1–2% of the general population. In patients requiring surgery for this disease, mitral valve repair is the standard of care. In many patients, the typical sign of the degenerative pathology is an excess of valve tissue (Barlow’s disease), which is the extreme form of myxomatous degeneration. In others, especially in older patients, the valve tissue does not show this alteration but is thinner and translucent (fibroelastic deficiency).
Anatomical and functional alterations that typically characterise degenerative MR are leaflet prolapse into the atrium during systole and annular dilatation. The chordae tendineae can be thickened or thinned and they can show elongation or ruptures. The posterior mitral leaflet and afferent chordae tendineae are more frequently affected by the degenerative process compared with the anterior leaflet.
The most frequent anatomical and pathological alteration is elongation or rupture of the chordae tendineae of the posterior leaflet.
Timing for correction of Degenerative Mitral Regurgitation
According to current guidelines, mitral repair is indicated in patients with severe MR who are symptomatic and in those showing initial signs of LV dysfunction (end-systolic diameter >40mm, ejection fraction <60%). Surgical treatment is encouraged in asymptomatic patients with preserved LV function when atrial fibrillation occurs or when systolic pulmonary artery pressure is >50mmHg at rest or >60mmHg during exercise. In addition, mitral repair is reasonable for asymptomatic patients with preserved LV function in whom the likelihood of successful repair without residual MR is >90%.3
However, in the individual patient the decision-making regarding the optimal time for surgery may be difficult. Indeed some asymptomatic patients may be candidates for early repair before the occurrence of structural and functional changes in the LV and left atrial chambers, which are predictors of poor post-operative outcome.4 Others with advanced age, relevant co-morbidities and/or complex valve lesions may be closely followed in order to safely postpone surgery (watchful waiting approach).5
Surgical Techniques of Mitral Repair in Degenerative Mitral Regurgitation
Mitral repair requires greater experience compared with valve replacement and patients with a mitral valve deemed reparable should be referred to centres with a high volume and extensive experience in mitral repair. Prior surgical methods associated with suboptimal results such as chordal shortening and the non-use of an annuloplasty ring have been recognised6 with beneficial effects on the evolution of mitral valve reconstructive surgery. Moreover new technical solutions have been added to the fundamental methods of repair first described by Carpentier, including the use of artificial chordae made of expanded polytetrafluoroethylene7 and the edge-to-edge (E-to-E) technique.8 Therefore, nowadays >90% of degenerative lesions can be repaired successfully in expert centres by use of contemporary techniques.
From a technical point of view, valve repair for degenerative MR includes a large array of valvular, subvalvular and annular procedures whose choice depends on the lesions identified during the preoperative echocardiograms and the intraoperative surgical examination of the mitral apparatus.
Posterior Leaflet Prolapse
In patients with isolated prolapse of the middle scallop (P2) of the posterior leaflet, which is encountered in the majority of patients with degenerative mitral regurgitation, repair usually involves quadrangular resection of this scallop. Annulus folding is performed at the implantation base of the resected segment and the remaining portions (P1 and P3) of the posterior leaflet are then brought together and sutured directly without exerting excessive tension on them. In this way the continuity of the posterior leaflet is restored. Nowadays, in many institutions, annular plication tends to be avoided by using techniques such as the sliding plasty or the folding plasty, which are indicated when the valve tissue is too redundant in order to decrease the risk of post-operative dynamic obstruction to the LV outflow tract.
This complication (systolic anterior motion, SAM) occurs in 5–10% of MR cases treated by simple quadrangular resection. The best strategy to avoid it is to reduce the height of the posterior leaflet to <15mm so that the coaptation point of the two valve leaflets can be moved posteriorly. Indeed, a sliding plasty or a folding plasty can be used for this purpose. In the sliding plasty after quadrangular resection the height of the posterior leaflet is reduced by incisions in the basal portions of P1 and P3, followed by reapproximation of the free edges. In the folding plasty, after resection of the prolapsing segment, the edge of the residual cut leaflet is folded (or rotated) toward the annulus and reattached to it. Continuity of the posterior leaflet is then restored by suturing together the remaining portions.
In cases of posterior leaflet prolapse without redundant leaflet tissue, limited resection or artificial chordal replacement with Gore-Tex expanded polytetrafluoroethylene sutures may be appropriate. In particular, in some institutions, the use of neochordae to correct severe degenerative MR due to posterior leaflet prolapse, has been progressively preferred to the standard resection approach (the so called ‘respect rather then resect’ approach). The neochordae are sutured to the fibrous portion of the papillary muscle and then to the free margin of the prolapsing portion of the posterior leaflet and the repair is completed by a ring annuloplasty.
