Cardiovascular Imaging

Aortic stenosis

narrowing of the valve between the left ventricle and the ascending aorta causing a pressure gradient during systole. It is usually caused by limitation of motion of the aortic valve cusps (valvular aortic stenosis) but can also occur in the aorta within a few cm of the valve, supravalvular aortic stenosis, or beneath the valve, subvalvular aortic stenosis. Commonly, aortic stenosis and aortic regurgitation coexist but one of the lesions is usually dominant. Left ventricular systolic pressure is elevated. Left ventricular wall stress is frequently increased; left ventricular hypertrophy tends to equalize wall stress even in the presence of considerable increase in left ventricular systolic pressure during the compensated state. Inadequate hypertrophy and myocardial failure in advanced disease is associated with marked increase in wall stress, left ventricular dilatation and eventually subendocardial myocardial ischaemia. The causes of valvular stenosis include congenital abnormalities such as bicuspid and unicuspid valves and deformed tricuspid valves. Acquired abnormalities include rheumatic fever and degenerative scarring and calcification.

Imaging

Plain radiography varies from entirely normal to severe cardiomegaly and pulmonary oedema. The radiograph of neonates with critical aortic stenosis shows pulmonary oedema or pulmonary venous hypertension and cardiomegaly. In both older children and adults there is usually mild or no cardiomegaly and no evidence of pulmonary venous hypertension. The most frequent feature of the plain radiograph is dilatation of the ascending aorta (poststenotic dilatation); aortic enlargement does not usually involve the arch or descending aorta (Fig. 1). Aortic valvular calcification bears a rough relationship to the severity of valvular stenosis in patients under 60 years of age. Calcification is readily identified on fluoroscopy but only dense calcification is recognized on plain radiography. Ascending aortography demonstrates restriction of systolic opening (doming) of the thickened aortic valve and a jet of unopacified blood entering the opacified ascending aorta. The aortogram also reveals the extent of dilatation of the ascending aorta. It also displays any diastolic reflux of contrast media into the left ventricle due to associated aortic regurgitation. The severity of valvular aortic stenosis cannot be accurately judged from angiography but rather is reflected by the pressure gradient measured across the valve. Left ventriculography displays the limitation of excursion and thickening of the valve in valvular stenosis. Left ventriculography typically shows normal to slightly reduced left ventricular volumes and increased ejection fraction. Left ventricular wall thickness and myocardial mass are increased.

Echocardiography, two-dimensional and Doppler, is the most frequently employed modality for the diagnosis and assessment of severity of aortic stenosis. Transoesophageal Echocardiography can precisely define the morphology and motion of the valve and the morphology of the subvalvular region. Colour flow mapping displays the high velocity jet across the valve. It also demonstrates the presence of associated regurgitation. Doppler sampling of the velocity of flow across the aortic valve is used to estimate the severity of aortic stenosis employing the modified Bernoulli equation. The peak velocity across the stenotic valve recorded by Doppler echocardiography is used in the following formula: peak pressure gradient = 4 · peak velocity2. Magnetic resonance imaging (MRI) and computed tomography (CT) can demonstrate the precise dimensions of the dilated ascending aorta (Fig. 2). This is useful in monitoring aortic size in patients who develop aneurysmal dilatation as a complication of aortic stenosis. Although not generally used in the evaluation of aortic stenosis, cine MRI can define the high velocity jet across the aortic valve. Velocity-encoded cine MRI has been effective for measuring the peak velocity and pressure gradient using the modified Bernoulli equation. Cine MRI is a precise method for quantifying left ventricular volumes and myocardial mass in aortic stenosis.

CBH

CBH

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Fig.1

Chest radiograph of a 43-year-old man shows dilated ascending aorta and normal heart size.
Aortic stenosis, Fig.1
Aortic stenosis, Fig.2