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MMCTS
(April 25, 2005). doi:10.1510/mmcts.2004.001024 Copyright © 2005 European Association for Cardio-thoracic Surgery
Procedure Mitral valve replacement with mechanical or bioprosthetic valveDepartment of Cardiac Surgery, Level 4, Queen Elizabeth Building, 10 Alexandra Parade, Glasgow Royal Infirmary, Glasgow G31 2ER, UK * Corresponding author: * Tel.: +44-141 211 4730; fax: +44-141 552 0987. E-mail: d.j.wheatley{at}clinmed.gla.ac.uk
A presentation on the technique of mitral valve replacement is shown: exposure and excision of the diseased mitral valve is demonstrated along with the use of sub-annular and supra-annular interrupted suture techniques for securing biological and mechanical mitral prostheses. A brief overview of the literature is presented.
Key Words: Mitral valve disease • Mitral valve replacement • Bioprosthesis • Mechanical valve
The first successful prosthetic mitral valve replacement was achieved by Nina Braunwald at the National Institute of Health in 1960 [1]. A year later, Albert Starr and Lowell Edwards published their results for what was to become the first commercially available prosthesis – the Starr-Edwards ball and cage mitral valve [2]. This was the gold standard until the Bjork-Shiley tilting disk valve (1969) and then the St. Jude Medical bi-leaflet valve (1977) emerged onto the market. These second and third generation valves had superior haemodynamic profiles and fewer valve related adverse events. Alongside the development of mechanical prosthetic mitral valves, improvements in fixation techniques using gluteraldehyde (1968) led to the development of the Hancock and Carpentier-Edwards porcine bioprosthetic mitral valves. These valves proved to be popular but by the early 1980s structural valve deterioration, particularly in younger patients became an increasingly recognised complication. Improved tissue fixation techniques and use of bovine pericardial leaflets have improved the longevity of bioprosthetic mitral valves, although their use in the mitral position is usually limited to the elderly or those with specific contra-indications to anticoagulation. Unstented mitral heterograft or allograft valves have the theoretical attraction of truly mimicking the natural mitral valve but at present they cannot be widely recommended.
Indications for mitral valve replacement take account of symptomatic status, occurrence of thromboembolic episodes or endocarditis, deterioration in pulmonary haemodynamics and myocardial function. Mitral valve replacement is not as satisfactory as a good repair procedure and the possibly of a repair should be carefully considered before recommending replacement. Recommendations for the management of valvular heart disease and the use of different valve prostheses are based on the guidance document from the American College of Cardiology/American Heart Association taskforce [3]1. Patient age, co-morbidity, the risks of valve related mortality, morbidity and re-operation must always be given full evaluation by the patient and surgeon when selecting a cardiac valve substitute.
Anaesthetic technique and monitoring of cardiovascular and respiratory systems are the same as for any major cardio-pulmonary procedure. Transoesophageal echocardiography has become an important adjunct to mitral valve surgery. It has a well recognised role in guiding mitral repair procedures and assessing their effectiveness, but in the setting of mitral valve replacement it is helpful in confirming unsuitability for a repair procedure (calcific masses or severe chordal fusion), it will confirm normal prosthetic valve function following replacement, and it aids in the de-airing process [4], (Video 1). Vertical sternotomy is the commonest and generally recommended approach to the heart. It gives optimal access to the aorta, allows other concomitant procedures to be undertaken readily, and allows de-airing and defibrillation to be undertaken in a way that is familiar to all surgeons. This approach may give a restricted view of the mitral valve since the left atrium is located posteriorly and the mitral valve is best viewed from the back of the heart (hence the good view obtained by transoesophageal echocardiography!). A right thoracotomy approach often gives a better view of the mitral valve, but poor access to the aorta and the left ventricle may create difficulties with cannulation, de-airing and defibrillation. When "closed mitral valvotomy" performed through a left thoracotomy gives an unsatisfactory result necessitating immediate progression to mitral valve replacement, this can be achieved through the same left lateral approach, albeit with less satisfactory views of the mitral valve. Cannulation for cardiopulmonary bypass should be undertaken before any manipulation of the heart to avoid cardiovascular instability and the risk of dislodging atrial thrombus (Video 2). To aid access to the left atrium it is advisable to place separate venous cannulae into the superior and inferior vena cavae. These should not cross within the right atrium – inserting the inferior cannula close to the inferior vena cava aids exposure of the left atrium by the atrial retractor. Caval snares ensure good venous drainage. Our institution uses the SarnsTM aortic and venous return catheters (MMCTSLink 20) (Video 3).
