Mitral valve disease: when should we call in the cardiac surgeon?

Spectrum of mitral valve disease amenable to surgery

Mitral regurgitation is the second most frequent indication for valve surgery in Europe and, although the incidence of rheumatic mitral stenosis is decreasing in Europe, degenerative mitral stenosis is becoming more common in an ageing European population [1]. The aetiology and epidemiology of mitral valve disease have been discussed in recent articles in this series and will not be covered in detail here. However, in brief, the most common cause of primary (organic) mitral regurgitation is degenerative (mitral valve prolapse or flail leaflet); other causes include coronary artery disease, infective endocarditis, and connective tissue disease. Secondary (functional) mitral regurgitation owing to mitral annular dilatation is a consequence of dilated or ischaemic cardiomyopathy. Mitral regurgitation usually develops and progresses as a chronic condition; however, acute severe mitral regurgitation may arise after myocardial infarction and papillary muscle rupture, acute chordal rupture, or infective endocarditis. The most common aetiology of mitral stenosis is a late complication of acute rheumatic fever. Other causes include degenerative, congenital and, more rarely, radiation-induced or drug-induced ones (e.g., anorectic drugs).

Indications for mitral valve surgery

Mitral valve surgery is indicated for severe valvular disease, usually symptomatic, although surgery is also indicated in selected asymptomatic cases. The severity of valvular disease is judged according to standard echocardiographic criteria. Cardiac magnetic resonance imaging may be helpful in certain settings, e.g., chronic ischaemic mitral regurgitation where assessment of myocardial perfusion and viability is also important [2].

The most common indication for mitral valve surgery is symptomatic chronic severe primary mitral regurgitation, usually owing to degenerative valve disease, with a left ventricular ejection fraction (LVEF) of >30% (Class I recommendation); mitral valve surgery is indicated in symptomatic patients with severe LV systolic dysfunction (LVEF <30%, and/or left ventricular end-systolic diameter [LVESD] >55 mm) refractory to medical therapy when there is low comorbidity, preferably mitral valve repair (Class IIa) although mitral valve replacement may be considered in these patients when the likelihood of successful repair is low (Class IIb).

Certain patient characteristics are associated with a worse postoperative outcome independent of symptoms. Surgery is therefore indicated in asymptomatic patients with LV dysfunction (LVESD ≥45 mm and/or LVEF ≤60%, Class I) and in those with preserved LV function (LVESD >45 mm and LVEF >60%) and atrial fibrillation secondary to mitral regurgitation, or pulmonary hypertension (systolic pulmonary pressure at rest >50 mmHg, Class IIa).

Asymptomatic patients with preserved LV function (LVEF >60%, LVESD 40-44 mm) and either a flail leaflet and/or significant left atrial dilation (≥60 mL/m² body surface area [BSA]) in sinus rhythm should be considered for surgery when a durable repair is likely and surgical risk is low, as judged by the Heart Team (see below), and the surgery is performed at a Heart Valve Centre (Class IIa). The role of surgery for asymptomatic severe primary mitral regurgitation with preserved LV function and no other indications for surgery compared with active monitoring has not been studied in a randomised controlled clinical trial. A recent meta-analysis of the limited published propensity-matched studies comparing early surgery with active monitoring found better long-term mortality for early surgery, with no difference in operative mortality or repair rate between early surgery and surgery “triggered” by current criteria [3].

Acute severe mitral regurgitation owing to papillary muscle rupture in the setting of inferior (right coronary artery territory) or posterior (circumflex artery) STEMI is poorly tolerated and often leads to acute pulmonary oedema and cardiogenic shock. Emergency mitral valve replacement is indicated after initial stabilisation and assessment. Acute severe mitral regurgitation owing to acute chordal rupture or endocarditis may also lead to acute pulmonary oedema, but surgery may be delayed for days up to weeks with appropriate heart failure and/or antibiotic therapy.

Indications for surgery for chronic severe secondary ischaemic mitral regurgitation are more limited. Surgery is indicated in patients with severe secondary mitral regurgitation undergoing coronary artery bypass surgery with an LVEF >30% (Class I). Surgery should be considered in symptomatic severe secondary mitral regurgitation with an LVEF <30% provided there is an option for revascularisation and recruitable viable myocardium (Class IIa). Finally, surgery may be considered for symptomatic severe secondary mitral regurgitation and an LVEF >30% despite optimal medical therapy and low surgical risk (Class IIb), as judged by the Heart Team. The role of concomitant mitral valve surgery for moderate ischaemic mitral regurgitation in patients undergoing coronary artery bypass grafting remains controversial with a recent study finding no evidence of benefit for concomitant valve surgery [4].

