Mitral valve disease

Author : Elyse Foster, M.D. Professor of Clinical Medicine and Anesthesia San Francisco, CA

2008-07-28

What is the mitral valve?

The mitral valve is one of four valves separating the chambers of the heart. The valve derives its name from the Latin word ‘mitra’ due to its anatomical resemblance to a bishop’s mitre or ceremonial cap. It is located between the left atrium and the left ventricle and is termed one of the two atrioventricular valves in the heart, the other being the tricuspid valve separating the right atrium from the right ventricle.

Structurally, the mitral valve consists of 2 parts or “cusps,” the anterior and posterior leaflets, and is sometimes referred to as a bicuspid valve (Figure 1 – Anatomy of Mitral Valve). The leaflets are held in place by the mitral valve annulus, a ring of tissue that separates the left atrium from ventricle. The leaflets themselves are unequal in size and shape, with the anterior leaflet shaped like an oval disc covering 2/3 of the opening, and the posterior leaflet resembling a crescent shape. The points at which the two leaflets join together at the annulus are termed commissures. The leaflets are also supported by a complex apparatus below the valve, which is situated within the left ventricle. There are two special bands of muscle bands, which arise directly from the left ventricular called the postero-medial and antero-lateral papillary muscles. These papillary muscles attach to the valve leaflets via numerous threadlike tissues called chordae tendineae.

What is the function of the mitral valve?

During the normal cardiac cycle, the mitral valve opens into the left ventricle during diastole, (the relaxation phase of the heart cycle), to allow blood flow from the left atrium into the ventricle. The normal mitral valve opening measures 4-6cm2 and allows blood to flow freely from left atrium to ventricle when the valve is open in diastole. This filling of the left ventricle occurs in two phases. The first, or early phase, occurs passively since the left atrial pressure is higher than left ventricular pressure, and accounts for 80% of ventricular filling. The second phase occurs during left atrial contraction in late diastole, when more blood is forced into the ventricle. This ‘atrial kick’ accounts generally for 20% of total left ventricular filling.

During ventricular contraction (systole), the pressure in the left ventricle rises above the pressure of the left atrium and forces the valve leaflets to close. The leaflets are held in the closed position rather than swinging the wrong direction into the left atrium due to the contraction of the papillary muscles and traction on the valve by the chordae tendineae. This allows systolic blood flow to exit the left ventricle via the aortic valve into the aorta rather than flowing back into the left atrium. The closing of the valve leaflets is heard on cardiac examination as a high frequency sound called the S1.

Mitral Stenosis

Mitral stenosis is an uncommon valvular disease that occurs when the mitral valve leaflets do not open normally, obstructing blood flow from the left atrium into the ventricle during diastole.

a) What causes mitral stenosis?

Mitral stenosis is due to rheumatic heart disease in the majority of cases. Rheumatic fever is a consequence of streptococcal disease, usually occurring after untreated episodes of strep throat. With good medical treatment of strep throat, the incidence of rheumatic fever in the United States has decreased and mitral stenosis has become relatively uncommon. Rheumatic heart disease leads to thickening and calcium deposition on the valvular apparatus, likely as a result of repeated bouts of inflammation. Typical changes include fusion of the commissures, which leads to narrowing of the valve opening, calcification of the valve leaflets, and shortening of the chordae tendineae. Other rare causes of mitral stenosis are congenital valvular abnormalities, infection of the mitral valve called endocarditis, and excessive calcium deposition seen in advanced kidney disease.

b) How does mitral stenosis affect the heart?

With mitral stenosis there is progressive narrowing of this valve area from the normal size of 4 - 6cm2. When the area decreases to 2cm2 or below, the mitral valve will hinder blood flow and lead to significant obstruction of left ventricular filling. This obstruction leads to an abnormal increase in pressure in the left atrium. Because the left atrium is directly connected to the circulation in the lung, elevation in left atrial pressure directly elevates the pressure in the lungs. Higher pressure, in turn, forces water to leak out of the capillaries into the lung tissue, causing fluid build-up in the lungs (‘pulmonary edema’), which impedes oxygen uptake and causes difficulty breathing. Over time, elevations in left atrial pressure cause the atrium to enlarge, and progressive narrowing in the valve area eventually leads the right ventricle to fail due to the high pulmonary (lung) pressures. Because the left ventricle is the only chamber that is ‘protected’ from high pressures, it usually has normal size and function in mitral stenosis.

c) What are the symptoms of mitral stenosis?

