The aortic valve controls the direction of blood flow from the left ventricle to the aorta. When in good working order, the aortic valve does not impede the flow of blood between these two spaces. Under some circumstances, the aortic valve becomes narrower than normal, impeding the flow of blood. This is known as aortic valve stenosis, or aortic stenosis, often abbreviated as AS.
When the aortic valve becomes stenotic, it causes a pressure gradient between the left ventricle (LV) and the aorta. The more constricted the valve, the higher the gradient between the LV and the aorta. For instance, with a mild AS, the gradient may be 20 mmHg. This means that, at peak systole, while the LV may generate a pressure of 140 mmHg, the pressure that is transmitted to the aorta will only be 120 mmHg. So, while a blood pressure cuff may measure a normal systolic blood pressure, the actual pressure generated by the LV would be considerably higher.
In individuals with AS, the left ventricle (LV) has to generate an increased pressure in order to overcome the increased afterload caused by the stenotic aortic valve and eject blood out of the LV. The more severe the aortic stenosis, the higher the gradient is between the left ventricular systolic pressures and the aortic systolic pressures. Due to the increased pressures generated by the left ventricle, the myocardium (muscle) of the LV undergoes hypertrophy (increase in muscle mass). This is seen as thickening of the walls of the LV. The type of hypertrophy most commonly seen in AS is concentric hypertrophy, meaning that all the walls of the LV are (approximately) equally thickened.
Of the various forms of aortic stenosis, the calcific type is predominant. Since calcific aortic stenosis shares many pathological features and risk factors with atherosclerosis, and since atherosclerosis may be prevented and/or reversed by cholesterol lowering, there has been interest in attempting to modify the course of calcific aortic stenosis by cholesterol lowering with statin drugs. Although a number of small, observational studies demonstrated an association between lowered cholesterol and decreased progression, and even regression, of calcific aortic stenosis, a recent, large randomized clinical trial, published in 2005, failed to find any predictable effect of cholesterol lowering on calcific aortic stenosis. A 2007 study did demonstrate a slowing of aortic stenosis with the statin rosuvastatin. However, what is likely to be considered the definitive trial, published in the New England Journal of Medicine in 2008, failed to find any beneficial effect of intensive cholesterol lowering on the course of aortic stenosis .
When symptomatic, aortic stenosis can cause dizziness, syncope, angina and congestive heart failure. More symptoms indicate a worse prognosis. Treatment requires replacement of the diseased valve with an artificial heart valve.
Congestive heart failure (CHF) carries a grave prognosis in patients with AS. Patients with CHF that is attributed to AS have a 2 year mortality rate of 50%, if the aortic valve is not replaced.
Syncope (fainting spells) in the setting of heart failure increases the risk of death. In patients with syncope, the 3 year mortality rate is 50%, if the aortic valve is not replaced.
It is unclear why aortic stenosis causes syncope. One popular theory is that severe AS produces a nearly fixed cardiac output. When the patient exercises, their peripheral vascular resistance will decrease as the blood vesels of the skeletal muscles dilate to allow the muscles to receive more blood to allow them to do more work. This decrease in peripheral vascular resistance is normally compensated for by an increase in the cardiac output. Since patients with severe AS cannot increase their cardiac output, the blood pressure falls and the patient will syncopize due to decreased blood perfusion to the brain.
A second theory as to why syncope may occur in AS is that during exercise, the high pressures generated in the hypertrophied LV cause a vasodepressor response, which causes a secondary peripheral vasodilation which in turn causes decreased blood flow to the brain. Indeed, in aortic stenosis, because of the fixed obstruction to bloodflow out from the heart, it may be impossible for the heart to increase its output to offset peripheral vasodilation.
A third mechanism may sometimes be operative. Due to the hypertrophy of the left ventricle in aortic stenosis, including the consequent inability of the coronary arteries to adequately supply blood to the myocardium (see "Angina" below), arrhythmias may develop. These can lead to syncope.
Finally, in calcific aortic stenosis at least, the calcification in and around the aortic valve can progress and extend to involve the electrical conduction system of the heart. If that occurs, the result may be heart block - a potentially lethal condition of which syncope may be a symptom.
Angina in the setting of heart failure also increases the risk of death. In patients with angina, the 5 year mortality rate is 50%, if the aortic valve is not replaced.
Angina in the setting of AS is secondary to the left ventricular hypertrophy (LVH) that is caused by the constant production of increased pressure required to overcome the pressure gradient caused by the AS. While the myocardium (i.e. heart muscle) of the LV gets thicker, the arteries that supply the muscle do not get significantly longer or bigger, so the muscle may become ischemic (i.e. doesn't receive an adequate blood supply). The ischemia may first be evident during exercise, when the heart muscle requires increased blood supply to compensate for the increased workload. The individual may complain of exertional angina. At this stage, a stress test with imaging may be suggestive of ischemia.
