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The area of a normal mitral valve is 4-6 cm2. This area is sufficient to allow a great deal of blood to flow from the left atrium (LA) to the left ventricle (LV). When any disease process decreases this area, mitral stenosis (MS) develops. As MS progresses over time, the pressure in the LA rises. This pressure is transmitted to the pulmonary veins, the pulmonary vasculature, and eventually to the right side of the heart. This transmitted pressure is responsible for nearly all of the clinical manifestations of MS. Etiologies By far the number one cause of MS is rheumatic valvular disease (RVD). This accounts for approximately 98% of all MS. RVD is more common in females than in males and up to 50% of the time, a person with newly diagnosed MS will not report a history of rheumatic fever as a child. The incidence of MS has drastically benn reduced in the United States since the advent of antibiotics resulting in the aggressive treatment of Streptococcus pyogenes pharyngitis, the primary cause of rheumatic heart disease. Many other causes if impaired LV filling exist and they can mimic MS in both hemodynamics and symptoms. LA myxomas are relatively common and occur as a part of the Carney complex. These slow growing tumors obstruct the mitral valve causing MS. Pulmonary vein stenosis after attempted radiofrequency ablation of atrial fibrillation is becoming more common and this mia mimic MS. The anterior leaflet of the mitral valve is the most common site for a infectious vegitation to form in patients with infective endocarditis. If the vegitation grows to a large enough size, as often occurs in fungal endocarditis, obstruction of LV filling may occur and symptoms of MS may develop. Some other rare causes of true MS include congenital MS, severe mitral annular calcification, and prosthetic valve dysfunction. A complete list of cuases of MS including those that mimic MS is below. The rest of this review focuses on MS due to RVD. Pathophysiology In RVD, an autoimmune attack on the mitral valve produces thickening of the valve leaflets. The mitral valve is often describes as having a "fish-mouth" appearance. Whether the worsening of MS over time is secondary to a smoldering rheumatic process or simply mitral valve damage due to high pressures and turbulence is unknown. Both most likely contribute to the progressive nature of MS. Over time as the MS worsens and the mitral valve area decreases, a pressure gradient develops between the LA and the LV to help assist diastole. The elevated LA pressures are transmitted to the pulmonary veins and the right side of the heart. On exertion as cardiac output increases and the velocity of flow through the mitral valve increases, the transmitted pressure gradient becomes exponentially larger. This is explained by the modified Bernoulli equation: Pressure gradient = 4v2 Thus if the velocity (v) of flow is doubled, the transmitral pressure gradient increases by 4 times. The resultant large increase in LA pressure is primarily responsible for the exertional symptoms seen in MS. Signs and Symptoms Mitral stenosis is often asymptomatic early in disease until the mitral valve area decreases enough to cause a large increase in LA pressure. The first symptoms of MS occur on exertion as explained above. This occurs since the mitral valve area is fixed, the cardiac output is unable to increase enough above resting (a low cardiac reserve is present), and high pressures are transmitted to the pulonary vasculature since LA pressures increase exponentially on exertion. This tramsmission of pressures results in exertional dyspnea. Fatigue and inability to exercise are also common complaints. Signs of left heart failure such as PND and orthopnea can occur. Symptoms also occur in other situations that require an increased cardiac output for the same reason such as pregnancy, anemia, and thyrotoxicosis. Most patients with moderate to severe MS will have some degree left atrial enlargement (LAE) due to the chronic increased LA pressures. This predisposes them to atrail fibrillation. Since people with MS rely on atrial contraction for abot 20% of their cardiac output and since tachycardia decreases diastolic filling time, the onset of atrial fibrillation with a rapid ventricular rate and loss of atrial contraction results in significant symptoms of low cardiac output and heart failure. These include fatigue, dyspnea, lightheadedness, and even syncope. In the absence of atrial fibrillation, patients with MS still have an increased risk of thrombus formation in their LA due to stagnation of blood. This may lead to embolic events including stroke, acute myocardial infarction, acute mesenteric ischemia, or "Blue to syndrome". Hemoptysis may occur due to sudden rupture of a broncial vein. Thie phenomenon is termed "pulmonary apoplexy". Ortner syndrome may occur when a massively enlarged LA compresses the left recurrent laryngeal nerve leading to a hoarse voice. Chest pain from right sided heart strain may occur due to severe venous pulmonary hypertension. Other signs of right heart failure such as right upper quadrant pain (due to hepatic congestion) and peripheral edema may occur. Physical examination Inspection of the jugular venous pulsations may reveal a prominent wave due to vigorous atrial contraction of a prominent v wavedue to tricuspid regurgitation that develops due to pulmnary hypertension. The presence of "mitral facies" refers to a pinkish-purple discoloration of the cheeks produced by a chronic low cardiac output state combined with systemic vasoconstriction. This sign is rare and non-specific. Palpation may reveal a palpable S1 over the apex. This finding is pathognomonic for MS. A diastolic thrill may rarely be appreciated at the apex with the patient in the left lateral decubitus position. Auscultation of heart sounds will reveal an accentuated S1 early in MS and soft S1 in severe MS (see Heart Sounds). This occurs since the increased LA pressures in early MS forces the mobile portion of the mitral valve leaflets far apart. At the onset of ventricular systole, they are forced closed from a relatively far distance resulting in a loud S1. When MS becomes more severe and the mitral valve leaflets become significantly more calcified, the mobility of the leaflets decline and they are unable to be separated a great deal, resulting in