Aortic Stenosis

Aortic Stenosis (AS) is a heart condition caused by the incomplete opening of the aortic valve.

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.

Pathophysiology


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.

Etiology
Causes of aortic stenosis include acute rheumatic fever, bicuspid aortic valve and congenital anomalies. As individuals age, calcification of the aortic valves may occur and result in stenosis.

Physical examination
It is most often diagnosed when it is asymptomatic. It is found on routine examination of the heart. A fairly loud systolic, crescendo-decrescendo murmur is heard loudest at the upper right sternal border, and radiates to the carotid arteries. The murmur increases with squatting, decreases with standing and isometric muscular contraction, which helps distinguish it from hypertrophic obstructive cardiomyopathy (HOCM). Respiration has no effect on the loudness of the murmur. The more severe the degree of the stenosis, the later the peak occurs in the crescendo-decrescendo of the murmur. 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.

Symptoms and signs of aortic stenosis
When symptomatic, aortic stenosis can cause syncope, angina and congestive heart failure. More symptoms indicate a worse prognosis. Treatment requires replacement of the diseased valve with either a porcine aortic valve or a cadaveric aortic valve, or an prosthetic aortic valve.

Congestive heart failure
Congestive heart failure (CHF) is 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.

CHF in the setting of AS is due to a combination of systolic dysfunction (a decrease in the ejection fraction) and diastolic dysfunction (elevated filling pressure of the LV).

Syncope
Syncope 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.

While it is unclear why aortic stenosis would cause syncope, the most 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 causes a vasodepressor response, which causes a secondary peripheral vasodilatation that will then cause decreased perfusion to the brain.

Angina
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 of the LV gets thicker, the arteries that supply the muscle does not get significantly longer or bigger, so the muscle may become ischemic. The ischemia may first be evident during exercise, when the 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 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.

Associated symptoms
In Heyde's syndrome, aortic stenosis is associated with angiodysplasia of the colon. Recent research has shown that the stenosis causes a form of von Willebrand disease by breaking down its associated coagulation factor (factor VIII-associated antigen, also called von Willebrand factor), due to increased turbulence around the stenosed valve.

Calculation of valve area
There are many ways to calculate the valve area of aortic stenosis. The most commonly used methods involve measurements taken during echocardiography. For interpretation of these values, the area is generally divided by the body surface area, to arrive at the patient's optimal aortic valve orifice area.

Planimetry
Planimetry is the tracing out of the opening of the aortic valve in a still image obtained during echocardiographic acquisition during ventricular systole, when the valve is supposed to be open. While this method directly measures the valve area, the image may be difficult to obtain due to artifacts during echocardiography, and the measurements are dependant on the technician who has to manually trace the perimeter of the open aortic valve.

The continuity equation
The continuity equation states that the flow in one area must equal the flow in a second area if there are no shunts in between the two areas. In practical terms, the flow from the left ventricular outflow tract (LVOT) is compared to the flow at the level of the aortic valve. Using echocardiography, the peak velocity at the level of the aortic valve and in the LV outflow tract can be measured, and the area of the LV outflow tract can be measured. From these, it is easy to calculate the area of the aortic valve.

The weakest aspect of this calculation is the variability in measurement of LVOT area, because it involves squaring the LVOT dimension. Some doctors prefer to read just the velocity ratio, to eliminate this.

The Gorlin equation
The Gorlin equation states that the aortic valve area is equal to the flow through the aortic valve during ventricular systole divided by the systolic pressure gradient across the valve times a constant. The flow across the aortic valve is calculated by taking the cardiac output (measured in liters/minute) and dividing it by the heart rate (to give output per cardiac cycle) and then dividing it by the systolic ejection period measured in seconds per beat (to give flow per ventricular contraction).

The Gorlin equation is related to flow across the valve. Because of this, the valve area may be erroneously calculated as stenotic if the flow across the valve is low (ie: if the cardiac output is low). The measurement of the true gradient is accomplished by temporarily increasing the cardiac output by the infusion of positive inotropic agents, such as dobutamine.

The Hakki equation
The Hakki equation1 is a simplification of the Gorlin equation, relying on the observation that in most cases, the numerical value of. The resulting simplified formula is: