James René Herlong

Technique

Transthoracic echocardiography is an ultrasound technique that is optimized for imaging the moving heart. Standard imaging windows that are free of interference from the lungs are illustrated in Figure 64-1. These windows allow imaging of the heart in multiple planes. These planes are based on the axes of the heart and not on the axes of the body (Fig. 64-2).

In each acoustic window, the heart is imaged in orthogonal planes. Because the heart is a three-dimensional structure and because ultrasonography is a tomographic imaging technique, slow sweeps in each view are necessary to understand the complex relationships between various segments of the heart. Three- and four-dimensional echocardiography are becoming ever more robust, but they are not yet capable of high-resolution imaging of the entire heart. Currently, the utility of these techniques is largely in the assessment of the cardiac valves, particularly the atrioventricular valves.

Assessment of ventricular systolic function is performed in every echocardiographic examination. Among the various techniques of quantifying left ventricular systolic function, the left ventricular shortening fraction is the most easily accomplished and universally used. Figure 64-3 illustrates this technique. Standards for left ventricular dimensions and shortening fraction according to body surface area are available in standard references. Assessment of left ventricular diastolic function is less exact, and techniques continue to be developed to evaluate this rather elusive entity. The method with the most promise is tissue Doppler imaging, which uses the Doppler principle to measure the high amplitude but low velocity signals derived from myocardial motion.

Doppler echocardiography uses color and spectral Doppler in the same manner as in vascular ultrasound imaging. One important difference is that when using color Doppler, the color map is always set so that flow toward the transducer is red, whereas flow away from the transducer is blue. Color Doppler imaging is useful for screening for valvar stenosis or regurgitation, septal defects, and arterial and venous stenoses. Color Doppler should be used to complement two-dimensional imaging, not to replace it, because color Doppler obscures anatomy.

In a typical echocardiographic examination, all cardiac valves are assessed by spectral Doppler, as are the aortic arch at the isthmus and any identified septal defects. As with all Doppler applications, the most accurate assessment is obtained by aligning the angle of interrogation exactly along the direction of flow. The simplified Bernoulli equation states that the pressure gradient across an area is approximately equal to four times the measured Doppler velocity squared (4ν²). By using this equation, gradients are estimated in each of the locations that are assessed by spectral Doppler. In general, peak instantaneous pressure gradients across semilunar valves and in the arterial system should be less than 15 mm Hg, and mean gradients in veins and across atrioventricular valves should be less than 3 mm Hg.

Other differences from noncardiac ultrasound involve sedation and image archiving. Because accurate assessment of anatomy and function requires images free of patient movement and because accurate assessment of pressure gradients requires the patient to be in a resting state, sedation often is necessary in patients younger than about 3 years. Because assessment of function and complex anatomy requires viewing moving images, echocardiograms are archived as video clips rather than as still-frame images.

Transesophageal echocardiography in the pediatric population is used primarily in the operating room during surgery for congenital heart disease and in the interventional cardiac catheterization laboratory. Techniques are similar to those previously described.

Formulating a Diagnosis

Once the images have been obtained, a comprehensive diagnosis must be constructed, which is accomplished by using the segmental approach to cardiac anatomy (see Chapter 63). A common template for reporting the findings of an echocardiogram is presented in Box 64-1. Note that the scheme is logical and largely follows the blood flow through the various segments of the heart.

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