Anterior Leaflet Prolapse
Repairs of the anterior leaflet, either in isolation or with concomitant posterior leaflet repair, are more complex procedures than repair of posterior leaflet prolapse alone. Various techniques may be used, including limited triangular resection of the anterior leaflet, chordal transposition, papillary muscle repositioning, artificial chordal replacement and the E-to-E repair.
A limited prolapse of the anterior leaflet can be easily treated by triangular resection of the prolapsing segment, followed by direct suture of the remaining leaflet portions. Resection should never involve an area greater than 10% of the total area of the anterior leaflet as it may otherwise distort the anatomy reducing the coaptation surface and impairing its mobility. When correctly applied the results of this technique are similar to those of the other repair procedures adopted for anterior leaflet prolapse.
One of the techniques that has been most widely used to correct anterior leaflet prolapse is chordal transposition. This technique consists of detaching a secondary chorda, with an adequate length and structure, from its implantation point on the ventricular side of the anterior leaflet and reimplanting it on the free margin of the same leaflet near the prolapsing segment.
Alternatively, marginal chordae of the segment of the posterior leaflet located in front of the prolapsing one can be used. In this case a segment of the posterior leaflet with the respective chordae is detached and then reattached using a 4.0 prolene suture on the free margin of the prolapsing segment of the anterior leaflet (the ‘flip-over’ technique). The defect on the posterior leaflet is then sutured in the same way as a standard quadrangular resection. The advantage of chordal transposition compared to the use of artificial chordae is that the former already have the correct length, while the major challenge for a surgeon when using the latter is to determine the right length. One of the main disadvantages of the flip-over approach is the fact that a valve segment not affected by the pathology needs to be resected in order to transpose the chordae tendineae of the posterior leaflet.
Papillary Muscle Repositioning
Papillary muscle repositioning has been conveniently used to correct anterior and bi-leaflet prolapse due to chordal elongation particularly in the difficult setting of global mixomatous degeneration of the mitral valve (Barlow’s disease). This technique consists in separating the anterior head of the papillary muscle from the other heads and taking it down into the left ventricle. This is achieved by putting a stitch in the fibrous segment of the anterior head and tying it to the fibrous segment of the posterior head. Since the chordae arising from the anterior head are anchored to the anterior leaflet, by shortening its length it is possible to correct anterior leaflet prolapse.
Implantation of Artificial Chordae
The use of artificial chordae tendineae (neochordae) is probably the most widely used technique to treat anterior leaflet prolapse, and many groups have documented excellent results with this approach. When using this technique it is essential to comply with the normal anatomy of the mitral subvalvular apparatus and the physiological distribution of the chordae tendineae: those arising from the anterior papillary muscle are distributed in the lateral half of the anterior and posterior leaflets, while those originating from the posterior papillary muscle anchor the median half.
The material most commonly used in surgical practice are 4.0 or 5.0 polytetrafluoroethylene (PTFE) neochordae (suture Gore-Tex, WL Gore & Associates, Flagstaff, AZ, US). Artificial chordae are fastened to the fibrous portion of the papillary muscle on one end and to the free margin of the prolapsing portion of the anterior leaflet on the other. Several artificial chordae are usually needed depending on the extension of the prolapsing segment. The main technical difficulty lies in determining the proper length of the neochordae. In case of isolated prolapse of the anterior leaflet, the best way to determine the correct length is to use the height of the non-prolapsing posterior leaflet as a reference. In the case of b-ileaflet prolapse or prolapse of several segments, the point of reference to be used is the lateral commissure, unless it is affected by the degenerative process. Modified artificial chordae with a premeasured loop have recently been introduced to facilitate the choice of the proper length. When using neochordae it should be considered that in the case of a very dilated left ventricle there is a higher risk of relapse of prolapse some time after surgery, due to the occurrence of reverse LV remodelling, which can make the length of the neochorda become too long.
