Aortic Stenosis - Treatment
Aortic stenosis is a condition in which one of the valves in the heart (the aortic valve) has become stenotic (narrowed/constricted) and does not open normally. When this occurs, the ability of the heart (left ventricle) to pump blood out of the heart to the aorta and other arteries is impaired. The body's organs may receive an insufficient supply of oxygen-rich blood, and blood may "back up" into the lungs, causing shortness of breath.
Anatomy
The aortic valve has three "leaflets" that open and close. It functions as a one-way valve, that is, it allows blood to flow only out of the left ventricle and into the aorta when the left ventricle contracts. When the heart relaxes between contractions, the aortic valve closes, preventing blood in the aorta from returning to the left ventricle.
If the aortic valve becomes diseased, over time the leaflets thicken and the valve becomes calcified. In the past, this type of degeneration of the valve was seen most commonly in patients who had rheumatic fever during childhood. Currently, rheumatic fever is rarely a cause of aortic valve dysfunction. About 1 percent of the population is born with only two valve leaflets. For reasons not yet well understood, these persons are at increased risk for developing aortic stenosis, often becoming symptomatic when they are in their 40s and 50s. The most common cause of aortic stenosis currently is age-associated degeneration and calcification of the aortic valve, which becomes symptomatic in patients when they are in their 70s and 80s.
When the valve leaflets thicken and the valve becomes calcified, the leaflets no longer open normally, and the volume of blood ejected from the left ventricle is reduced. The heart's ability to supply the body with blood decreases, and blood "backs up" into the lungs. To overcome increasing resistance to expelling blood caused by aortic stenosis, the left ventricle initially compensates by thickening, i.e., increasing muscle mass, to help generate enough force to eject blood through the stenotic aortic valve into the aorta. This helps overcome aortic stenosis for a while, but the left ventricle increasingly dilates (increases in diameter), stiffens, and gradually loses its ability to generate enough contractile force to compensate for the stenotic aortic valve. If aortic stenosis remains untreated, the left ventricle becomes further dilated and less able to contract and expel blood into the aorta.
Treatment: Valve Replacement Surgery
The development of any one of the three classic symptoms (angina, shortness of breath, syncope/fainting) indicates aortic stenosis severe enough to be treated with valve replacement surgery. Valve replacement, the main treatment for aortic stenosis, involves open-heart surgery. Since patients with aortic stenosis often also have blockages in the coronary arteries (coronary artery disease), surgeons typically treat any significant blockages by performing a bypass operation at the same time. Thus, most patients who will undergo valve replacement surgery are first referred for cardiac catheterization to detect blockages in the coronary arteries.
Some patients with aortic stenosis have no symptoms and have otherwise normal heart function and size. Most doctors agree that these patients do not require immediate valve replacement surgery. Treatment for these patients is usually careful medical follow-up, scheduled check-ups for assurance that symptoms have not developed, and echocardiograms at intervals of every three months to every year to assess the pumping function and size of the heart. Once any of the symptoms of aortic stenosis develops, one should inform his or her doctor immediately, not wait until the next scheduled checkup.
There are two reasons for proceeding with valve replacement surgery. First, once patients begin to develop symptoms, aortic stenosis may become fatal over time. Thus angina, shortness of breath, or syncope/fainting are universally accepted as strong indications to proceed with valve replacement surgery.
Second, if findings during an echocardiogram or cardiac catheterization suggest that the heart is beginning to decompensate due to the strain of pumping blood across a stenotic aortic valve, valve replacement surgery is indicated. The two main signs of decompensation are (1) dilation of the heart and (2) loss of vigorous contraction of the heart (falling ejection fraction). Normally, the left ventricle of the heart ejects approximately 60 percent of the blood contained within the left ventricle with each beat (contraction). This percentage of blood that is ejected out of the left ventricle into the aorta with each beat is called the ejection fraction. As the heart loses the ability to contract vigorously, the ejection fraction falls. Patients with an ejection fraction of approximately 40 percent to 45 percent are said to have mildly depressed ejection fractions; those with ejection fractions of approximately 30 percent to 40 percent are said to have moderately depressed ejection fractions, and those with ejection fractions of 10 percent to 25 percent are said to have severely depressed ejection fractions. Even a mild fall in the ejection fraction is an indication to proceed with valve replacement surgery. The more the ejection fraction falls, the greater the indicator for valve replacement surgery.
Replacement of the aortic valve requires open-heart surgery, in which the sternum (breast bone) is opened, allowing access to the heart. The heart is actually stopped during critical parts of the operation, and a special machine pumps oxygenated blood throughout the body. A small part of the heart is then opened, the diseased valve is removed, and a new valve is sewn in.
There are three basic types of valves used to replace the diseased heart valve. A porcine valve is made of tissue from a pig. The advantage of a porcine valve is that it poses no significant risk for blood clots on the valve; thus, patients do not need blood thinner medication. The disadvantage is that after approximately ten years some of these valves degenerate and must be replaced.
A mechanical valve is fashioned from metal and synthetic materials. The most commonly used mechanical valve, St. Jude's valve, consists of two semicircular discs that open with each contraction of the left ventricle and close when the ventricle relaxes. The advantage of a mechanical valve is that it is quite durable, often lasting more than 20 years. The disadvantage is that there is a small potential for a blood clot to form on the valve. This blood clot can break off, travel to the brain, and cause a stroke. To prevent this complication, patients who receive mechanical heart valves are treated with warfarin (Coumadin®), a blood thinner that decreases the chance for blood clot formation.
A homograft valve is an aortic valve that has been taken from a human organ donor. These valves are not associated with a significant risk of blood clot formation and, thus, do not require blood thinner therapy. Although no long-term follow-up data are yet available, it is thought that these valves may be quite durable.
There have been significant advances in the way valve replacement surgery is now performed, and in most patients the risks for major complications are acceptably low, approximately 3% to 5% in otherwise relatively healthy patients. Major complications include bleeding, infection, kidney failure, stroke, heart attack, and death.
After Surgery
After surgery, tubes that have been placed in the body to help one breathe, to monitor pressures in the heart and arteries, and to prevent blood from accumulating in the lungs are removed during the first day or two. Most patients remain in the hospital for a week after surgery and take approximately 3 to 4 weeks to recover completely. Most patients are able to resume leisure activities and many return to work. Patients who receive a mechanical valve must continue to take the blood thinner warfarin (Coumadin®) to decrease the chances of blood clot formation on the valve.
Although the risks vary with the type of replacement heart valve, all artificial valves carry some increased risk for developing infection on the valve. The potential for bacteria to enter the bloodstream exists during any type of invasive procedure. Therefore, patients who have an artificial heart valve and are to undergo any type of invasive procedure are first treated with antibiotics (this precautionary measure is called endocarditis prophylaxis).
Because an artificial valve occasionally begins to malfunction over the course of many years, physicians may schedule periodic echocardiograms to check the valve.