Eventually, however, the muscle will require more blood supply at rest than can be supplied by the coronary artery branches. At this point there may be signs of ventricular strain pattern (ST segment depression and T wave inversion) on the EKG, suggesting subendocardial ischemia. The subendocardium is the region that becomes ischemic because it is the most distant from the epicardial coronary arteries.
Aortic stenosis is most often diagnosed when it is asymptomatic and can sometimes be detected during routine examination of the heart and circulatory system. Good evidence exists to demonstrate that certain characteristics of the peripheral pulse can rule in the diagnosis. In particular, there may be a slow and/or sustained upstroke of the arterial pulse, and the pulse may be of low volume. This is sometimes referred to as pulsus tardus et parvus. There may also be a noticeable delay between the first heart sound (on auscultation) and the corresponding pulse in the carotid artery (so-called 'apical-carotid delay'). Similarly, there may be a delay between the appearance of each pulse in the brachial artery (in the arm) and the radial artery (in the wrist).
An easily heard systolic, crescendo-decrescendo (i.e. 'ejection') murmur is heard loudest at the upper right sternal border, and radiates to the carotid arteries bilaterally. The murmur increases with squatting, decreases with standing and isometric muscular contraction, which helps distinguish it from hypertrophic obstructive cardiomyopathy (HOCM). The murmur is louder during expiration, but is also easily heard during inspiration. The more severe the degree of the stenosis, the later the peak occurs in the crescendo-decrescendo of the murmur.
The 2nd heart sound tends to become softer as the aortic stenosis becomes more severe. This is a result of the increasing calcification of the valve preventing it from "snapping" shut and producing a sharp, loud sound. Due to increases in left ventricular pressure from the stenotic aortic valve, over time the ventricle may hypertrophy, resulting in a diastolic dysfunction. As a result, one may hear a 4th heart sound due to the stiff ventricle. With continued increases in ventricular pressure, dilatation of the ventricle will occur, and a 3rd heart sound may be manifest.
Finally, aortic stenosis often co-exists with some degree of aortic insufficiency. Hence, the physical exam in aortic stenosis may also reveal signs of the latter, for example an early diastolic decrescendo murmur. Indeed, when both valve abnormalities are present, the expected findings of either may be modified or may not even be present. Rather, new signs emerge which reflect the presence of simultaneous aortic stenosis and insufficiency, e.g. pulsus bisferiens.
According to a meta analysis, the most useful findings for ruling in aortic stenosis in the clinical setting were slow rate of rise of the carotid pulse(positive likelihood ratio ranged 2.8-130 across studies), mid to late peak intensity of the murmur(positive likelihood ratio, 8.0-101), and decreased intensity of the second heart sound(positive likelihood ratio, 3.1-50).
Peripheral signs include:
As noted above, the calcification process which occurs in aortic stenosis can progress to extend beyond the aortic valve and into the electrical conduction system of the heart. Evidence of this phenomenon may include heart block that is apparent on the ECG but otherwise undetectable.
The aortic valve area can be calculated non-invasively using echocardiographic flow velocities. Using the velocity of the blood through the valve, the pressure gradient across can be calculated by the modified Bernoulli's equation:
Gradient = 4(velocity)² mmHgA normal aortic valve has no gradient. If the mean gradient is <25 mm Hg, the stenosis is mild; if the mean gradient is between 25 mm Hg and 50 mm Hg, the stenosis is moderate; if the mean gradient is >50 mm Hg the stenosis is severe; and when the gradient is greater than 70 mm Hg, the stenosis is critical. A normal aortic valve area is >2 cm2. If the valve area is between 1.3 and 2.0 cm2, the stenosis is mild; if the valve area is between 1.0 and 1.3 cm2, the stenosis is moderate; if the valve area is between 0.7 and 1.0 cm2, the stenosis is moderate-severe; areas of less than 0.7 cm2 constitute severe aortic stenosis.
Notwithstanding the foregoing, the American Heart Association has recently changed its recommendations regarding antibiotic prophylaxis for endocarditis. Specifically, as of 2007, it is recommended that such prophylaxis be limited only to 1. those with prosthetic heart valves, 2. those with previous episode(s) of endocarditis, and 3. those with certain types of congenital heart disease.
Since the stenosed aortic valve may limit the heart's output, people with aortic stenosis are at risk of syncope and dangerously low blood pressure should they use any of a number of common medications. Ironically, these same medicines are used to treat a variety of cardiovascular diseases, many of which may co-exist with aortic stenosis. Examples include nitroglycerin, nitrates, ACE inhibitors, terazosin (Hytrin), and hydralazine. Note that all of these substances lead to peripheral vasodilation. Normally, however, in the absence of aortic stenosis, the heart is able to increase its output and thereby offset the effect of the dilated blood vessels. In some cases of aortic stenosis, however, due to the obstruction of blood flow out of the heart caused by the stenosed aortic valve, cardiac output cannot be increased. Low blood pressure or syncope may ensue.
For infants and children, balloon valvuloplasty, where a balloon is inflated to stretch the valve and allow greater flow, may also be effective