Many patients with mitral stenosis are initially symptom-free until the valve area narrows beyond a certain threshold. Thereafter, symptoms are related mainly to the development of progressive heart failure. Initially, patients may feel short of breath with exercise and gradually become more limited in activity, and may eventually progress to having trouble breathing at rest or during sleep. Pulmonary edema can also cause coughing, wheezing, and blood-tinged phlegm. When the right ventricle starts to fail there may be leg swelling and abdominal swelling. Rarely, enlargement of the left atrium can cause voice hoarseness due to nerve compression. Left atrial enlargement may also lead to an abnormal heart rhythm called atrial fibrillation and patients often present with palpitations or irregular heart beat. This rhythm can lead to clots within the left atrium, which can break off causing a stroke.

On physical examination, mitral stenosis causes a murmur during diastole that is often described as a low-frequency rumble. The calcified and restricted valve leaflets can produce a high-pitched sound when they open, called an opening snap.

d) How is mitral stenosis diagnosed?

Echocardiography, an ultrasound examination of the heart, is used to make the diagnosis of mitral stenosis. Two-dimensional moving images of the heart show poor separation of the mitral valve leaflets and characteristic doming of the leaflets during diastole (Figure 2 – 2D echo image of doming MV). Sometimes the appearance of the leaflet is said to resemble a hockey stick (arrow). Other echocardiographic techniques include Doppler ultrasound, which allows measurement of the pressure gradient across the mitral valve. The information obtained by echocardiography can be used to estimate the severity of the obstruction. This is done by calculating the actual area of the mitral valve opening, as well as measuring the pressure drop or gradient across the mitral valve (Table 1 – grading of MS).

Another test, which is sometimes used to estimate the severity of mitral stenosis is cardiac catheterization. This allows direct measurement of the pressure gradient across the mitral valve via catheters placed inside different chambers of the heart, which transduce direct pressure measurements (Figure 3 – LV/PCWP tracing in MS).

e) How is mitral stenosis treated?

The treatment of mitral stenosis generally depends on the level of associated symptoms (Figure 4 – modified diagram of ACC/AHA guidelines on management strategy for MS). In patients with very mild symptoms, medical therapy with diuretics and salt restriction may control fluid retention due to heart failure. In patients with more than mild symptoms however, consideration should be given to opening up the stenotic mitral valve with surgery or catheter-based techniques. The catheter-based procedure to treat mitral stenosis is called a percutaneous balloon mitral valvuloplasty. During this procedure, a catheter is placed across the mitral valve, and a balloon is inflated which dilates the mitral valve orifice. This procedure is most successful in mitral stenosis without extensive calcium deposits on the valve itself or significant associated mitral regurgitation (leakiness). In most patients this is very successful.

In patients with severe calcification of the valve, or significant associated regurgitation, surgery may be the preferred treatment. Surgery usually involves replacement of the mitral valve with a prosthetic valve, although in rare cases the valve can be repaired or the obstruction relieved without having to replace the valve itself.

If there is associated atrial fibrillation with mitral stenosis, as described above, treatment to control the heart rate is indicated. Given the risk of blood clots and potential stroke, treatment with blood thinners should be considered.

