Aortic valve stenosis

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

2008-07-28

What is aortic valve stenosis?

Aortic valve stenosis is one of the most common types of heart valve disease. It is often just referred to as aortic stenosis, or AS. To understand this condition, it is important to first understand how the aortic valve is related to the heart.

The human heart is a four-chamber muscular pump that keeps blood flowing through the entire body. Each chamber has a valve, which is a one-way door that normally allows blood to flow forward but not backward. The aortic valve is situated between the heart and the aorta, the largest blood vessel which feeds all of the body’s organs and muscles (see figure 1).

In aortic stenosis, the aortic valve does not open fully. When the valve opening is narrowed, it is a condition called stenosis. Overall, up to 5% of adults over 70 years old have aortic stenosis, but it can occur in younger people or even be present at birth. There are other conditions called subvalvular and supravalvular aortic stenosis, which involve narrowing or blockage just under or just above the aortic valve, respectively, but do not involve the valve itself. These conditions result in similar adverse effects on the heart and circulation, but will not be covered specifically in this discussion.

How does aortic stenosis damage the heart?

A normal aortic valve has three leaflets that open widely to allow blood to flow easily out of the heart (see figure 2). Normally when the aortic valve opens, the pressure between the heart and the aorta becomes equal as blood flows into the aorta. As the valve leaflets become diseased, however, they start to stick together and do not open as widely, and it becomes more difficult to pump blood out of the heart. This generates increasingly higher pressures inside the heart compared to the blood pressure in the aorta. This is called a pressure gradient. Initially, the muscular walls of the heart adapt by developing hypertrophy, which means the heart walls thicken and become more muscular to pump more forcefully (see figure 3). The hypertrophy compensates for the dysfunctional valve and the heart is still able to pump the volume of blood needed for adequate circulation. However this comes at a great cost.

The remodeled heart becomes stiffer, and requires more oxygen to feed its thick muscular walls. After a period of stability, the high pressure system overwhelms the heart and the walls start to stretch out like an enlarging water balloon. Then the heart’s ability to pump decreases to the point where it is unable to meet the demands of the body; initially this occurs only during exertion, but eventually it occurs even when the person is at rest.

What are the causes of aortic stenosis?

There are three main causes of aortic stenosis:

1. Degenerative calcification:  Calcific aortic stenosis occurs when calcium builds up on the aortic valve.  Normally calcific aortic stenosis is a disease of the elderly, with symptoms developing around 70 to 80 years of age.  However, advanced kidney disease, radiation exposure, and other conditions can cause acceleration of the calcification process.  Risk factors include age, male gender, high blood pressure, high cholesterol, diabetes, and smoking.  There are also genetic factors that predispose certain individuals to developing calcific AS that are not clearly understood yet.  There is no guaranteed way to prevent calcific AS, but some of the risk factors can be prevented and treated.

2. Congenital bicuspid or unicuspid valve:  This is a birth defect where the aortic valve only has one (unicuspid) or two (bicuspid) leaflets instead of the usual three (see figure 4).  Bicuspid aortic valve occurs in 1 - 2% of the population and is twice as common among men.  It is often associated with other abnormalities of the aorta.  Unicuspid valve is extremely rare and usually presents in infancy or childhood because symptoms develop early.  There is increasing evidence that congenital valve defects are hereditary.  Because a bicuspid valve is usually asymmetric, it is prone to excess mechanical stress.  Thus, damage and subsequent calcification of the valve occurs at an earlier age than in a normal three-leaflet valve.  Approximately 75% of bicuspid aortic valves will develop significant enough AS to cause symptoms, which usually develop between 50-60 years of age. 

3. Rheumatic heart disease:  This is a long-lasting consequence of rheumatic fever, which can occur after a strep throat infection.  Rheumatic heart disease is the most common cause of aortic stenosis worldwide, but in the Western world it is increasingly rare because antibiotic treatment of strep throat prevents rheumatic fever.  In rheumatic heart disease, the heart muscle and valves become irritated and injured over many years, and the valve leaflets become fused together, resulting in a gradual narrowing of the valve orifice.  Rheumatic heart disease usually affects multiple valves, and symptoms tend to develop between 20 to 50 years of age.

What are the symptoms of aortic stenosis?

