Aortic valve stenosis

Aortic valve stenosis is a heart condition caused by narrowing 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.

When the aortic valve becomes stenotic, it causes a pressure gradient between the left ventricle (LV) and the aorta. The more stenotic 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 considered high.

Causes of aortic stenosis include acute rheumatic fever, bicuspid aortic valve and congenital anomalies.

Table of contents

1 Physical examination 2 Symptoms and signs of aortic stenosis 2.1 Congestive heart failure 2.2 Syncope 2.3 Angina 3 Associated symptoms 4 Calculation of valve area 4.4 Planimetry 4.5 The continuity equation 4.6 The Hacke equation

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.

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 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 wil syncopize due to decreased blood persufion 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.

Planimetry

Planimetry is the tracing out of the opening of the aortic valve in a still image obtained during echocardiographic aquisition 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 echocardiographic measurements, the peak velocity at the level of the aortic valve and in the LV outflow tract (a serrogate for flow in these areas) can be measured, and the area of the LV outflow tract can be measured. From this, it is easy to calculate the area of the aortic valve.

	Example: An individual undergoes trans thoracic echocardiography for the evaluation of a systolic ejection murmur with delayed carotid upstroke noted on physical examination. During echocardiography, the following measurements were made. LVOT diameter of 2 cm, peak velocity in the LVOT of 1 cm/sec, and a peak velocity at the level of the aortic valve of 2.9 cm/sec. What is the aortic valve area?
	Answer: An LVOT diameter of 2 cm gives a LVOT area of π. This gives

The Hacke equation

The Hacke equation is a simplification of the Gorlin equation to calculate the aortic valve area based on the cardiac output and the peak pressure gradient across the valve.

	Example: An individual undergoes left and right cardiac catheterization for the evaluation of aortic stenosis. Measurements includes an aortic pressure of 120/60, LV pressure of 170/15, cardiac output of 3.5 liters/minute. What is the aortic valve area?
	Answer: The peak gradient between the LV and aorta is 50 mmHg. This gives