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Mitral regurgitation (MR), mitral insufficiency or mitral incompetence is a valvular heart disease. It is the abnormal leaking of blood through the mitral valve, from the left ventricle into the left atrium of the heart, when the left ventricle pumps blood to the rest of the body. It is the most common form of valvular heart disease.

Etiology

The mitral valve is composed of the valve leaflets, the mitral valve annulus (which forms a ring around the valve leaflets), the papillary muscles (which tether the valve leaflets to the left ventricle, preventing them from prolapsing into the left atrium), and the chordae tendineae (which connect the valve leaflets to the papillary muscles). A dysfunction of any of these portions of the mitral valve apparatus can cause mitral regurgitation.

The most common cause of primary mitral regurgitation in the United Statesmarker (causing about 50% of primary mitral regurgitation) is myxomatous degeneration of the valve. Myxomatous degeneration of the mitral valve is more common in females, and is more common in advancing age. It is due to a genetic abnormality that results in a defect in the collagen that makes up the mitral valve. This causes a stretching out of the leaflets of the valve and the chordae tendineae. The elongation of the valve leaflets and the chordae tendineae prevent the valve leaflets from fully coapting when the valve is closed, causing the valve leaflets to prolapse into the left atrium, thereby causing mitral regurgitation.

Ischemic heart disease causes mitral regurgitation by the combination of ischemic dysfunction of the papillary muscles, and the dilatation of the left ventricle that is present in ischemic heart disease, with the subsequent displacement of the papillary muscles and the dilatation of the mitral valve annulus.

Secondary mitral regurgitation is due to the dilatation of the left ventricle, causing stretching of the mitral valve annulus and displacement of the papillary muscles. This dilatation of the left ventricle can be due to any cause of dilated cardiomyopathy, including aortic insufficiency, nonischemic dilated cardiomyopathy and Noncompaction Cardiomyopathy.

Pathophysiology

Comparison of acute and chronic mitral regurgitation
Acute mitral regurgitation Chronic mitral regurgitation
Electrocardiogram Normal P mitrale, atrial fibrillation, left ventricular hypertrophy
Heart size Normal Cardiomegaly, left atrial enlargement
Systolic murmur Heard at the base, radiates to the neck, spine, or top of head Heard at the apex, radiates to the axilla
Apical thrill May be absent Present
Jugular venous distension Present Absent


The pathophysiology of mitral regurgitation can be broken into three phases of the disease process: the acute phase, the chronic compensated phase, and the chronic decompensated phase.

Acute phase

Acute mitral regurgitation (as may occur due to the sudden rupture of a chordae tendineae or papillary muscle) causes a sudden volume overload of both the left atrium and the left ventricle. The left ventricle develops volume overload because with every contraction it now has to pump out not only the volume of blood that goes into the aorta (the forward cardiac output or forward stroke volume), but also the blood that regurgitates into the left atrium (the regurgitant volume). The combination of the forward stroke volume and the regurgitant volume is known as the total stroke volume of the left ventricle.

In the acute setting, the stroke volume of the left ventricle is increased (increased ejection fraction), this happens because of more complete emptying of heart.however as it progresses the LV volume increases and the contractile function deteriorates and thus leading to dysfunctional LV and a decrease in ejection fraction . The mechanism by which the total stroke volume is increased is known as the Frank-Starling mechanism.

The regurgitant volume causes a volume overload and a pressure overload of the left atrium. The increased pressures in the left atrium inhibit drainage of blood from the lungs via the pulmonary veins. This causes pulmonary congestion.

Chronic compensated phase

If the mitral regurgitation develops slowly over months to years or if the acute phase can be managed with medical therapy, the individual will enter the chronic compensated phase of the disease. In this phase, the left ventricle develops eccentric hypertrophy in order to better manage the larger than normal stroke volume. The eccentric hypertrophy and the increased diastolic volume combine to increase the stroke volume (to levels well above normal) so that the forward stroke volume (forward cardiac output) approaches the normal levels.