Mitral valve surgery is only indicated for severe mitral stenosis in patients not judged suitable for percutaneous mitral commissurotomy (PMC) by the Heart Team (Class I recommendation) because of unfavourable characteristics or contraindications to PMC that include severe pulmonary hypertension, permanent atrial fibrillation and/or persistent left atrial appendage thrombus, previous commissurotomy, high echocardiographic anatomic scores (Wilkins, Cormier), and concomitant aortic or tricuspid valvular disease or significant coronary artery disease [2].

Preoperative assessment and the role of the Heart Valve Team

Patients for mitral valve surgery need careful assessment to help determine the operative risk, because significant comorbidities such as chronic obstructive airways disease, cerebrovascular disease, and renal disease become more common in an ageing population. In some patients, the long-term prognosis is affected more by their comorbidities than by their valvular disease and, in asymptomatic patients in particular, the benefits of intervention have to be carefully weighed against the risks. The assessment of operative risk has been facilitated by the development of scoring systems to estimate the risks of cardiac surgery. In the EuroSCORE II risk scoring system [5], a number of parameters have been identified on univariate analysis as influencing the outcome of surgery, as shown in Table 1.

Table 1. Variables associated with mortality for cardiac surgery (EuroSCORE II) [5].

^a Critical preoperative state is defined as ventricular tachycardia or fibrillation, aborted sudden death or cardiac massage, ventilation prior to surgery, inotropic support, ventricular assist device/balloon pump preoperatively or acute renal failure (anuria or oliguria <10 mL/h).

All patients should have routine haematological and biochemical assessment, coronary angiography, and up-to-date echocardiography. Right heart catheterisation may be useful when significant pulmonary hypertension has been identified on echocardiography, particularly in long-standing rheumatic mitral valve disease. Patients should have a dental assessment including an orthopantogram. Other investigations such as formal lung function tests or carotid duplex studies may be indicated according to an individual patient’s history and clinical assessment.

A recent consensus report has emphasised the importance of a multidisciplinary Mitral Valve Heart Team based within a Heart Valve Centre [6] for the assessment and management of patients with mitral valve disease requiring intervention. The Heart Team should include a cardiac surgeon with subspecialty expertise in mitral and tricuspid valve repair, a cardiologist with specialist expertise in valve disease, a specialist in echocardiography, a specialist in other imaging modalities (CT, cardiac MR), and an interventional cardiologist. Cardiac centres referring patients with mitral valve disease should either host or have access to such a specialised Heart Team. Each patient referred for intervention should be reviewed by the Heart Team to determine whether the valve anatomy is favourable for repair. Mitral valve repair should only be undertaken by expert surgeons within Heart Valve Centres. Higher annual individual surgeon and unit volumes are associated with higher repair rates and lower operative mortality [7]. Targets for surgical outcomes for repair of primary degenerative mitral regurgitation are shown in Table 2.

Table 2. Outcome targets for mitral valve repair for degenerative disease.

Adapted with permission from Chambers et al [6].

Surgical approach and operative procedure

Valve repair is preferable to valve replacement (Class I recommendation). The suitability and success of valve repair rather than replacement depends on valve pathology, the pathophysiological consequences, and surgical expertise and institutional experience [2]. The advantages of valve repair include the avoidance of long-term anticoagulation, prosthetic valve dysfunction, and paravalvular leak, with lower procedural risks and better long-term outcome. The preferred technique for mitral valve replacement is “chordal-sparing” replacement with preservation or neochordal reconstruction of the suvalvular apparatus. This technique has been shown to yield better long-term outcomes with respect to ventricular function and survival [8].

The mitral valve may be approached through a lower partial sternotomy, right thoracotomy, or a port access approach through the right chest using specialised instruments and a thoracoscopic camera for guidance. Robotic mitral valve surgery is feasible and safe but limited to specialised centres owing to the high costs of a surgical robot. Depending on the exposure, these minimal-access approaches may require peripheral cannulation for cardiopulmonary bypass, with specialised surgical equipment for venting and arresting the heart and clamping the aorta. There is a recognised learning curve for these newer surgical approaches. Nonetheless, the short-term and midterm outcomes of minimally invasive mitral valve surgery are equivalent to conventional surgery with respect to mortality, mitral valve repair rate and durability, and stroke. Minimally invasive mitral valve surgery has also been reported to produce less bleeding and blood transfusion, shorter ventilation time and intensive care stay, and less sternotomy-related morbidity [9].