Mitral Regurgitation

Mitral regurgitation is very common. It occurs when the two leaflets of the mitral valve do not close completely, which allows blood to leak back through the closed valve. This happens during ventricular contraction when pressure in the ventricle is higher than that in the atrium, and causes some of the blood to leak back into the atrium, rather than exiting the ventricle forward into the aorta. Very small amounts of mitral regurgitation are common even when the valve itself is normal or nearly normal. This is referred to as “physiological” mitral regurgitation and increases with age.It is present in as many as 40% of people over age 60. Physiological mitral regurgitation needs to be distinguished from pathological mitral regurgitation, which is clearly abnormal. Pathological regurgitation can also be divided into two categories, primary when the valve itself is abnormal or secondary when abnormal function of the left ventricle affects the valve function. These important differences affect treatment.

a) Etiology:

Mitral regurgitation can result from abnormal functioning of any part of the mitral valve itself or of the structures in the heart that support it, including the leaflets, annulus, chordae tendineae, and papillary muscles (Table 2 – etiology of MR by anatomy).

Leaflet abnormalities can be due to abnormality of the valve tissue. One relatively common type is called ‘myxomatous’ degeneration in which the leaflets are replaced by scar tissue and become floppy and redundant (see section on mitral valve prolapse below). Other conditions affecting the leaflets include infection (‘endocarditis’), rheumatic heart disease that also causes mitral stenosis, and rare congenital abnormalities.

The annulus, or ring, in which the valve sits may also be abnormal with enlargement or an abnormal accumulation of calcium. Enlargement or dilation can occur when the annulus stretches beyond the normal size of the mitral valve, such that the leaflets can no longer cover the entire opening. Annular dilatation usually goes along with enlargement of the left ventricle, which occurs in heart failure. Calcification of the mitral annulus is seen in normal aging, but can be particularly pronounced in patients with high blood pressure, kidney disease, and rheumatic heart disease.

The chords that support the valve leaflets may occasionally rupture so that the tip of the valve “flails” into the left atrium. This most often occurs when there is myxomatous degeneration of the valve. These chords also thicken and shorten in rheumatic heart disease.

Abnormalities of the papillary muscle are most often due to decreased blood supply when there are blockages of the arteries supplying them with blood (coronary artery disease). The papillary muscles get their blood supply from branches off the coronary arteries, and so any obstruction of the coronary arteries due to cholesterol or a blood clot during a heart attack can lead to papillary muscle dysfunction. In the most severe case, infarction of the papillary muscle during a heart attack can lead to rupture of the muscle.


b) How does mitral regurgitation affect the heart?

Mitral regurgitation can affect the heart in two different ways depending on the time course of the disease, and can be divided into sudden onset (acute) versus slowly developing (chronic) mitral regurgitation. If mitral regurgitation occurs suddenly, as would happen in the case of a heart attack for example, the heart has not had a chance to ‘remodel’ to accommodate the increased blood flow. In this case, the back flow of blood into the left atrium during ventricular contraction can cause a sudden rise in left atrial pressure. Because the left atrium is connected to the lung circulation, the pressure rises in the pulmonary vessels. Higher pressure in the lungs causes water to leak out of the capillaries into the lung tissue, leading to fluid build-up in the lungs (‘pulmonary edema’) and difficulty breathing. In severe acute mitral regurgitation, the blood flow forward into the aorta can be decreased to the point at which not enough blood is exiting the heart. This can lead to low blood pressure and limited oxygen supply to the body, a life-threatening condition called cardiogenic shock, requiring emergency treatment.

In chronic mitral regurgitation that occurs over time, the heart gradually ‘remodels’ to accommodate the extra volume load. The backward blood flow causes extra volume load in the left atrium and ventricle, and over time both tend to dilate to compensate. This initially is helpful. As the chambers grow they can keep the pressure in the left atrium and ventricle from rising. However, over time this remodeling can damage the heart as it leads to less efficient heart function. Eventually the ventricular dilation can lead to decreased function of the heart muscle and congestive heart failure.


c) What are the symptoms of mitral regurgitation?

Acute mitral regurgitation can cause sudden pulmonary edema in which the lungs fill with fluid. The patient has severe difficulty breathing, cough, wheezing, and blood-tinged phlegm. This can sometimes be confused with a severe attack of asthma. In rare cases of cardiogenic shock, patients are extremely ill with low blood pressure causing lightheadedness, confusion, or decreased responsiveness.