In the left atrium, the volume overload causes enlargement of the chamber of the left atrium, allowing the filling pressure in the left atrium to decrease. This improves the drainage from the pulmonary veins, and signs and symptoms of pulmonary congestion will decrease.

These changes in the left ventricle and left atrium improve the low forward cardiac output state and the pulmonary congestion that occur in the acute phase of the disease. Individuals in the chronic compensated phase may be asymptomatic and have normal exercise tolerances.

Chronic decompensated phase

An individual may be in the compensated phase of mitral regurgitation for years, but will eventually develop left ventricular dysfunction, the hallmark for the chronic decompensated phase of mitral regurgitation. It is currently unclear what causes an individual to enter the decompensated phase of this disease. However, the decompensated phase is characterized by calcium overload within the cardiac myocytes.

In this phase, the ventricular myocardium is no longer able to contract adequately to compensate for the volume overload of mitral regurgitation, and the stroke volume of the left ventricle will decrease. The decreased stroke volume causes a decreased forward cardiac output and an increase in the end-systolic volume. The increased end-systolic volume translates to increased filling pressures of the ventricular and increased pulmonary venous congestion. The individual may again have symptoms of congestive heart failure.

The left ventricle begins to dilate during this phase. This causes a dilatation of the mitral valve annulus, which may worsen the degree of mitral regurgitation. The dilated left ventricle causes an increase in the wall stress of the cardiac chamber as well.

While the ejection fraction is less in the chronic decompensated phase than in the acute phase or the chronic compensated phase of mitral regurgitation, it may still be in the normal range (ie: > 50 percent), and may not decrease until late in the disease course. A decreased ejection fraction in an individual with mitral regurgitation and no other cardiac abnormality should alert the physician that the disease may be in its decompensated phase.

Symptoms and signs

The symptoms associated with mitral regurgitation are dependent on which phase of the disease process the individual is in. Individuals with acute mitral regurgitation will have the signs and symptoms of decompensated congestive heart failure (ie: shortness of breath, pulmonary edema, orthopnea, paroxysmal nocturnal dyspnoea), as well as symptoms suggestive of a low cardiac output state (ie: decreased exercise tolerance). Cardiovascular collapse with shock (cardiogenic shock) may be seen in individuals with acute mitral regurgitation due to papillary muscle rupture or rupture of a chordae tendineae.

Individuals with chronic compensated mitral regurgitation may be asymptomatic, with a normal exercise tolerance and no evidence of heart failure. These individuals may be sensitive to small shifts in their intravascular volume status, and are prone to develop volume overload (congestive heart failure).

Findings on clinical examination depends of the severity and duration of mitral regurgitation. The mitral component of the first heart sound is usually soft and is followed by a pansystolic murmur which is high pitched and extends, as the name suggests, the whole of systole. The murmur usually radiates to the axilla. The loundness of murmur does not correlate well with the severity of regurgitation. Patients may also have a third heart sound.

Patients with mitral valve prolapse often have a mid-to-late systolic click and a late systolic murmur.

Diagnostic studies

There are many diagnostic tests that have abnormal results in the presence of mitral regurgitation. These tests suggest the diagnosis of mitral regurgitation and may indicate to the physician that further testing is warranted. For instance, the electrocardiogram (ECG) in long standing mitral regurgitation may show evidence of left atrial enlargement and left ventricular hypertrophy. Atrial fibrillation may also be noted on the ECG in individuals with chronic mitral regurgitation. The ECG may not show any of these finding in the setting of acute mitral regurgitation.

The quantification of mitral regurgitation usually employs imaging studies such as echocardiography or magnetic resonance angiography of the heart.

Chest x-ray

The chest x-ray in individuals with chronic mitral regurgitation is characterized by enlargement of the left atrium and the left ventricle. The pulmonary vascular markings are typically normal, since pulmonary venous pressures are usually not significantly elevated.

Echocardiography

transesophageal echocardiogram of mitral valve prolapse
The echocardiogram is commonly used to confirm the diagnosis of mitral regurgitation. Color doppler flow on the transthoracic echocardiogram (TTE) will reveal a jet of blood flowing from the left ventricle into the left atrium during ventricular systole.