Techniques for mitral valve repair for degenerative disease are well established with excellent long-term outcomes with respect to reoperation. More than 90% of degenerative mitral valves are suitable for repair using a combination of techniques: resection or plication of prolapsing or redundant leaflet tissue; chordal replacement with expanded polytetrafluoroethylene neochords; or annuloplasty, usually with implantation of a prosthetic ring or band to support the repair and prevent further annular dilatation [10].

Surgical repair for rheumatic mitral valve disease is more limited, depending on the extent and chronicity of rheumatic changes. Most patients require mitral valve replacement for a durable result from surgery, but closed and open commissurotomy may be performed in selected mitral stenosis cases with good long-term results [11].

The optimal technique for chronic ischaemic mitral regurgitation remains uncertain. Mitral valve repair with a downsizing annuloplasty [12] or an asymmetric annuloplasty ring [13] is associated with a high rate of recurrent mitral regurgitation and poorer long-term outcome related to the underlying ischaemic cardiomyopathy. A recent study comparing mitral valve repair with valve replacement found no difference in survival at two years, but valve repair was associated with more frequent recurrent mitral regurgitation and heart failure-related adverse events [14].

Choice of valve prosthesis

Biological or bioprosthetic valves for mitral valve replacement are made from glutaraldehyde-fixed porcine aortic leaflet or bovine pericardial tissue with a proprietary anti-calcification treatment mounted in an alloy frame. Mechanical valves are made from pyrolytic carbon and offer the advantage of excellent durability, but the disadvantages of long-term anticoagulation to prevent thromboembolism and the associated risk of bleeding. Earlier randomised studies comparing mechanical with biological valves found no difference in outcomes with respect to mortality, prosthetic valve endocarditis, or thromboembolism, but a recent large cohort study has found that there is a long-term mortality benefit associated with mechanical mitral valve replacement up to the age of 70; biological valves had a higher rate of reoperation, mechanical valves a higher risk of significant bleeding complications [15]. The choice of valve prosthesis for an individual patient depends on several factors including, most importantly, patient preference, age and life expectancy, metabolic factors predisposing to calcification and early structural valve deterioration (e.g., chronic kidney disease), any increased bleeding risk or contraindication to anticoagulation, expectation of pregnancy, previous infection, and risk of reoperation [2].

A mechanical prosthesis is recommended for patients <65 years and a bioprosthesis for patients >70 years or those in whom life expectancy is shorter than the expected bioprosthetic valve durability. Actuarial freedom from structural valve deterioration for a modern bovine pericardial mitral bioprosthesis has been reported as 100% at 10 years in patients >70 years at implantation [16], compared with 78±5%, 47±7% and 19±7% at 10, 15, and 20 years, respectively, in patients <65 years at initial surgery [17].

Anticoagulation is required for all currently available mechanical mitral valve prostheses. The intensity of anticoagulation depends on prosthesis valve characteristics, e.g., bileaflet or tilting disc, and patient factors such as a history of thromboembolism, atrial fibrillation, left atrial enlargement, and left ventricular dysfunction. The target international normalised ratio (INR) is 3.0 for modern bileaflet mechanical mitral valve prostheses (e.g., Medtronic, St. Jude, On-X, LivaNova) [18].

Operative risks and long-term outcomes

The risk of operative mortality is 1-2% overall for mitral valve repair and 5-6% for mitral valve replacement. Other common postoperative complications are rare in reported series: re-exploration for bleeding ~1%, sternal wound infection ~1%, respiratory failure ~5%, renal failure ~1%, stroke <2%, sepsis ~1% [19]. Endocarditis is a rare, late complication of mitral valve surgery reported in ~1% patients after a median 10-year follow-up [20].

Repair rates better than 90%, or even close to 100% in selected series, are reported by centres worldwide with expertise in valve repair. The cumulative reoperation rate is <1%/year, better for isolated posterior leaflet repair (0.5%), and worse for bileaflet (0.9%) or anterior leaflet (1.6%) repairs [19,20].

Conclusions

Mitral valve surgery is indicated for symptomatic severe primary mitral regurgitation and asymptomatic severe primary mitral regurgitation in patients with signs of left ventricular dysfunction, atrial fibrillation or pulmonary hypertension. Surgery is indicated in selected patients with asymptomatic severe primary mitral regurgitation with preserved left ventricular function and in sinus rhythm provided there is a high likelihood of durable repair by an experienced mitral surgeon within a Heart Valve Centre. Mitral valve repair is preferable to mitral valve replacement. Mitral valve surgery is indicated in patients with severe mitral stenosis and unfavourable characteristics for PMC. Mitral valve cases should be discussed by a multidisciplinary Mitral Valve Heart Team to determine the optimal interventional approach. Such discussion will become more important as newer, less invasive interventions become established.