The E-to-E technique is another method of mitral valve repair that has been introduced in the armamentarium of mitral valve repair in the early nineties as a simple method to conveniently correct MR in presence of some complex lesions.9,10 The idea behind the E-to-E approach is that the competence of a regurgitant mitral valve can be effectively restored with a ‘functional’ rather than an ‘anatomical’ repair. The key point is to identify the location of the regurgitant jet. Exactly at that point, the free edge of one leaflet is sutured to the corresponding edge of the opposing leaflet, thereby eliminating the incompetence of the mitral valve. When the regurgitant jet is in the central part of the valve, the E-to-E repair produces a mitral valve with a double orifice configuration. Depending on the location of the suture, the two orifices can have similar or different sizes. When the regurgitant jet is located in the proximity of a commissure, the E-to-E procedure leads to a single orifice mitral valve with a relatively smaller area. The technique appeared to be attractive because of its simplicity, reproducibility and effectiveness even in complex settings. Several institutions around the world adopted the E-to-E technique in selected patients with MR due to different aetiologies and mechanisms.11–14
A 4-0 polypropylene continuous mattress suture is passed first, followed by an over-and-over continuous suture with the same 4-0 polypropylene. A flexible or semi-rigid prosthetic ring is almost invariably implanted to increase the coaptation surface of the leaflets, reduce the stress on the E-to-E suture and stabilise the repair. The global mitral valve area is assessed by direct inspection and, in case of doubt, by introducing Hegar dilators into the valve orifices. In normal size patients only a global valve area of more than 2.5cm2 is accepted. Transoesophageal echo-doppler performed after weaning from cardiopulmonary bypass typically shows no residual MR and two diastolic flows through the double orifice mitral valve. The valve area is commonly assessed by a planimetric method using the trans-gastric short-axis view. Using the E-to-E technique, even complex mitral valves can be currently repaired with a very short cross-clamp time. The technique is very versatile and it has been used with very satisfactory results in patients with segmental prolapse of the anterior leaflet,15 commissural prolapse16,17 and bi-leaflet prolapse of facing segments in the context of a global mixomatous degeneration of the mitral valve (Barlow’s disease). In this last setting both leaflets are prolapsing, due to chordal elongation or rupture. When the prolapse is involving facing segments (more commonly A2 and P2), the E-to-E technique can be conveniently applied.18 The E-to-E technique can be a useful adjunct to the undersized annuloplasty to treat functional mitral regurgitation in patients with dilated cardiomyopathy as it will be outlined later in this article. Finally, it has also been used as a ‘rescue’ procedure in patients with significant residual MR after conventional mitral repair and to prevent or treat SAM.
Remodelling of the mitral annulus by means of annuloplasty is one of the key concepts in mitral repair surgery. Annular dilatation is almost always present in patients with degenerative MR, typically involving the posterior annulus since the anterior portion is anchored to the heart’s fibrous skeleton. Besides annular dilatation, in degenerative MR there is also an alteration in the shape of the annulus leading to a reduced leaflet coaptation. The aim of annuloplasty is to restore normal annulus dimensions and shape, to prevent further dilatation and to increase the coaptation surface of the leaflets leading to increased mitral repair durability. Therefore, all the previously described surgical techniques should be followed by a ring annuloplasty.
The final repair is assessed initially by visual inspection and by injecting saline through the mitral valve to look for regurgitation (the ‘saline test’), and then by intraoperative transoesophageal echocardiography after the patient is weaned from cardiopulmonary bypass. Patients should not leave the operating theatre with more than 1+ mitral regurgitation on transesophageal echocardiography.
Hospital mortality after isolated mitral repair for degenerative MR in high-volume centres19 is <1%.20 If the procedure is performed before the onset of symptoms and LV dysfunction, patient survival and quality of life after the procedure perfectly matches that of the general population of the same age.21,22 By contrast, patient survival is reduced if the procedure is carried out in patients with symptoms of congestive heart failure and in presence of reduced LV ejection fraction (LVEF).21
Mitral valve repair failure rates, defined by recurrence of moderate or severe mitral regurgitation or re-operation for mitral regurgitation, are determined by the aetiology, lesion and repair techniques. Most early failures are the result of technical issues, and the presence of residual mitral regurgitation greater than mild immediately after surgery is a strong predictor of this event. Late failures primarily relate to progression of the degenerative disease with the occurrence of new leaflet prolapse/flail and, less commonly, leaflet retraction or infection.
Recent studies have documented a risk of recurrence of moderate or severe mitral regurgitation after repair of 1–2% per year, particularly in patients with anterior or bi-leaflet prolapse, which are technically more difficult to treat.23,24 Indeed, in most of the published series, the best results have been obtained in patients with isolated prolapse of the posterior leaflet treated with quadrangular resection associated with annuloplasty with a freedom from reoperation at 20 years of 97%. On the other hand, less favourable outcomes have been consistently and repeatedly reported in patients with MR due to anterior leaflet and bi-leaflet prolapse.25–27 However, particularly in the last decade, after the introduction in the surgical armamentarium of the artificial chordae and of the edge-to-edge technique, several groups have reported comparable long-term results in patients with posterior, anterior and bi-leaflet prolapse,15,28 and it is likely that, with continued refinement of mitral repair strategies, anterior and bi-leaflet prolapse will be completely neutralised as incremental risk factors for recurrent MR after repair.
