Echocardiography in Congenital Heart Disease

Transcription

1 Chapter 44 Echocardiography in Congenital Heart Disease John L. Cotton and G. William Henry Multiple-plane cardiac imaging by echocardiography can noninvasively define the anatomy of the heart and the great vessels by delineating the configuration and the position of the cardiac structures and the spatial interrelations of these structures. The information obtained can be used to accurately diagnosis and for prognosis in complex congenital heart disease. In many pediatric cardiac tertiary care centers, echocardiography is the only diagnostic test performed before neonatal congenital heart surgery. With advances such as pulsed and color Doppler echocardiography and improvements in the size and capabilities of transducers and other imaging equipment, pediatric echocardiography has gained rapid acceptance. The technology allows real-time three-dimensional imaging, assessment of myocardial function, and precise definition of cardiac anatomy from the fetal stage through adulthood. For all of these reasons, echocardiography has become the standard noninvasive diagnostic imaging modality for pediatric cardiology. Transthoracic multiplane imaging by twodimensional (2-D) echocardiography defines the anatomy of the heart and the great vessels. Analysis of each cardiac segment allows complete definition of the configuration and the position of the cardiac structures and their spatial interrelations. The cardiac chambers and the intracardiac valves are shown with high resolution. Because tortuous vessels may be difficult to define by a slice technology such as echocardiography or magnetic resonance imaging, color Doppler echocardiography is customarily used to provide a map of blood velocity and direction that complements the 2-D image. Small septal defects and fistulous connections may be recognized only by perturbations in blood flow when the anomaly is too small to be visualized clearly. Pulsed and continuous wave Doppler echocardiography provide excellent time resolution that allows precise quantification of blood velocity. Positional and velocity information are combined to assess the presence and the severity of a valvular obstruction or insufficiency, the position and the size of jets associated with septal defects, and abnormal flow in large vessels in congenital lesions such as anomalous systemic and pulmonary venous return, coarctation of the aorta, and patent ductus arteriosus. Transesophageal echocardiography (TEE) allows imaging planes different from those obtained in a standard transthoracic study. Miniaturization of transducer components now allows TEE to be performed in infants who weigh as little as 2.5 kg. Structures not well visualized by transthoracic echocardiography (TTE), primarily those located posteriorly are well visualized by TEE. In the older child or a child in whom there are poor transthoracic windows, TEE can also be very useful in the evaluation of congenital heart disease. Oftentimes during cardiac surgery it is important to address specific issues. The presence of abnormalities such as anomalous pulmonary venous return, pulmonary vein stenosis, and the presence of atrial baffle flow can be determined with intraoperative TEE. In addition, an immediate intraoperative or postoperative assessment of the adequacy of surgical repair is possible. These perioperative examinations are usually targeted, however, and are not a substitute for a complete preoperative transthoracic evaluation. Fetal echocardiography is the newest frontier in pediatric echocardiographic imaging. With the use of transvaginal transducers, detailed fetal cardiac anatomy can be seen as early as 12 weeks of gestation. Transabdominal imaging can be performed by 16 weeks, although the 429