Aortic stenosis usually progresses without noticeable symptoms for many years. When they eventually do develop, there are three primary symptoms:

1. Chest pain: Chest pain due to insufficient oxygen supply to the heart muscle is called angina, and occurs in two-thirds of patients with severe aortic stenosis. The quality of the chest pain is usually pressure-like rather than sharp or stabbing. Angina also commonly occurs in people with coronary artery disease, when blood vessels that feed the heart muscle get blocked with cholesterol, leading to heart attacks. In aortic stenosis, angina occurs because the abnormally thickened heart muscle has increased oxygen requirements, but delivery of oxygen via blood flow is limited by the narrowed valve. Hence oxygen supply does not meet demand, and the oxygen-starved muscle produces pain. In addition, about 50% of older people with aortic stenosis also have coronary artery disease, which further exacerbates angina. Angina usually occurs with exertion; when it happens at rest or with minimal movement, this signifies more advanced disease.

2. Fainting: Fainting and lightheadedness are referred to as syncope and presyncope, respectively. These symptoms tend to occur with exertion; again here, when they happen at rest, it is a more ominous sign. With exercise, the body requires an increase in blood flow, which is limited in aortic stenosis. The result is a precipitous drop in blood pressure and decreased oxygen delivery to the brain, which causes one to pass out. Another explanation is that patients with abnormal hearts are prone to abnormal heart rhythms (arrhythmias), which can also lead to lightheadedness and fainting. If the arrhythmia continues for more than a few seconds, it can lead to sudden cardiac death where the heart stops completely.

3. Heart failure: Congestive heart failure is often an advanced-stage finding that consists of shortness of breath, inability to lie flat, and swelling of the legs. Usually shortness of breath, also called dyspnea, develops first with exertion. Patients gradually reduce their activity level to adjust for their limitations, and often do not even notice that they have new symptoms. Over many years, the heart tires out from pumping against a tiny orifice. As the pump fails, blood gets backed up into the blood vessels in the lungs, and the pressure build-up causes fluid to collect in the lungs, and eventually all the way back to the legs. In end-stage aortic stenosis, the heart pumps so poorly that it cannot sustain adequate circulation, and blood pressure then falls to dangerously low levels. Vital organs like the brain, kidneys, and liver suffer from lack of blood flow and are starved of oxygen and nutrients. Thus when the heart fails, the other organs also fail. This state is called cardiogenic shock. If a patient reaches this point, death is imminent unless there is an emergency intervention.

What is the prognosis of aortic stenosis?

Once symptoms develop, mortality is high if aortic stenosis is left untreated (see figure 5). Some data have shown that without surgery average survival after onset of angina is five years, after onset of syncope is three years, and after onset of congestive heart failure is two years. Other recent studies suggest less clear distinctions, and that all symptomatic patients have poor outcomes if surgery is not performed. The development of symptoms also increases the risk of sudden death to 15-20%.

However, aortic stenosis can be effectively treated by aortic valve replacement surgery– the only effective treatment– and long-term survival after valve replacement is close to the normal healthy population. In the absence of symptoms, aortic stenosis has a good prognosis, but patients need to be monitored by a physician regularly as most patients will eventually develop symptoms.

How do you diagnose and monitor aortic stenosis?

Since aortic stenosis can be completely asymptomatic for years, it is often first noticed incidentally by routine examination of the heart. The most obvious finding is a harsh-sounding murmur that is heard with a stethoscope over the upper chest. This is the sound of turbulent blood flow across the narrow aortic valve opening. In addition, the carotid artery pulse in the neck can feel weaker and delayed. In advanced aortic stenosis, one can feel an abnormally enlarged heart by placing a hand on the chest wall. There can also be extra abnormal heart sounds. A careful interview with a patient can also elicit early symptoms of aortic stenosis such as decreased exercise capacity.

An ultrasound of the heart, called an echocardiogram or echo for short, is the test of choice to diagnose and characterize aortic stenosis. An echo is a painless, non-invasive, and radiation-free test that takes less than 30 minutes to perform and can be interpreted by a cardiologist immediately. Usually an abnormal finding on physical examination prompts the physician to order this test, but sometimes it is ordered for a different reason and aortic stenosis is found incidentally. The echo can provide a wealth of information, including congenital defects, the severity of aortic stenosis, the size and function of the heart, and other abnormal conditions (see figure 6). It can measure the size of the aortic valve orifice area, as well as the pressure gradient across the aortic valve. These numbers are used to classify aortic stenosis into mild, moderate, severe, and critical stages (see figure 7).

