COARCTATION OF THE AORTA
This relatively common condition is the result of localized narrowing of the descending aorta near the origin of the ductus arteriosus. The lesion consists of a ridge-like indentation of the posterolateral wall of the aorta resulting from thickening and infolding of the aortic media. It is typically located just distal to the origin of the left subclavian artery and the specific location may be “preductal†or “postductal†depending on the position of the ridge of tissue relative to the ductus (or ligamentum) arteriosus. It is often associated with other forms of congenital heart disease, especially bicuspid aortic valve and mitral valve malformations.
Demonstration of Aortic coarctation from the suprasternal notch. A: The descending aorta can be seen with a shelf-like narrowing in the mid-portion (arrow). B: Color Doppler imaging demonstrates acceleration of flow and turbulence at the level of the coarctation
Echocardiographic detection of coarctation requires both an index of suspicion and careful recording of the descending aorta from the suprasternal window. In children, the evaluation of this portion of the aorta is relatively straightforward. In adults, however, the assessment can be technically demanding and both false-negative and false-positive results occur. The goal is to record the arch and descending aorta in the long axis from the suprasternal notch. False-negative results usually result from an inability to image the most distal portion of the arch (where the narrowing occurs). False-positive findings are the result of a tangential imaging plane through the vessel, creating the illusion of narrowing. The origins of the carotid and subclavian arteries serve as landmarks when localizing the juxtaductal area. The location of the left subclavian artery relative to the coarctation is an important factor in surgical management. If an area of stenosis is suspected, care should be taken to ensure proper beam alignment. If the aortic lumen can be seen beyond the narrowing, the likelihood of a false-positive result is reduced . Dilation and exaggerated pulsation of the proximal aortic arch are further evidence of significant coarctation.
When two-dimensional echocardiographic imaging is diagnostic of (or suspicious for) coarctation, Doppler imaging should be performed to aid in the diagnosis and to provide an estimation of the pressure gradient. As a first step, color Doppler imaging can be used to detect acceleration and turbulence within the region of narrowing. The absence of Doppler evidence of acceleration and turbulence of flow should alert the examiner to the possibility of a false-positive two-dimensional echocardiographic result. Color Doppler imaging also permits more accurate alignment of the continuous wave Doppler beam.
Continuous wave Doppler imaging demonstrating peak systolic pressure gradient of 35 mmHg across the coarctation.
To estimate the peak pressure gradient, the Bernoulli equation can be used. When this equation is applied to aortic coarctation, however, it may be inappropriate to ignore the proximal aortic flow velocity. As a general rule, if this proximal velocity is less than 1.5 m/sec, it can be ignored and the simplified equation can be used. If it is greater than 1.5 m/sec, the expanded Bernoulli equation is necessary. In this way, a more accurate pressure gradient is obtained. The persistence of a high-velocity flow signal into diastole is another useful clue to the severity of the stenosis. A pressure gradient throughout the cardiac cycle indicates a more severe form of obstruction compared with a pressure gradient that is confined to systole. Then, the presence of a diastolic gradient is confirmed with continuous wave Doppler imaging. Because coarctation gradients are flow dependent, low-level exercise, usually in the form of leg lifts, can be performed to assess the response to stress. In many cases, exercise will not cause a significant increase in the peak gradient, but will result in the development or increase in the diastolic gradient. In borderline cases, this response can be helpful in clinical decision making.