2 ECHOCARDIOGRAPHY IN optimal time for fetal echocardiography is approximately 18 weeks. Abnormalities detected at 18 weeks by echocardiography may be important in the decision for further imaging, chromosomal testing, or even termination of the pregnancy. As with TTE of infants, fetal echocardiography can identify intracardiac anatomy, blood flow across all the valves in the heart, size and orientation of the great vessels, cardiac function, and cardiac rhythm. The order and the windows used in a fetal echocardiogram depend on the position of the fetus, the amount of fluid in the uterus, and the size and motion of the baby. TRANSTHORACIC IMAGING IN PEDIATRICS Each pediatric echocardiography laboratory has a specific protocol for acquiring a complete study of the cardiac anatomy in children. Because patient cooperation is needed, in young children and infants all images may not be obtained using standardized positions. Some centers use conscious sedation for all patients under a certain age to ensure uniformity of studies. Another option is video sedation : child-friendly videos played during the study to distract the patient and allow time to obtain diagnostic images. As long as clear pictures are obtainable, scanning can be performed with the patient sitting in a parent s lap, feeding, or even in a stroller. This approach substantially decreases the number of patients who must be sedated. The protocol for a complete study includes views from the four major echocardiographic windows: parasternal, apical, subxiphoid, and suprasternal. Each window provides the image of the heart from a different angle, allowing multiple, corroborating views of the same structures. The image from each window begins from a standard reference view; then a sweep of the heart is made, first with 2-D scanning and then with color Doppler. The color Doppler mapping defines the location for pulsed Doppler scanning in each plane. Once the pertinent information is obtained, the transducer is rotated 90 to perform an orthogonal sweep. The sonographer and the interpreting physician can reconstruct multiple 2-D images into a three-dimensional representation of the cardiac anatomy. Cardiac function and blood flow are calculated from Doppler mapping and from the 2-D images obtained. For example, pulmonary and aortic flow are calculated from the mean velocity and diameter of the vessel at the area of interest as follows: Blood flow = (mean flow velocity) 5 (time) 5 (cross-sectional area of vessel) Peak instantaneous gradients are calculated from a simplified Bernoulli equation, using peak flow velocity within the stenotic jet in the following formula, where V is the peak flow velocity measured by spectral Doppler scanning: Peak pressure gradient = 4V 2 This gradient is used to estimate pressures in the different cardiac chambers. Several different methods can be used to quantify left ventricular (LV) function. LV fractional shortening (FS) is a measurement of the percentage of change in LV diameter: FS = (LV end-diastolic dimension LV endsystolic dimension) / (LV end-diastolic dimension) Ejection fraction is similarly calculated, using measured LV volumes. In the discussion that follows, echocardiographic examinations are described for some common congenital heart lesions, with emphasis on the information needed to plan surgical intervention and the best techniques to obtain this information. ATRIAL SEPTAL DEFECT Transthoracic echocardiography is often sufficient to define the size and the location of an atrial septal defect (Fig. 44-1). Pulsed and color Doppler echocardiography identify the direction and the amount of shunting at the atrial level. Other findings can confirm the presence of a hemodynamically significant shunt. For instance, RV volume overload can produce diastolic bowing of the ventricular septum to the left during diastole, with the left ventricle assuming an elliptical shape. Partial anomalous pulmonary 430

3 ECHOCARDIOGRAPHY IN Figure 44-1 Atrial Septal Defect Left Atrium Atrial septal defect Right atrium 431

4 ECHOCARDIOGRAPHY IN veins can be identified by 2-D echocardiography and should be sought in patients with an atrial septal defect so that they can be corrected at the time of surgery. The flow of these veins can be traced by color Doppler. TTE and subxiphoid echocardiography are usually sufficient to define the anatomy of the atrial septum and the pulmonary veins in infants and small children. For older children and adults, TTE may be needed for full anatomic definition. Cardiac catheterization is not usually needed in the evaluation of atrial septal defects. VENTRICULAR SEPTAL DEFECT Multiple views are needed to visualize the entire interventricular septum. TTE with 2-D imaging will usually demonstrate the size and the location of the interventricular communications. Color Doppler can be used to determine the direction of shunting across the ventricular septal defect (VSD) (Fig. 44-2). By measuring the direction and the velocity of flow across the defect, pulsed and continuous wave Doppler can be used to estimate the pressure gradient across the defect. Cardiac catheterization is not required before surgery unless the physical and noninvasive findings are atypical or contradictory. ATRIOVENTRICULAR SEPTAL DEFECT Echocardiography is also an important tool for the preoperative assessment of atrioventricular septal defects (AVSDs) (Fig. 44-3). 2-D imaging defines atrioventricular (AV) valve morphology. If the superior bridging leaflet is divided and has attachments to the crest of the ventricular septum, it is considered a type A valve. Straddling of central superior bridging leaflet attachments to a papillary muscle in the right ventricle defines a type B valve. If the superior bridging leaflet has no attachments to the crest of the interventricular septum and the valve leaflet is free-floating, it is considered a type C valve. Superior bridging leaflet septal attachments can obstruct the ventricular portion of the defect restricting shunting or cross the LV outflow tract either of which can cause obstruction to aortic blood flow. Anterolateral papillary muscle insertions tend to be rotated counterclockwise in AVSDs and sit much closer to the posteromedial papillary muscle, which may create a parachute -like deformity of the left portion of the AV valve. Any significant length of suturing of the superior and inferior leaflets during surgical repair risks creating LV inflow obstruction. In the intermediate form of AVSD, it is not uncommon to find shortened and immobile leaflets with thick, chordal attachments that limit the ability to properly fashion a functioning AV valve. Echocardiographic findings can sometimes anticipate this insufficiency of valvular tissue. Color, continuous wave, and pulsed Doppler echocardiography assess potential gradients across the outflow tracts and show the direction of shunting across the septal defect. Color Doppler interrogation of the AV valve usually reveals some degree of insufficiency. A doubleorifice mitral valve, present in about 5% of AVSDs, can be identified by echocardiography. The usual ostium primum atrial septal defect (with or without a shunt at the ventricular level) is also well visualized with 2-D echocardiography. The VSD component of AVSDs is usually single and in the inlet position; however, multiple defects can be ruled out with close color Doppler interrogation of the septum. COARCTATION OF THE AORTA Echocardiography can be valuable in making the diagnosis of coarctation of the aorta (Fig. 44-4). The characteristic narrowing of the aorta with a posterior ledge can be identified with 2-D imaging but may be difficult to appreciate if a patent ductus arteriosus is present. When a pressure gradient is present, a highvelocity jet will be present at the coarctation site. At the distal transverse arch, there is diastolic and systolic forward flow. Damped pulsatile flow is seen in the thoracic aorta. Often some degree of hypoplasia of the distal transverse aortic arch exists. 2-D echocardiography can usually distinguish coarctation of the aorta from interrupted aortic arch, but angiography may be necessary if the findings are ambiguous. It is also important to evaluate the patient for other anomalies that commonly present with coarctation of the aorta. As noted previously, VSDs can be well defined with echocardiography. Bicuspid aortic valve and mitral valve abnormalities should be carefully examined by 2-D imaging, color flow, and pulsed Doppler probing. LV outflow tract 432

