Aortic Stenosis
Definition
Aortic Stenosis is defined as subtotal obstruction in the channel of left ventricular outflow. It occurs as three types: valvular (85%), subvalvular (10%), and supravalvular (5%). Congenital aortic stenosis accounts for 5-10% of all congenital heart defects and occurs more frequently in males.
Pathoanatomy
There are three types of AS:
Valvular:
The normal aortic valve has three cusps and an area of 2 cm^2/m^2 of BSA. In valvular AS, the structural abnormality may include abnormal cusp formation - cusps may be fused, thickened, or dysplastic, or the valve may be bicuspid, unicuspid, or have more than 3 leaflets. The aortic calcification that causes acquired aortic stenosis is extremely rare in childhood. Poststenotic dilatation of the ascending aorta often accompinies aortic stenosis.
Subvalvular:
Rarely present at birth, late onset may be due to the presence of ther congenital defects. Left ventricular outflow tract (LVOT) narrowing takes three forms:
1. A discrete fibrous membrane located within 1 cm of the aortic valve.
2.A thick muscular ridge of collar.
3.Diffuse narrowind of the entire LVOT.
Patients with subvalvular stenosis present with dilated and tortuos coronary vessels and have an increased risk of coronary artery disease due to increased LV pressures that are generated past the areas of stenosis.
Supravalvular:
Occurs above the Sinuses of Valsalva, manifests in three ways:
1. An hourglass shaped internal constriction of the ascending aorta.
2. Diffuse narrowing of the entire ascending aorta.
3. A discrete subvalvular membrane.
Pathophysiology
Severity is usually determined by a systolic pressure gradient across the valve and also by the calculated area. Critical AS may occur after closing of ductus arteriosus when the infant's left ventricle is unable to generate adequate flow through LVOT. Right to left shunt through the ductus arteriosus with VSD may provide perfusion to the lower body. Left ventricle may hypertrophy and oxygen supply would thus decrease. AS may also be complicated by aortic insufficiency (AI).
Hemodynamics
The resting cardiac output (CO) and stroke volume (SV) are usually within normal limits. LV failure will result in decreased CO and an increase in left atrial, left ventricular end diastolic, and pulmonary vascular pressures. AS will result in pressure overload with normal systolic function systolic function, however, abnormal left ventricular early diastolic filling will occur. Even in the presence of normal coronary arteries the blood flow to the myocardium is significantly compromised and the blood flow the the left ventricle is entirely diastolic. This may result in coronary vasodilation with decreased blood flow to the subendocardium resulting in ischemic events.
Clinical Manifestations
Newborn infants with acute aortic stenosis present with signs of circulatory collapse, cyanosis, or congestive heart failure (CHF). Hypotension, tachycardia, respiratory distress, irritability, and poor peripheral perfusion are nonspecific signs of physiologic distress, RV hypertrophy will be present.
Older infants exhibit symptoms of CHF, difficulty feeding, and poor weight gain. Hepatomegaly may be present, as well peripheral edema and decreased peripheral pulses. Respiratory distress may be present.
A condition known as Williams Synrome is associated with nearly half the cases of supravalvular aortic stenosis. Manifestations include narrowing of the pulmonary and systemic arteries, mental retardation, hearing difficulties, and abnormalities of the teeth and cervical spine.
Surgical Inteventions
Surgery is indicated when the aortic gradient exceeds 30 mm Hg, or if aortic regurgitation develops secondary to the stenotic condition.
Congenital aortic stenosis must be considered a medical emergency in a seriously ill newborn. Immediate surgical intervention is necessary, and may come in the form of balloon valvuloplasty, this, however, may result in complications relating to the small size and age of the patient. Open repair under direct vision is usually preferred. Ultimately, valve replacement will be necessary, but it is often delayed due to the growth of the patient and to anticoagulation difficulties.
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