Patients with chronic mitral regurgitation may have no symptoms for many years. However, over time symptoms of congestive heart failure may develop. Initially, patients may feel short of breath with exercise and gradually become more limited in activity, and may eventually progress to having trouble breathing at rest or during sleep. Other symptoms of heart failure include leg swelling, abdominal bloating, fatigue, and other symptoms of pulmonary edema as above.

On physical examination, mitral regurgitation is heard as a heart murmur during ventricular contraction or systole, and is often described as a ‘blowing’ murmur.


d) How is mitral regurgitation diagnosed?

Echocardiography, or ultrasound examination of the heart is used in making the diagnosis of mitral regurgitation, and is also crucial in determining the cause and severity of the valve disease. Color Doppler, an echocardiographic technique that visualizes blood flow through the heart, can detect mitral regurgitation as a jet of blood flowing from left ventricle to atrium during ventricular contraction (Figure 5 – color jet MR). The severity of mitral regurgitation is estimated by the appearance of the color jet, as well as other blood flow characteristics. Two-dimensional moving images by echocardiography also allow examination of the mitral valve itself and may help in determining the underlying cause of the leak. Sometimes a transesophageal echocardiogram is performed, during which a special ultrasound probe is inserted into the esophagus and stomach rather than obtaining images externally on the chest (‘transthoracic’). This allows for better visualization of the mitral valve apparatus when transthoracic image quality is suboptimal.

Cardiac catheterization can also be used to diagnose mitral regurgitation. During this procedure, a catheter is inserted into an artery or vein in the groin and positioned in different chambers of the heart, where it can measure the pressure. In mitral regurgitation, the pressure in the left atrium (estimated from a position called the pulmonary capillary wedge pressure) is elevated and often shows a characteristic waveform called a v-wave. This represents the pressure rise during ventricular contraction when the regurgitant blood enters the left atrium and produces an abnormally large pressure deflection (Figure 6 – v-wave).


e) How is mitral regurgitation treated?

Acute severe mitral regurgitation can be treated medically with drugs that reduce pressure in the aorta in order to encourage more blood to exit the ventricle rather than leaking back into the atrium, however most patients require urgent surgery.

The treatment of chronic mitral regurgitation is complicated, and depends on symptoms, severity, underlying cause, and patient characteristics such as age and other medical conditions (Figure 7 - modified diagram of ACC/AHA guidelines on management strategy for chronic MR). In general, patients with symptoms and moderate to severe mitral regurgitation due to abnormalities of the valve itself (“primary MR”) should be considered for surgical therapy. Patients without symptoms and severe mitral regurgitation warrant careful monitoring of left ventricular function by serial echocardiographic exams. Surgery may be indicated if the left ventricle becomes dilated or if function of the left ventricle falls even slightly, even if the patient continues to be free of symptoms. In these patients who show early signs of reduced heart function, treatment with medication may lead to unnecessary and potentially harmful delays in surgery.

When the mitral regurgitation is “secondary” the treatment needs to be aimed at the underlying problem, which is abnormal function of the left ventricle. Medical therapy with angiotensin-converting-enzyme inhibitors and other heart failure medications may improve mitral regurgitation in patients with underlying left ventricular dysfunction and can be tried before considering surgery. In these patients, surgical attempts to reduce the mitral regurgitation have had mixed results.

Surgical therapy for mitral regurgitation may involve repair of the valve or replacement with a prosthetic valve. In the case of ischemic mitral regurgitation, addressing the underlying coronary artery blockage with bypass surgery or catheter-based interventions is also important.

Mitral Valve Prolapse

Mitral valve prolapse is also known as click-murmur syndrome, floppy valve syndrome, myxomatous degeneration, and Barlow’s syndrome. It is the most common cause of mitral regurgitation in the United States and affects approximately 2% of the population.

a) Etiology and effects on the heart:

Mitral valve prolapse is due to myxomatous degeneration of the valve, which leads to redundant and stretchy leaflets, and chordae tendineae that have lost some of their strength. As the valve is redundant and more floppy, it can prolapse or bulge back the wrong way into the left atrium during ventricular contraction, rather than staying in the midline closed position. Because the leaflets do not stay closed, mitral regurgitation can occur and is typically seen late in systole after the valve has had time to prolapse. The degree of mitral regurgitation with prolapse can vary from very mild to severe, which usually occurs when chords have ruptured.


b) What are the symptoms of mitral valve prolapse?

