8 Mitral valve disease and mitral valve surgery

James Barnard

Introduction

Operations on the mitral valve are technically very satisfying for the surgeon and they offer significant benefits for patients, in terms of treating symptoms of heart failure and prolonging quality of life. There are various operative techniques for repair of the valve, for replacement and for minimally invasive intervention. There is also much ongoing research looking into the benefits of new treatment modalities.

This chapter describes the anatomical structure of the mitral valve, the pathophysiology, the aetiology of mitral valve disease and the symptoms and signs that patients present with. It then goes on to describe the surgical treatments that are currently available.

Anatomical structure of the mitral valve

The function of the mitral valve is to maintain forward flow of blood between the left atrium and the left ventricle. It has anterior and posterior leaflets. It is an asymmetrical oval in shape, with one side flattened slightly where the valve is in contact with the aortic valve and with the anterior leaflet occupying most of the cross-sectional area of the valve when it is closed. The one-way mechanism of the mitral valve is facilitated by string-like structures called chordae tendineae, which originate from the papillary muscle and attach to different areas on the ventricular surface of the mitral valve leaflet. These chords are classified as primary, secondary and tertiary, depending on their area of attachment to the mitral valve leaflet (McCarthy, Ring & Rana 2010).

Each leaflet is divided into three scallops: P1, P2 and P3 of the posterior leaflet, starting from anterolateral to posteromedial commissure; and the corresponding area on the anterior leaflet, divided into A1, A2 and A3. The mitral annulus is saddle-shaped and is divided into the anterior and posterior annulus according to the leaflet attachment. The mitral annulus is an anatomical border between the ventricle and the atrium. The anterior leaflet of the valve attaches to the fibrous skeleton of the heart. There are two more prominent and fibrous areas, called the right and left trigones, which are located above the anterolateral and posteromedial commissures. There is an absence of any well-defined structure where the posterior leaflet attaches to the annulus. The inter-trigonal distance and the height of the anterior leaflet are the two important clinical landmarks used when sizing annuloplasty rings in mitral valve repair procedures (Ender et al. 2011).

There are two main papillary muscles, the anterolateral and the posteromedial, which give a secure foundation for the chordae tendineae that attach to both anterior and posterior leaflets. The anterolateral papillary muscle is large and usually has a single head, whereas the posteromedial papillary muscle is flat with two or more heads. The blood supply to the anterolateral papillary muscle is usually from the left anterior descending coronary artery and the circumflex coronary arteries, whereas the blood supply to the posteromedial papillary muscle is from a single source. This single vessel blood supply explains why this papillary muscle may rupture if there is a myocardial infarction involving the posterior descending coronary artery (Jain et al. 2013).

Pathophysiology

The two main haemodynamic consequences of pathology affecting the mitral valve are mitral regurgitation and mitral stenosis.

Mitral regurgitation

MR results in a backflow of blood into the left atrium through an incompetent valve, causing a volume overload. Over time, this results in an eccentric hypertrophy of the left ventricle and ultimately a reduction of left ventricular contractile function. There is also a gradual increase in left ventricular end-systolic volume and left atrial and pulmonary venous pressures, which eventually lead to congestive heart failure (McCarthy, Ring & Rana 2010).

Mitral regurgitation (MR) can be caused by a variety of aetiologies. But before discussing its causes, it may be useful to look at Carpentier’s classification of mitral regurgitation (see Figure 8.1), which helps in formulating a management plan according to the underlying pathology (Carpentier 1983).

Figure 8.1: Pathological examples of Carpentier’s Classification (adapted from Carpentier 1983)

MR can be acute or chronic. Acute MR is characterised by acute volume overload. It differs from the chronic volume overload of chronic MR in which atrial and ventricular enlargement, together with ventricular wall hypertrophy, can maintain cardiac output. Acute MR is not tolerated very well and it can cause haemodynamic compromise and acute pulmonary oedema, whereas chronic MR is well tolerated and patients generally present with symptoms before severe heart failure manifests (Mokadam, Stout & Verrier 2011).

Common causes of acute MR include papillary muscle rupture (post myocardial infarction), infective endocarditis and chordal rupture. Common causes of chronic MR include rheumatic fever, myxomatous degeneration and secondary MR.

