Aortic Valve Stenosis

Overview

Aortic Valve Stenosis occurs due to incomplete opening of the aortic valve thus preventing proper emptying of the left ventricle into the systemic circulation during systole.

Etiologies

Sclerosis and calcification of the aortic valvular cusps are the ultimate pathological changes which lead to aortic stenosis. These morphological changes thicken and harden the cusps, resulting in poor opening of the aortic valve, and thus stenosis of the aortic valve outlet. Chronic hemodynamic shear forces and turbulent flow across these cusps are likely the root cause of progressive sclerosis and thickening. In individuals with anatomically normal aortic valves, such changes may take years to result in sclerotic and calcific pathology and thus disease manifests late in life, if ever. However, in individuals with preexisting aortic valvular deformities which enhance shear forces and turbulence, pathological progression is accelerated and disease may manifest much earlier in life. Below we discuss the most common etiologies that lead to aortic stenosis.

Age related hemodynamic wear-and-tear of the aortic valve is the most common etiology of aortic stenosis. In fact, the aortic valves of nearly a third of all individuals older than 65 display some sclerotic pathology although much fewer display clinical signs of aortic stenosis. When clinical consequences do arise, they generally manifest late in life, in the seventh and eighth decade.

Some patients display congenital malformations of their aortic valves, such as a Congenital Bicuspid Aortic Valve, which result in abnormal patterns of hemodynamic stress. Chronic exposure of the aortic valve to such stresses from an early age results in more rapid progression of sclerotic and calcific pathology, which can lead to signs of frank stenosis in adult life.

Some patients who develop Rheumatic Heart Disease as a complication of Rheumatic Fever develop malformations of their aortic valves during adult life. Once again, such malformation likely expose the aortic valves to additional hemodynamic stresses which accelerate progression of sclerotic and calcific pathology. Consequently, clinical signs of aortic stenosis may develop during the adult years.

Pathophysiology

Stenosis of the aortic outlet generates significant resistance to left ventricular outflow of blood during systole. Consequently, a significantly higher blood pressure gradient between the left ventricle and the aorta is required to maintain cardiac output at a normal level. This substantially increases the afterload on the left ventricle and in doing so significantly raises the cardiac oxygen demand.
Because all etiologies of aortic valve stenosis develop over years, sufficient time exists for the heart to undergo significant remodeling to alleviate the chronically high afterload. Remodeling initially follows a pattern of concentric ventricular hypertrophy in which the ventricle becomes significantly thickened. The increased thickness of the ventricular wall reduces myocardial tension, courtesy of the Law of Laplace, and thus serves to reduce afterload. With such changes the normal ventricle is able to maintain cardiac output in most situations for years; however, as time progresses or the stenosis becomes more severe, the following pathophysiological sequelae may arise.

Although ventricular hypertrophy reduces the myocardial tension required for successful ventricular ejection, it also stiffens the left ventricle, reducing its passive compliance for filling during diastole. Consequently, substantially increased diastolic filling pressures are required to successfully fill the ventricle. Because most of the blood flow to the myocardium occurs during diastole, increased diastolic filling pressures significantly reduce the blood pressure gradient for cardiac perfusion, and thus reduce cardiac blood flow. Reduced cardiac perfusion together with increased cardiac oxygen demand from the hypertrophied ventricular myocardial mass can result in transient episodes of myocardial ischemia, especially during physical activity.

The increased diastolic filling pressures mentioned above are initially achieved by enhancing left atrial systole (See: Cardiac Cycle) rather than a sustained increase in left atrial pressure. This avoids backward transmission of left atrial pressure to the pulmonary circulation but requires significant hypertrophy of the left atrium. Importantly, because such patients are thus highly dependent on successful atrial contraction, atrial fibrillation can cause rapid clinical deterioration. Eventually, however, the left ventricle cannot sustain the increased systolic pressures required to maintain cardiac output and Left Heart Failure ensues. As the left heart fails, high sustained left atrial pressures are required to fill the ventricle, resulting in backward transmission of pressure into the pulmonary circulation, consequent pulmonary edema, and the clinical syndrome of left heart failure.

Clinical Consequences

Aortic Stenosis can remain subclinical for many years as the heart possesses significant functional reserve and left ventricular hypertrophy can successfully compensate for the increased afterload. Eventually, a set of prototypical symptoms manifests and result from the episodic myocardial ischemia and progressive left heart failure described above.

The mismatch generated by the reduced cardiac blood flow and increased cardiac oxygen demand render the heart highly prone to episodes of myocardial ischemia. This manifests as anginal chest pain very similar in quality to that observed in stable angina.

The stenotic aortic valve prevents the heart from substantially increasing cardiac output when required. This is especially true during bouts of physical activity when the systemic vascular resistance can decline due to increased muscle activity. Consequently, a hallmark of aortic stenosis is exertional syncope.

As mentioned, left atrial hypertrophy can prevent mean left atrial pressures from rising during the early stages of disease. However as time wears on and stenosis becomes more severe, the failing left heart is unable to eject sufficient blood volume during systole. This requires increasingly sustained high diastolic left atrial pressures to fill the ventricle which is transmitted retrogradely into the pulmonary circulation, resulting in pulmonary edema, and thus dyspnea. Over time, the full clinical syndrome of left heart failure develops.

Signs

Heart Murmur: Aortic stenosis often results in a low-pitched, rough-sounding murmur that rises after S1, is loudest in the middle of systole, and disappears before S2
Pulses Parvus et Tardus: Peripheral pulses are often weak (parvus), reflecting a reduced pulse pressure, and late (tardus)
S4: Strong atrial contraction against a hypertrophied, stiff left ventricle often results in a fourth heart sound.

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