Because of the inability in getting accurate images of the left atrium and the pulmonary veins on the transthoracic echocardiogram, a transesophageal echocardiogram may be necessary to determine the severity of the mitral regurgitation in some cases.

Factors that suggest severe mitral regurgitation on echocardiography include systolic reversal of flow in the pulmonary veins and filling of the entire left atrial cavity by the regurgitant jet of MR.

Quantification of mitral regurgitation

Determination of the degree of mitral regurgitation
Degree of mitral regurgitation Regurgitant fraction Regurgitant Orifice area
Mild mitral regurgitation 20 percent
Moderate mitral regurgitation 20 - 40 percent
Moderate to severe mitral regurgitation 40 - 60 percent
Severe mitral regurgitation > 60 percent > 0.3 cm2
The degree of severity of mitral regurgitation can be quantified by the percentage of the left ventricular stroke volume that regurgitates into the left atrium (the regurgitant fraction).

Methods that have been used to assess the regurgitant fraction in mitral regurgitation include echocardiography, cardiac catheterization, fast CT scan, and cardiac MRI.

The echocardiographic technique to measure the regurgitant fraction is to determine the forward flow through the mitral valve (from the left atrium to the left ventricle) during ventricular diastole, and comparing it with the flow out of the left ventricle through the aortic valve in ventricular systole. This method assumes that the aortic valve does not suffer from aortic insufficiency.

regurgitant fraction   =   \frac{V_{mitral} - V_{aortic}} {V_{mitral}}


where Vmitral and Vaortic are respectively the volumes of blood that flow forward through the mitral valve and aortic valve during a cardiac cycle.

Another way to quantify the degree of mitral regurgitation is to determine the area of the regurgitant flow at the level of the valve. This is known as the regurgitant orifice area, and correlates with the size of the defect in the mitral valve. One particular echocardiographic technique used to measure the orifice area is measurement of the proximal isovelocity surface area (PISA). The flaw of using PISA to determine the mitral valve regurgitant orifice area is that it measures the flow at one moment in time in the cardiac cycle, which may not reflect the average performance of the regurgitant jet.

Treatment

The treatment of mitral regurgitation depends on the acuteness of the disease and whether there are associated signs of hemodynamic compromise.

In acute mitral regurgitation secondary to a mechanical defect in the heart (ie: rupture of a papillary muscle or chrordae tendineae), the treatment of choice is urgent mitral valve replacement. If the patient is hypotensive prior to the surgical procedure, an intra-aortic balloon pump may be placed in order to improve perfusion of the organs and to decrease the degree of mitral regurgitation.

If the individual with acute mitral regurgitation is normotensive, vasodilators may be of use to decrease the afterload seen by the left ventricle and thereby decrease the regurgitant fraction. The vasodilator most commonly used is nitroprusside.

Individuals with chronic mitral regurgitation can be treated with vasodilators as well. In the chronic state, the most commonly used agents are ACE inhibitors and hydralazine. Studies have shown that the use of ACE inhibitors and hydralazine can delay surgical treatment of mitral regurgitation. The current guidelines for treatment of mitral regurgitation limit the use of vasodilators to individuals with hypertension, however.

There are two surgical options for the treatment of mitral regurgitation: mitral valve replacement and mitral valve repair. Minimally-invasive surgical techniques for mitral valve repair are showing promise in clinical development.

Indication for surgery

Indications for surgery for chronic mitral regurgitation
Symptoms LV EF LVESD
NYHA II - IV > 60 percent 45 mm
Asymptomatic or symptomatic 50 - 60 percent ≥ 45 mm
Asymptomatic or symptomatic 50 percent and ≥ 45 mm
Pulmonary artery systolic pressure ≥ 50 mmHg
Indications for surgery for chronic mitral regurgitation include signs of left ventricular dysfunction. These include an ejection fraction of less than 60 percent and a left ventricular end systolic dimension (LVESD) of more than 45 mm.

References

  1. Mitral valve regurgitation at Mount Sinai Hospital
  2. Harrison's Internal Medicine 17th edition


See also



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