MR may also be classified according to whether the leak across the mitral valve is a result of primary leaflet pathology or due to pathology that is not related to the leaflet (i.e. secondary MR). Secondary MR can be further divided into MR that is due to ischaemic heart disease and MR that is due to other causes, which can be termed functional MR. Functional MR is the result of gradual dilatation of the annular-ventricular apparatus with alterations in the left ventricular geometry that result in MR (Yamauchi et al. 2013).

Mitral stenosis

Mitral stenosis is characterised by a restriction in flow from the left atrium to the left ventricle. This results in an increase in pressure in the left atrium and an increase in pulmonary vascular pressure, which eventually leads to pulmonary hypertension and right ventricular failure. The progress of mitral stenosis is slow and it usually takes a couple of decades before the patient becomes symptomatic (Mokadam, Stout & Verrier 2011).

The most common cause of MS is rheumatic heart disease. Other causes include calcification of the mitral valve leaflets, mitral annular calcification, congenital MS, endomyocardial fibroelastosis, malignant carcinoid syndrome, systemic lupus erythematosus and Whipple disease, Fabry disease (rare genetic lysosomal disease) and rheumatoid arthritis (Shah & Sharma 2018). The pathological effects of rheumatic fever can take several decades to produce symptoms and signs of heart disease as a result of valve damage. Usually contracted at a young age, only around 50% of patients progress to rheumatic fever, which is more common in women than men (Carapetis et al. 2018).

The pathophysiology of mitral stenosis may also manifest in some circumstances with an anatomically normal mitral valve but with a left atrial myxoma, cor triatum or a significant stenosis of a pulmonary vein. Mitral stenosis caused by an atrial myxoma can be hazardous, as the patient may have complete, or near complete, dynamic obstruction of the mitral valve orifice with severe haemodynamic compromise.

Signs and symptoms of mitral valve disease

Patients with mitral valve disease can be asymptomatic initially, especially with mitral stenosis which has a latent period of many years. But some patients may develop symptoms of dyspnoea, palpitations, fatigue or weakness, orthopnoea and paroxysmal nocturnal dyspnoea. If left untreated, patients may develop pulmonary hypertension and subsequently right heart failure. Increased volume and pressure overload cause left atrial dilatation and atrial fibrillation, which may give rise to thrombus formation in the left atrial appendage and a risk of thromboembolism. Rarely, an enormously enlarged left atrium may cause pressure symptoms like dysphagia, hoarseness (Ortner’s syndrome) and left lung collapse (Sarin & Bhardwaj 2016).

Signs of mitral regurgitation

Signs associated with MR are apical thrill, displaced apex beat, and a pansystolic or holosystolic murmur. This murmur is best heard over the mitral area, in expiration, in a lateral decubitus position, with the diaphragm of the stethoscope. The murmur may radiate to the left axilla. It is also worth knowing that the intensity of the murmur is a poor predictor of severity of mitral regurgitation. Further signs may be present if there is pulmonary hypertension, including a third heart sound, a diastolic flow murmur and evidence of a right ventricular heave on palpation of the chest (Bhattacharya & Sharma 2018).

Signs of mitral stenosis

Various signs are present in mitral stenosis patients, such as low volume pulse (decreased filling of left ventricle), tapping but non-displaced apex beat in contrast to displaced apex beat in MR, and irregularly irregular pulse (atrial fibrillation). One of the most prominent signs may be the loud mitral component (M1) of the first heart sound (S1) which is made by the mitral and tricuspid valves closing. The sound is louder because of the increased force closing the mitral valve.

Other signs include an opening snap (high-pitched additional sound heard after the A2 [aortic] component of the second heart sound [S2], which correlates to the forceful opening of the mitral valve). Back pressure through the left atrium and the pulmonary vasculature may result in pulmonary hypertension, and this can potentially result in a loud second heart sound (S2), due to the closure of the pulmonary valve under force. Classically, a mid-diastolic low-pitched rumbling murmur with presystolic accentuation is described after the opening snap is heard. The murmur is detected optimally at the apical area on the chest wall and is listened for with the bell of the stethoscope, with the patient tilted towards their left side.