Asymptomatic patients should get regular echo testing to monitor progression of aortic stenosis. On average, aortic stenosis progresses by an increase in mean pressure gradient of 7mmHg per year, and a decrease in aortic valve area of 0.1cm2 per year. The speed of the blood flow through the aortic valve can predict approximately when the patient will develop symptoms. Expert guidelines recommend echo testing every year for asymptomatic severe aortic stenosis, every one to two years for moderate AS, and every three to five years for mild AS.

If an echo is inconclusive about the severity of aortic stenosis, cardiac catheterization is the next step. This requires putting a catheter into an artery in the leg and threading it up through the aorta into the heart to take pictures and measurements. It is usually done with the patient awake, with numbing medicine given locally as in a tooth extraction. Also, if an echo shows evidence of an abnormal aorta in addition to aortic stenosis, the patient may require more precise evaluation with a computed tomography (CT) scan or magnetic resonance imaging (MRI).

In asymptomatic patients, exercise treadmill stress testing is sometimes useful to clarify whether a patient develops symptoms with exertion. Exercise stress testing is contraindicated in patients with symptomatic aortic stenosis, because it can be dangerous. If stress testing is needed in these patients, they can get a different kind of stress test that does not require exercise.

How do you treat aortic stenosis?

Aortic valve replacement surgery is the only definitive treatment of aortic stenosis (http://www.sts.org/sections/patientinformation/valvesurgery/aorticvalve/index.html). Once patients develop symptoms, surgery should be done promptly to replace the valve. Current guidelines clearly recommend valve replacement in symptomatic severe AS, or asymptomatic severe AS in patients who are either undergoing heart surgery for another reason, or have decreased pump function of the heart. The mortality risk with valve surgery is approximately 3% to 8% in patients less than 70 years old, and 3% to 16% in older patients. Despite a higher mortality rate from valve surgery in older patients, it is still better than their mortality rate from aortic stenosis without surgical intervention.

Prior to surgery, patients should be thoroughly evaluated to determine if they are appropriate candidates for surgery. Older patients (generally men older than 45 years and women older than 55 years of age) should undergo cardiac catheterization to look for coronary artery disease, which would require doing bypass surgery at the time of valve replacement.

Aortic valve replacement can be done with bioprosthetic valves (pig or cow valves), or mechanical valves (synthetic metal valves). Mechanical valves last the longest but require a lifelong blood-thinning medication called warfarin. Bioprosthetic valves do not require warfarin, but only last about 10 years. Bioprosthetic valves are usually preferred for elderly patients for whom blood thinners would be dangerous because of their increased risk of bleeding or falling, and for women of child-bearing age, since anticoagulation during pregnancy is difficult. The choice should be individualized for each patient (see figure 8). Currently the standard operation for aortic valve replacement is open-heart surgery, but minimally invasive surgery may be offered in special cases.

After aortic valve replacement, patients should take antibiotics prior to dental and other procedures to prevent infection of their new valve. Mechanical valves are prone to blood clots, which can cause valve malfunction or travel up to the brain and cause a stroke. Thus, patients with mechanical valves need to take the medication warfarin daily to keep their blood thinned. Warfarin requires monthly blood tests for monitoring.

Are there non-surgical treatment options for aortic stenosis?

Medications

No medications have been proven to delay or reverse progression of aortic stenosis. There have been observations that progression of aortic stenosis is associated with high cholesterol levels. Studies have been done to evaluate whether cholesterol-lowering medications, called statins, can slow the progression of aortic stenosis. So far the results have been inconclusive from relatively short term clinical studies, and currently statin therapy is not recommended for aortic stenosis alone. However, longer studies are ongoing and it is possible that we will learn that statins do slow the progression of aortic stenosis. In patients with bicuspid aortic valve and an enlarged aortic root, beta blockers, which decrease heart rate and blood pressure, are recommended to protect the aorta, as long as there is no significant backward leak of the valve. However, this does not delay or reverse the aortic stenosis.

