MITRAL VALVE REPLACEMENT

Mitral valve replacement
Definition
Mitral valve replacement is a surgical procedure in which the diseased mitral valve of the heart is replaced by a mechanical or biological tissue valve.

Purpose
The mitral valve can become diseased, preventing it from adequately controlling the direction of the flow of blood between the left atrium and left ventricle. It also can become insufficient (regurgitant) and allow blood to flow backwards into the left atrium from the left ventricle during ventricular contraction (systole). In addition, the mitral valve can become stenotic (narrowed), preventing the flow of blood from the left atrium into the left ventricle during ventricular filling (diastole). In mitral valve prolapse, one or more of the mitral valve’s cusps protrude back into the left atrium during ventricular contraction. Mitral valve replacement is performed to remove the diseased valve and provide a new mechanical valve or biological tissue valve that correctly controls the direction of blood flow.

Demographics
Approximately 65,000 valve repairs and replacements are performed in the United States each year. Twice as many women as men are affected by mitral valve stenosis. About 60% of patients with mitral valve stenosis have had rheumatic fever. After rheumatic fever there is usually a latency period of 10–20 years before symptoms of mitral valve stenosis appear. The prevalence of mitral valve stenosis has declined in the United States because there has been a decline in the number of cases of rheumatic fever. Mitral valve stenosis may be present at birth (congenital); however, it rarely occurs alone but rather in conjunction with other heart defects.

Mitral valve prolapse is the most common condition of the heart valves, and is present in about 2% of the general population. Recent studies indicate that similar numbers of men and women have mitral valve prolapse. Having this condition does not guarantee that mitral insufficiency will develop. Patients with a history of rheumatic fever, coronary artery disease, infective endocarditis, or collagen vascular disease also may develop mitral insufficiency.

Description
Cardiac monitoring is instituted and general anesthesia is provided. The surgeon uses a sternotomy to access the heart and great blood vessels. Anticoagulation is given as cannulae are inserted into the large vessels of the heart, femoral vessels, or a combination. Cardiopulmonary bypass is instituted. The heart is arrested as the cross clamp is applied to the ascending aorta to stop blood flow through the organ. The surgeon opens the heart to visualize the mitral valve. He/she may expose the mitral valve by opening the right atrium and then opening the atrial septum. Another approach requires a large left atrium that can be opened directly, making the mitral valve visible.

Next, the surgeon cuts the diseased valve away from the valve annulus (outer ring). The annulus is sized so that the proper size of valve can be selected for the patient’s anatomy. Sutures are applied around the valve annulus, the valve is sutured into place, and tied into position. The atrial septum is closed with suture or left to heal naturally, and the heart is closed with sutures.

Deairing of the heart is performed prior to removal of the cross clamp. When the cross clamp is removed, deairing continues to ensure that no air is delivered to the systemic circulation. At this time a transesophageal echocardiogram (TEE) may be used to test that the valve is functioning correctly and that the heart is free of air.

Once the surgeon is satisfied that the valve is working correctly, cardiopulmonary bypass is terminated, anticoagulation is reversed, and the cannulae are removed from the vessels. The sternotomy is closed. Permanent stainless steel wires are used to hold the sternum bone together. The skin incision is closed with sutures, and sterile bandages are applied to the wound.

A heart valve is a structure within the heart that prevents the backflow of blood by opening and closing with each heartbeat. Replacement heart valves are either mechanical or biological tissue valves. For patients under the age of 65, the mechanical valve offers superior longevity, but the use of this type of valve requires that the patient take an anticoagulation drug for the rest of his/her life. The biological tissue valve does not require anticoagulation therapy, but this type of valve is prone to deterioration leading to reoperation, particularly in those under the age of 50. Women who may want to have children after a valve replacement should usually receive a biological tissue valve, because the anticoagulant (Coumadin/warfarin) most often prescribed for patients with mechanical valves is associated with fetal birth defects. Aspirin can be substituted for warfarin in certain circumstances.

Diagnosis / Preparation
Mitral valve stenosis is diagnosed by history, physical examination, listening to the sounds of the heart (cardiac auscultation), chest x ray, and ECG. Patients may have no symptoms of a valve disorder or may have shortness of breath (dyspnea), fatigue, or frank pulmonary edema. Other patients present with atrial fibrillation (a cardiac arrhythmia) or an embolic event. Doppler echocardiography is the preferred diagnostic tool for evaluation of mitral valve stenosis, and it can be performed in conjunction with non-invasive exercise testing by treadmill or bicycle. Cardiac catheterization is reserved for patients who demonstrate discrepancies in Doppler testing. Both left- and right-heart catheterization should be performed in the presence of elevated pulmonary artery pressures.

A diagnosis of mitral insufficiency requires a detailed patient history. Listening to the heart (auscultation) reveals the presence of a third heart sound. Chest x ray and ECG provide additional information. Again, Doppler echocardiography provides valuable information. Exercise testing with Doppler echocardiography can show the true severity of the disease. After initial findings, patients may be followed with repeat visits and testing to monitor disease progress. If the patient has reached NYHA Class III or IV, replacement is considered. Severe pulmonary hypertension with pulmonary artery systolic pressures greater than 60 mm Hg is considered an indication for surgery. Left ventricular ejection fraction (a measure of output) less than 60% also is an indication for surgery.