Cardiopulmonary bypass is established with moderate hypothermia (32°). Following cross clamping of the ascending aorta, cold blood cardioplegia is delivered via a MedicutTM cannula in the aorta (MMCTSLink 21). The left atrium should be opened (by short incision close to the right superior pulmonary vein) immediately upon commencement of cardioplegic administration to avoid left heart distension in the event of mild aortic regurgitation being present, and to remove the warm blood present in the left atrium (thus aiding cooling of the heart). Care is needed to avoid return of topical cold saline via cardiotomy suction (Video 3). The left atriotomy incision is made close to the heart (just behind the inter-atrial groove) best commenced at the junction of the right superior pulmonary vein with the left atrium (this incision continues the opening already made for atrial decompression during cardioplegic arrest). The incision is extended superiorly toward the left atrial roof, and inferiorly in front of the inferior pulmonary vein and behind the inferior vena cava [5] (Video 4).
Other approaches to the mitral valve are described and include the superior approach through the atrial roof, and the transeptal approach in which the right atrium is opened by vertical incision, which extends into the inter-atrial septum [6]. The mitral valve can be exposed using the self-retaining Cosgrove retractor – MMCTSLink 22 (which has the advantage of creating a good, stable operating field while freeing the surgical assistant), or using a hand-held atrial retractor such as the Cooley retractor – MMCTSLink 23 (which allows the assistant to adjust the view as appropriate). Grasping the posterior leaflet and pushing it posteriorly can improve the view of the mitral valve (Video 5). Excessive traction on the valve should be avoided – particularly keeping in mind the risk of overstretching and rupturing the adjacent posterior left ventricular wall.
The first consideration must be to confirm that a repair procedure is not feasible. Resection of the valve commences close to the junction of the anterior leaflet with the atrial floor, leaving a rim of 2–3 mm of atrial tissue. This gives an edge suitable for grasping in a Roberts forceps and allows the valve to be displaced as necessary for a view of the resection line around the leaflet base, as well as a view of the subvalvular mechanism (Video 5). An important consideration is the extent to which mitral-annular continuity can be preserved [7]. Although it may be possible to preserve all chordae, the commonest chordal preservation is by leaving as much of the posterior leaflet in place as is feasible. Large calcified masses of fused chordae often need to be resected simply to give enough space for the prosthesis. Excess tissue may need resection. The rim of retained leaflet tissue should be minimal and loose chordae, which may interfere with a mechanical valve, should be resected. Resection of chordae should be just above their insertion into the papillary muscle, avoiding transection of the papillary muscle itself (Video 6).
Each valve prosthesis manufacturer provides sizers that aid in selection of the appropriate prosthesis size –1 the commonest sizes being 29 mm or 31 mm (Video 7). Selection of type of prosthesis should have been made in consultation with the patient, taking account of the need for continuing valve-related therapy. In the case of mitral valve replacement the onset of atrial fibrillation is so common in the natural history that many centres would advocate universal use of mechanical prostheses unless strong contra-indications to anticoagulation exist.
Several effective techniques are advocated for anchoring the prosthesis. When access is good, and annular tissues are tough and fibrous, a continuous running suture technique has the advantage of speed [8]. This technique should only be used when the surgeon is confident of its ease and security. Placement of interrupted sutures with pledgets is more consistently reliable, particularly with restricted access and friable tissues. A double-ended 2/0 pledgetted suture with large needles (26 mm) is ideal. EthibondTM (MMCTSLink 24) and TicronTM (MMCTSLink 25) sutures are conveniently supplied in alternate colours to aid identification for tying. For implantation of a mechanical valve the pledgets are positioned on the atrial floor and the sutures emerge just below the annulus at the junction of fibrous tissue with myocardium. The sutures should be inserted 3–5 mm from the annulus and should emerge within fibrous tissue and not from myocardium. The distance between the two points of emergence of the double-ended suture should be about 6–8 mm and the next double-ended suture should start close by (1–2 mm). In practice, some 15–18 double-ended sutures are required. Excessive suture depth in the posterior-superior aspect of the annulus may damage the circumflex artery, and in the anterior-superior annulus misplaced sutures may injure the aortic valve (Videos 8 and 9).