Balloon valvuloplasty

Balloon valvuloplasty is a cardiac catheterization procedure where a balloon is inflated across the aortic valve to increase the size of the orifice and relieve the aortic stenosis. Initially there was hope that this could be offered as an alternative to valve replacement surgery, or to patients who were too old or frail to undergo surgery. However, studies so far have shown that in adults there is no improvement in long-term survival compared to doing nothing. Balloon valvuloplasty is mostly reserved for children and sometimes, young adults, with congenital aortic stenosis and little or no calcification of the valve. In this population, there is some evidence to support good outcomes. In adults it is only used in emergency situations as a temporary bridge to surgery. Valve replacement through a catheterization procedure is still experimental.

What special situations should I be aware of?

Antibiotics prophylaxis (infection prevention)

Patients with aortic stenosis have a moderate risk for endocarditis, or infection of the valve. People with congenital bicuspid valve and higher pressure gradients are at higher risk. Historically these patients have taken antibiotics prior to dental work or other procedures to prevent infection. However, as of 2007, the official American Heart Association guidelines determined that there was not enough evidence to support this practice, and they no longer recommend antibiotic prophylaxis in aortic stenosis without replaced valves. (http://www.americanheart.org/presenter.jhtml?identifier=3047051)

Exercise limitations
Patients with aortic stenosis may have to limit their exercise. Those with mild asymptomatic AS can participate in all competitive sports, but need annual echocardiograms to evaluate for progression of the condition. Patients with asymptomatic moderate AS can participate in low-intensity sports, such as golf. If they pass an exercise stress test, they can even participate in moderate-intensity activities, such as baseball or horseback riding. Patients with either severe AS or symptomatic moderate AS should not participate in any competitive sports.

Atrial fibrillation
When patients with AS develop atrial fibrillation, a common abnormal heart rhythm in the elderly and in anyone with heart disease, they may develop symptoms or become more unstable than patients without AS. Therefore atrial fibrillation should be promptly and aggressively controlled.

Dehydration

Patients with aortic stenosis can be more sensitive to dehydration than those without the condition. They can become very lightheaded or even faint more easily if they get dehydrated.

Bleeding risk
Patients who have moderate to severe aortic stenosis have an increased risk of bleeding, such as easy bruising, oozing from the gums after brushing teeth, and blood in the stool. This is because turbulent blood flow through the narrowed aortic valve disrupts certain proteins in the blood, called von Willebrand factor, that help blood to clot normally.

Pregnancy
Mild to moderate aortic stenosis is generally well-tolerated in pregnancy, but severe AS is poorly tolerated as there is increased risk of heart failure, arrhythmias, death, and fetal complications. All pregnant women with known bicuspid aortic valves should have imaging of their aorta to evaluate for associated abnormalities, because in late stages of pregnancy they can develop life-threatening complications. Women with severe AS or decreased heart function should avoid getting pregnant until their heart disease is treated. Prior to pregnancy, patients can be considered for either balloon valvuloplasty or valve replacement surgery, usually with a bioprosthesis (see above). If a patient is already pregnant, intervention is reserved only if the patient deteriorates because any surgery or procedure exposes the fetus to risk.

Noncardiac surgery
Severe aortic stenosis significantly increases the risk of any other surgery the patient may need. Therefore, it is important for the anesthesiologist and surgeon to know about this condition when planning for other surgeries. If symptomatic, patients with AS should definitely postpone non-urgent surgery until the valve is replaced. If a patient either declines valve replacement surgery or is not a candidate for it, then they can choose to proceed with other types of surgery, but should understand they have about a 10% mortality rate from the operation because of their aortic stenosis. If a patient is not a candidate for valve replacement, balloon valvuloplasty is a possible option to temporarily stabilize a patient and get them through surgery, but this method has not been proven to be beneficial and should be evaluated on a case by case basis.

Conclusion

Aortic stenosis can occur at any age but most commonly occurs in the elderly. With the aging of the population AS is increasing in frequency. It may be detected by the finding of a heart murmur. The most common symptoms are chest pain (angina), fainting (syncope) and shortness of breath (dyspnea). Echocardiography is the primary diagnostic technique. Symptomatic patients should have prompt definitive treatment, which almost always requires valve replacement.


Links

www.seemyheart.org
www.americanheart.org
www.echoincontext.com
www.uptodate.com
www.nlm.nih.gov/medlineplus/heartvalvediseases.html
www.sts.org/sections/patientinformation/valvesurgery/aorticvalve/index.html

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