Most patients with mitral valve prolapse are asymptomatic, and the condition is diagnosed based on a characteristic murmur heard on examination. In the past, mitral valve prolapse had been linked to many seemingly unrelated symptoms, such as panic attacks, chest pain, and syncope, although these associations are likely unrelated. There is an increased occurrence of abnormal heart rhythm such as atrial fibrillation, although the risk of a life-threatening heart rhythm is not increased. Other symptoms are related to the development of mitral regurgitation and associated symptoms of congestive heart failure.


c) How is mitral valve prolapse diagnosed and treated?

The diagnosis of mitral valve prolapse is often made on examination by a characteristic click and late systolic murmur. This is confirmed by echocardiography or ultrasound examination of the heart, which shows the prolapsing valve leaflet and any associated mitral regurgitation (Figure 8 – MVP on 2D echo). The treatment of mitral valve prolapse is determined by the degree of mitral regurgitation, and follows the same considerations as discussed in the previous section. In the past, patients with mitral valve prolapse were thought to have higher rates of valve infection and would receive antibiotics prior to dental procedures to prevent infection. However, recent practice guidelines advise against the use of routine antibiotics in patients with mitral valve prolapse, as prophylactic treatment may carry more risk of harm than good.

Summary

The mitral valve is located between the left atrium and left ventricle of the heart, and is crucial for the heart’s normal function.. It opens during ventricular relaxation to allow the ventricle to fill with blood flowing from the atrium, and closes during ventricular contraction to prevent blood from going backward, allowing blood to exit the heart into the aorta.

If either normal opening or closing are impaired in disease states, mitral stenosis or regurgitation can occur, respectively. Both conditions can cause symptoms of congestive heart failure when elevated pressures cause fluid build-up in the lungs and accompanying shortness of breath. The diagnosis is usually made by ultrasound examination of the heart, a test that can also guide appropriate therapy.

Links

www.americanheart.org
www.echoincontext.com
www.uptodate.com

References

1. Bonow RO, Carabello BA, Kanu C, de Leon AC, Jr., Faxon DP, Freed MD, Gaasch WH, Lytle BW, Nishimura RA, O'Gara PT, O'Rourke RA, Otto CM, Shah PM, Shanewise JS, Smith SC, Jr., Jacobs AK, Adams CD, Anderson JL, Antman EM, Faxon DP, Fuster V, Halperin JL, Hiratzka LF, Hunt SA, Lytle BW, Nishimura R, Page RL, Riegel B. ACC/AHA 2006 guidelines for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (writing committee to revise the 1998 Guidelines for the Management of Patients With Valvular Heart Disease): developed in collaboration with the Society of Cardiovascular Anesthesiologists: endorsed by the Society for Cardiovascular Angiography and Interventions and the Society of Thoracic Surgeons. Circulation. 2006;114(5):e84-231.

2. Bozkurt B, Mann DL. Cardiology patient page. Shortness of breath. Circulation. 2003;108(2):e11-13.

3. Carabello BA, Crawford FA, Jr. Valvular heart disease. The New England journal of medicine. 1997;337(1):32-41.

4. Flavell C, Stevenson LW. Take heart with heart failure. Circulation. 2001;104(18):E89-91.

5. Otto CM. Clinical practice. Evaluation and management of chronic mitral regurgitation. The New England journal of medicine. 2001;345(10):740-746.

6. Stern S, Behar S, Gottlieb S. Cardiology patient pages. Aging and diseases of the heart. Circulation. 2003;108(14):e99-101.

7. Turi ZG. Cardiology patient page. Mitral valve disease. Circulation. 2004;109(6):e38-41.