Suture placement for a bioprosthetic valve should be with the pledgets on the ventricular aspect of the mitral annulus and the sutures emerging on the atrial floor, with spacing and suture type as for a mechanical prosthesis. Care should be taken to keep the bioprosthesis moist with normal saline. With any stented bioprosthesis there is a serious risk of entrapment in the sutures of one or more stent posts. This risk is much reduced if the valve has a protective suture in place linking the post tips (such has that provided by the Carpentier-EdwardsTM porcine mitral valve (MMCTSLink 26) this should not be removed until the valve has been securely placed in position. If the manufacturer does not provide this protective mechanism, care should be taken to position each post through the mitral valve annulus, with a good view, and with the sutures under tension in order to confirm that there is no post entrapment (Videos 10 and 11).
Before placing sutures through the prosthesis sewing ring the valve should be orientated in its intended position (this simplifies or even obviates the need for subsequent adjustment). Once in place most mechanical valves can be rotated to optimise orientation but this should only be done using the manufacturer's purpose-made device. Attempting to twist valves with surgical instruments carries a risk of damaging the valve and similarly disc motion should not be confirmed by touching the leaflet with a surgical instrument – the manufacturer's valve testing probe should be used. Single disc valves are orientated with the disc mimicking the anterior leaflet – i.e. the greater opening is posteriorly placed. Bileaflet valves are orientated with the axis of opening in line with the middle of the aortic root. Local anatomy may modify the desired orientation – e.g. an unresected mass of calcium may risk disc impingement in certain valve orientations. Bioprosthetic valves are orientated with two posts at trigones – i.e. one leaflet attachment is positioned along the sub-aortic curtain, ensuring that a post does not intrude into the left ventricular outflow tract. Spacing of the sutures in the sewing ring should match that of the spacing in the annulus. Most sewing rings have markers to aid judgement – commonly allowing the sutures to be separated into 3 or 4 groups. Sutures can be held in groups by an assistant, fastened to the wound towels, or held in a purpose-made retaining device. Alternate colour sutures, or a single hitch knot made in each suture, are additional ways of avoiding suture entanglement, and simplifying identification for rapid tying. Generally, a minimum of 6 knots are recommended, with the first two being "slip knots" to aid in tightening (Videos 12, 13 and 14).
Our practice is to leave the left atrial appendage alone unless there is visible thrombus within it, in which case we would place a purse string suture from within the left atrium to occlude the appendage [9,10]. With mechanical valves, placement of a small Foley catheter (14F) across the valve with inflation of the balloon in the ventricle prevents full valve closure and is an aid to venting and de-airing. The balloon must be fully deflated prior to removal (Video 15).
The left atriotomy is closed with a running monofilament suture (we use 3/0 ProleneTM - MMCTSLink 27), commencing a suture at each end of the atriotomy and ending anterior to the right superior pulmonary vein – the high point of the left atrium and a suitable site for evacuation of air (Videos 16).
Once de-airing is complete the aortic cross-clamp is released. When regular left ventricular contractions are established (if necessary, by defibrillation) the Foley catheter balloon is deflated, the catheter is extracted and the atriotomy suture line is completed. A SarnsTM aortic needle vent catheter (MMCTSLink 28) is placed in the ascending aorta for several minutes to scavenge any residual air (Videos 16 and 17).
Discontinuation of cardiopulmonary bypass, haemostasis and chest wall closure follow.
Short and long-term outcomes following mitral valve replacement are summarised below [11,12,13,14]:
1 http://www.acc.org/clinical/guidelines/valvular/ 
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