Advanced mitral stenosis may result in signs of right-sided heart failure such as raised jugular venous pressure, parasternal heave, hepatomegaly, ascites and (as previously mentioned) pulmonary hypertension. A further systemic sign includes a malar flush in the patient’s face that is the result of back pressure and a build-up of carbon dioxide resulting in vasodilatation (Chandrashekhar, Westaby & Narula 2009).

Investigations

Radiographic signs that may be present in patients with mitral stenosis include a double atrial shadow and splaying of the carina secondary to a severely enlarged left atrium.

Echocardiography

Echocardiography is the gold standard for diagnosing mitral valve pathology. It is performed using either the trans-thoracic echo or trans-oesophageal echo approach (Biswas & Yassin 2015). It is important not only to make the diagnosis of mitral valve disease, but also to assess the nature of the pathology and the magnitude of the problem. Vital information required to assess the severity of MR includes the regurgitant volume, the regurgitant fraction, the jet area percentage, the size of the vena contracta and the effective orifice area (see Table 8.1) (Baumgartner et al. 2009; Zoghbi et al. 2017).

The criteria for ischaemic MR to be classified as severe, moderate or mild are different from those of degenerative MR (Vahanian et al. 2012). Important parameters to measure or assess mitral stenosis severity include effective orifice area, gradient across the valve and the degree of pulmonary hypertension (see Table 8.1) (Baumgartner et al., 2009; Zoghbi 2017).

Table 8.1: Assessing the severity of mitral regurgitation

*LA – left atrium, EROA – effective regurgitant orifice area; table obtained from EAE/ASE guidelines (based on Baumgartner et al. 2009).

Table 8.2: Assessing the severity of mitral stenosis

*PA – pulmonary artery; table obtained from EAE/ASE guidelines (based on Baumgartner et al. 2009).

Cardiac magnetic resonance imaging

Cardiac magnetic resonance (CMR) is well established as an excellent modality to assess LV volumes, ejection fraction and left ventricular (LV) mass. In MR it is particularly useful when calculating regurgitant volume to help quantify the degree of regurgitation. In degenerative mitral valve disease there is no need for routine CMR scanning. CMR is useful when assessing ischaemic MR which correlates with LV end-systolic volume and interpapillary muscle distance. CMR may also inform clinicians about myocardial muscle viability when concomitant revascularisation is being considered for ischaemia (Cavalcante 2016).

Coronary angiography

Coronary angiography is indicated in patients aged 40 years or above, who are scheduled for mitral valve repair or replacement (Vahanian et al. 2012). It defines coronary anatomy in relation to the mitral valve and can identify significant coronary disease to allow intervention at the time of surgery. The surgeon is better prepared to perform surgery if they are aware of the dominance of the coronary circulation, understanding that a left-dominant coronary artery circulation system is more susceptible to damage due to the proximity of the circumflex artery to the mitral annulus.

Cardiac chamber catheterisation

Another method of measuring the severity of MS is simultaneous left- and right-heart chamber catheterisation. Right-heart catheterisation (commonly known as Swan-Ganz catheterisation) gives the physician the mean pulmonary capillary wedge pressure, which reflects the left atrial pressure. Left- heart catheterisation gives the pressure in the left ventricle. Simultaneous pressure measurement allows determination of the gradient between the left atrium and left ventricle during ventricular diastole. This method of evaluating MS tends to overestimate the degree of gradient because of the time lag in the pressure tracings seen on the right-heart catheterisation and the slow Y descent seen on the wedge tracings. A more accurate method of measuring the gradient is at the time of a trans-septal puncture during right-heart catheterisation.

Medical therapy and mitral regurgitation

Mitral regurgitation in the acute setting is poorly tolerated and frequently requires urgent surgical intervention. Medical management includes afterload reduction with arteriolar vasodilators, use of an inotrope with a vasodilator if the patient is hypotensive, and use of an intra-aortic balloon pump.