a soft or even absent S1 heart sound. When pulmonary hypertension develops, the S2 heart sound will be accentuated. A left ventricular S3 is almost always absent in pure MS since LV early diastolic filling is impaired. The significantly increased opening pressures causes an opening snap (OS) to occur when the mitral valve leaflets suddenly tense and dome into the LV. This high frequency sound is best heard at the apex. The murmur of MS is low frequency ans is referred to as a "rumble". The first part of the murmur of MS relects the pressure gradient between the LA and the LV. It begins after S2 with the OS and then decreshendos (see picture below) ending in mid diastole. The second part of the murmur occurs just before S1 in a creshendo fashion. This part of the murmur is due to the increased flow of blood through the mitral valve that occurs during atrial contraction. It then makes sense that this aspect of the murmur would be absent if the patient is in atrial fibrillation since active LA contraction would be lost.  The severity of MS can be estimated on physical exam by the position of the OS in diastole and the length of the first part of the murmur. An OS that almost immediately follows S2 indicates severe MS while an OS that occurs later in diastole indicates milder MS. This happens since there is a much higher LA pressure in severe MS and a transmitral gradient develops immediately after the mitral valve opens. A longer murmur indicates more severe MS since it takes more time for borth blood to pass through the stenotic mitral valve and for the pressure gradient to dissipate once the mitral valve opens. Diagnosis The EKG in MS is often normal early in disease. The most common finding is left atrial enlargement (p-mitrale), however this finding disappears if the patient enters atrail fibrillation. Right heart strain may produce findings of right axis deviation and right ventricular hypertrophy on EKG. In pure MS, left ventricular hypertrophy would be absent. The chest radiograph will again show left atrial enlargement. This finding is often referred to as a "double density". Elevation of the left mainstem bronchus and a prominent pulmonary artery may also be seen. Both the EKG and the chest radiograph, however, are non-specific for MS. Echocardiography is the primary means to both diagnose and evaluate the severity of MS. Since transmitral velocities can be determined, the transmitral pressure gradient can be calculated using the modified Bernoulli equation as described above. The mitral valve area can be found sing the continuity equation and the pulmonary artery pressure can be calculated to assess the severity of pulomonary hypertension. Cardiac catheterization can also measure the mitral valve area using the Gorlin equation: MVA = MVF 37.7(MVG)½ MVA = mitral valve area MVF = mitral valve flow MVG = mean mitral valvular gradient Mitral valvular flow is proportional to cardiac output. This equation assumes that no mitral regurgitation is present and the mitral valve area remains constant. With this equation, it can be seen that if the MVA remains constand and the cardiac output increases, the mitral valve gradient will increase exponentially as previously described using the modified Bernoulli equation. Treatment The treatment of MS relies on the prevention or early recognition of rheumatic heart disease. Prohpylactic penicillin treatment of patients known to have rheumatic heart disease siccessfully reduces exacerbations and will limit the damage done to the mitral valve. Anticoagulation is of great importance to prevent the formation of a LA thrombus and embolic events. Even in the absence of atrial fibrillation, patients with certain risk factors including hypertension or hypercoaguable states should be anticoagulated. Antibiotic prophylaxis before dental procedures and certain surgeries is currently recommended to prevent bacterial endocarditis. Preload reduction with diuretics and salt restriction can relieve symptoms if MS if pulmonary hypertension is present. Many patients experience symptoms only when the heart rate is elevated, since tachycardia diecreases diastolic filling time significantly. Thus the use of beta-blockers can be beneficial at times, especially in patients with predominantyl exertional symptoms. Definitive treatment includes Percutaneous Ballon Mitral Valvotomy (PBMV). In this procedure, a catheter is inserted through the interatrial septum and into the stenotic mitral valve. A balloon is then inflated that fractures the calcium deposits and relieves the stenosis. Unlike valvuloplasty in the setting of aortic stenosis, PBMV is highly successful with a low rate of restenosis. Complications include residual mitral regurgitation, a persistent atrial septal defect, and rarely calcium embolization. PBMV is indicated for patients who are symptomatic with moderate to severe MS in the absence of a pre-existing LA thrombus and mitral regurgitation. PBMV is also indicated for patients who are asymptomatic with severe MS and an Abascal echocardiographic scor of 8 or less. Surgical approaches to the management of MS include closed commisurotomy, open commisurotomy, and mitral valve replacement. Closed commisurotomy is similar to PBMV in that the mitral valve is not directly visualized and a balloon is used to dilate the stenotic mitral valve. The same criteria that are used for PBMV are also used to assess which patients may benefit from closed commisurotomy. Open commisurotomy requires the use of a cardiopulmonary bypass machine which significantly increases both the cost and complication rate of the procedure. However the surgeon is able to debride calcifications on the mitral valve, remove LA thrombi if found, and remove the left atrial appendage, which is a common site for the formation of thrombi. Open commisurotomy is the treatment of choice in patients with known LA thrombi or MS with concurrent severe mitral valve calcifications. Mitral valve replacement is also an option for patients with symptomatic or severe MS requiring definitive therapy. This is usually reserved only for patients that are not a candidate for PBMV or commisurotomy due to the long-term complications associated with prosthetic valves. Patients with MS along with moderate to severe mitral regurgitation can benefit greatly from mitral valve replacement. |
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