5 ECHOCARDIOGRAPHY IN Figure 44-2 Ventricular Septal Defect Right ventricle Left ventricle Ventricular septal defect Aorta 433

6 ECHOCARDIOGRAPHY IN Figure 44-3 Atrioventricular Septal Defect Left atrium Right atrium Interatrial septal defect Left ventricle Interventricular septal defect Right ventricle Ventricular septum 434

7 ECHOCARDIOGRAPHY IN Figure 44-4 Coarctation of the Aorta Transverse aortic arch Coarctation 435

8 ECHOCARDIOGRAPHY IN obstruction and other forms of subaortic obstruction can be seen, including posterior infundibular malalignment in the presence of a VSD. TRANSPOSITION OF THE GREAT ARTERIES Echocardiography can provide a definitive diagnosis of transposition of the great arteries by demonstrating the origin of the aorta from the right ventricle and the pulmonary artery from the left ventricle. Cross-sectional imaging can determine the presence and size of the interatrial communication. Echocardiography can often define the origins of the coronary arteries, but considerable experience is needed to confidently assess more distal branching patterns. Pulsed and color flow Doppler will identify a patent ductus arteriosus and delineate the magnitude of shunting at the atrial and ventricular levels. Ventricular mass and volumes can be quantified with both 2-D and M-mode echocardiography. The shape of the interventricular septum in systole indicates the differential pressures between the right and the left ventricles because the septum will bow toward the chamber with the least wall stress. In addition, the LV outflow tract can be interrogated with pulsed and color flow Doppler for signs of obstruction. TETRALOGY OF FALLOT Diagnosis of tetralogy of Fallot requires delineation of the structures listed in Table Since most of these structures are well visualized with echocardiography, many infants do not need catheterization before repair of tetralogy of Fallot with a patent main pulmonary artery and continuity between the branches (Fig. 44-5). The RV outflow tract is usually well visualized by imaging in a combination of different echocardiographic planes. The diameters of the pulmonary valve annulus and the proximal pulmonary arteries are measured from parasternal, subxiphoid, and suprasternal views. Careful interrogation of the ventricular septum using both color flow and pulsed Doppler techniques can reveal any additional septal defects, which are seen with the greatest frequency in patients less than 1 year of age. Both the origins and the proximal branches of the right and left coronary arteries must be visualized because the origin of Table 44-1 Diagnosis of Tetralogy of Fallot Levels and severity of right ventricular outflow tract obstruction Pulmonary valve annulus size Main and branch pulmonary artery size Ventricular septal defect (single vs. multiple) Origin of the left anterior descending coronary artery Aortopulmonary collaterals Aortic arch anatomy the left anterior descending coronary from the right coronary artery and the presence of a prominent conal branch are infrequent associations that may significantly influence the surgical management of the RV outflow tract in patients who require an outflow patch. There is an increased incidence of right aortic arch in patients with tetralogy of Fallot. The presence of a right aortic arch is usually clearly demonstrated by a combination of plain chest radiography and TTE. Knowledge of this anomaly is critical before a staged shunt operation is considered. PULMONARY ATRESIA When an initial echocardiographic examination determines that pulmonary atresia with an intact ventricular septum is present (Fig. 44-6), it is very important to define the level of the RV outflow tract obstruction and the RV morphology, including the inlet, the outlet, and the trabecular components. The nature of the interatrial communication must be known in order to rule out existing or potential restriction to essential right-to-left shunting. Subxiphoid views of the interatrial septum will demonstrate the size and the position of the foramen ovale or of the septal defect. The flap valve of the foramen ovale is usually deviated toward the left atrium, but the flap valve moves back and forth during the cardiac cycle unless there is an obstructive communication. The flow dynamics across the atrial septum can be further defined by color flow and pulsed wave Doppler. Nonpulsatile flow with a velocity in the range of 2 m/sec is strongly suggestive of obstructive atrial communication, especially if the patient has hepatomegaly and evidence of a low-output state. 436