In patients with chronic MR, the aim is to reduce preload, reduce afterload and maintain sinus rhythm. In patients who are asymptomatic, a watch-and-wait approach may be taken, with regular valve surveillance to await symptoms or the development of LV dilatation, reduced LV function or new atrial fibrillation. These phenomena increase the balance in favour of undergoing surgery. Evidence shows that patients who undergo surgery while still in New York Heart Association (NYHA) functional class I to II groups live longer than patients who do not undergo surgery until NYHA functional class III to IV symptoms have developed (Asano et al. 2017).

There is a role for cardiac re-synchronisation therapy for patients with secondary MR who are not suitable for surgical intervention. Patients deemed inoperable or too high risk for surgical intervention with secondary MR can be considered for percutaneous mitral clip procedure if they still have debilitating symptoms despite the use of optimal medical therapy (Vahanian et al. 2012).

Medical therapy and mitral stenosis

Diuretics, nitrates and beta-blockers can improve symptoms to a degree and for a period but medical treatment in mitral stenosis is not very promising. Anti-coagulation is indicated if the patient develops atrial fibrillation with a target international normalised ratio (INR) between 2 and 3. Anti-coagulation should be considered in patients with left atrial thrombus, embolic phenomenon, large left atrial size and if the patient has significant spontaneous contrast mechanism secondary to significant stasis in the left atrium (Vahanian et al. 2012).

Indications for surgery in mitral regurgitation

Surgical indications are elaborated in the European Society of Cardiology guidelines on valvular heart diseases (Vahanian et al. 2012). There are numerous indications for surgery. However, severe mitral regurgitation with symptoms or other signs of heart failure (left ventricular dysfunction, atrial fibrillation and pulmonary hypertension) give a mandate for surgical intervention. Mitral valve repair is the preferred technique for primary severe MR. It is associated with less risk of mortality and better prognosis, compared to mitral valve replacement (Ramlawi & Gammie 2016).

Preoperative functional class is important when making decisions about mitral valve repair. There is evidence that NYHA functional class III and IV symptoms are associated with greater short- and long-term mortality, when compared with the patients who only have NYHA class I and II symptoms. This finding informs decision-making in patients with no symptoms or mild symptoms, and patients who have low operative risk, as it avoids the development of the more severe symptoms that correlate with poor outcome (Asano et al. 2017).

As far as secondary MR (including ischaemic MR) is concerned, there has been controversy regarding the best management. The 2012 guidelines from the European Society of Cardiology suggest that in patients without coronary artery disease medical therapy should be the first-line treatment for secondary MR (Vahanian et al. 2012). Patients with severe MR and a LVEF >30%, who are undergoing CABG, have a Class IC indication for surgery. Recent data suggest comparable or even better results with replacement of the mitral valve by preserving the subvalvular apparatus in ischaemic mitral regurgitation patients (Acker et al. 2014). Moreover, mitral valve replacement is preferred in cases of acute severe MR, secondary to papillary muscle rupture. Although in expert hands repair can be performed there is little evidence regarding robust long-term durability.

Indications for surgery in mitral stenosis

Percutaneous balloon mitral valvotomy (PBMV) is the first-line treatment for patients with symptomatic moderate to severe mitral stenosis if the valvular morphology is favourable, which can be determined by Abascal or Wilkins echocardiographic score (Wilkins et al. 1988). This scoring system was first described in 1990 to predict the morphological suitability of rheumatic mitral stenosis for PBMV. Surgery is recommended for scores >8–9. The scoring system has four main components which look at: (1) leaflet mobility; (2) leaflet thickening; (3) subvalvular thickening; and (4) leaflet calcification. PBMV is contraindicated in the presence of moderate mitral regurgitation and thrombus in left atrial appendage, and surgery is recommended for these patients.

Mitral valve endocarditis

Endocarditis vegetations may affect valve motion and lead to MR. Vegetations more commonly embolise from the mitral than the aortic valve and may cause significant damage to their destination. Stroke is the most commonly observed clinical manifestation of embolisation. Vegetation growth on a leaflet may also cause leaflet perforation or can cause the rupture of one of the chordae tendineae (see Figure 8.2). Infective endocarditis on a prosthetic valve usually begins on the sewing cuff and generally tracks outside of the valvular apparatus leading to dehiscence of the sewing ring (Pham et al. 2012).