Mitral Valve Repair for Functional Mitral Regurgitation
Functional MR results from changes in LV geometry leading to papillary muscle dislocation and consequent tethering of the leaflets.
Almost invariably the annulus is dilated and deformed, while the valve leaflets are morphologically normal.29 While valve repair for degenerative MR includes the large array of valvular, subvalvular and annular procedures previously described, surgical correction of functional MR in patients with ischaemic or non-ischaemic dilated cardiomyopathy, is mainly performed by means of a restrictive (undersized) annuloplasty aiming at reduction and reshaping of the annulus to eliminate the insufficiency of the valve.30–32
In the opinion of the majority of the surgeons the appropriate prosthetic ring in this clinical context should be complete and rigid.33,34 and at least two sizes under the one measured following the standard criteria35,36 in order to obtain a leaflet coaptation length of at least 8mm. The procedure is simple and easily reproducible. In appropriately selected patients a well performed restrictive annuloplasty is associated with a low operative mortality and is effective in eliminating MR, promoting LV reverse remodelling, reducing symptoms and improving quality of life. However, when the patient selection criteria are not respected, residual/recurrent MR can frequently occur, and such an event is strictly related with an unfavourable outcome in terms of heart failure and mortality in the follow-up.37,38 Patient selection for annuloplasty is therefore crucial. Indeed residual or recurrent valve insufficiency following mitral annuloplasty is likely in presence of preoperative complex/multiple regurgitant jets,39 severe tethering of the leaflets (coaptation depth >1.5cm), an angle between the posterior mitral leaflet and the annular plane in systole >45°,40 significant distal anterior mitral leaflet tethering (angle between the annular plane and a line that joins the anterior annulus and the coaptation point >25°).41 The outcome of annuloplasty alone is particularly unfavourable in the rare instances where severe leaflet tethering is associated with only mild annular dilatation since, in these circumstances, an effective reduction of the size of the mitral annulus is unlikely to be surgically obtained owing to the lack of significant preoperative dilatation. Moreover, the clinical history has a relevant role in selecting patients for annuloplasty.
It has been clearly documented that reverse remodelling following annuloplasty occurs much more frequently in patients with a short duration of symptoms of heart failure and a smaller pre-operative LV size.35,42 Therefore, undersized annuloplasty alone should only be carried out in the early phase of the disease, when the history of heart failure is short, the left ventricle is not excessively dilated and the well defined echocardiographic predictors of recurrence of MR after repair are absent.
Under certain circumstances other surgical procedures can be conveniently added to restrictive annuloplasty to enhance the effectiveness and durability of repair. Resection of the secondary chordae of the anterior leaflet,43 repositioning of the tip or the base of the papillary muscles,44–46 the addition of the edge-to-edge suture,47 the association of an external cardiac support48 and a concomitant LV restoration49 procedure have demonstrated a positive effect on the durability of annuloplasty in well-selected patients. Finally, when an effective and durable mitral valve repair is not expected owing to advanced LV remodelling and long-lasting heart failure, valve replacement with a bioprosthesis, preserving the integrity of the subvalvular apparatus, can be a reasonable alternative.
Current guidelines do not recommend correction of functional MR in patients with dilated cardiomyopathy if concomitant coronary revascularisation is not required.50,51 Suboptimal data regarding the postoperative outcome explain uncertainties in surgical indications. In addition, the absence of randomised trials showing the benefit of surgery over medical treatment is also a strong argument against recommending surgery. However, despite all of the above, correction of functional MR is generally advocated52,53 because the improvements in symptoms and quality of life after surgery are a major advantage of surgical treatment in patients who are usually experiencing repeated hospital admissions for congestive heart failure despite maximal medical therapy. Obviously, due to the clinical profile of these patients, a careful evaluation of the surgical risk versus the benefit of the operation is mandatory in the individual patient and definite surgical contraindications have to be identified. Heavy co-morbidities, severe right ventricular dysfunction and/or extreme LV dysfunction without contractile reserve under dobutamine infusion during echocardiography are absolute contraindications to surgery.