9 ECHOCARDIOGRAPHY IN Figure 44-5 Tetralogy of Fallot Right ventricle Interventricular septal defect Aorta Left ventricle Left atrium 437

10 ECHOCARDIOGRAPHY IN Figure 44-6 Pulmonary Atresia Right atrium Left atrium Right ventricle Left ventricle 438

11 ECHOCARDIOGRAPHY IN TOTAL ANOMALOUS PULMONARY VENOUS RETURN Two-dimensional echocardiography will accurately delineate pulmonary venous anatomy in circumstances in which total or partial anomalous pulmonary venous return is present. Color and pulsed Doppler examinations are necessary to confirm the presence of obstruction. Turbulent, nonpulsatile venous flow with a velocity of at least 2 ms signifies hemodynamically significant obstruction. The intracardiac anatomy should also be assessed by echocardiography because other significant congenital lesions occur approximately 30% of the time, including patent ductus arteriosus, atrial isomerism, VSD, single ventricle, transposition of the great arteries, and systemic venous anomalies. Total anomalous pulmonary venous return is strongly associated with complex congenital heart disease and asplenia. SINGLE VENTRICLE Echocardiography is a powerful tool in defining the anatomy of the univentricular heart (Fig. 44-7). Both the interatrial and interventricular communications are measured and obstructions noted with color-directed pulsed Doppler. If an outflow chamber (hypoplastic ventricle) is found to communicate with a dominant ventricle via a bulboventricular foramen (VSD), the dimensions of the interventricular communication must be obtained with two orthogonal views. With these dimensions, a prediction can be made about whether the connection may become obstructive in the future. This prediction is made on the basis of the cross-sectional area of the connection, normalized to the body surface area and its boundaries, muscular or membranous. Doppler examination of the subarterial outflow can detect even mild obstruction by an increase in blood flow velocity. When the aorta arises from the hypoplastic chamber, detection of even mild obstruction is particularly important because of the danger that subaortic obstruction will develop. Hence, serial studies are necessary, particularly following interventions that reduce ventricular preload or afterload (including medication use or surgical procedures). The anatomy of the AV valve is most clearly defined by echocardiographic imaging, and any stenosis or regurgitation should be quantified via color and pulsed Doppler flow imaging. In patients with a pulmonary artery band, echocardiography evaluates the band position, the morphology of the proximal pulmonary artery branches, and gradients at either level. Ventricular function can be estimated using echocardiography, but the accuracy of echocardiography in this circumstance may be limited by nonuniform ventricular geometry, particularly in patients with a single morphologic right ventricle. Because of large differences in preload and afterload in patients with a single ventricle, measures of contractility that are less load-independent, such as the velocity of circumferential fiber shortening, are of greater value than a simple ejection fraction. However, even these indices are not reliable with subaortic obstruction or when the ventricular geometry does not conform to a prolate ellipsoid, and alternate means for assessment of ventricular function (MRI or radionuclide ventriculography) are sometimes needed. A reliable means for assessing ventricular function serially is needed in order to optimally time the stages of surgical correction and/or palliation (see chapter 50). TRUNCUS ARTERIOSUS Echocardiography can visualize the large truncal root overlying a subarterial VSD (Fig. 44-8). The origin of the pulmonary arteries may be seen as a single trunk, or the arteries may be seen arising separately from the proximal truncal root. The number of truncal valve leaflets can be determined by a combination of 2-D imaging and Doppler echocardiography. The presence of valvular regurgitation or stenosis, or stenosis at the origin of the pulmonary artery or branches, can, and should, be evaluated by TTE. The appearance of a small ascending aortic portion and a larger pulmonary portion of the common truncus should prompt a careful examination of the aortic arch from the suprasternal, high parasternal, and subxiphoid transducer positions to determine whether an associated coarctation of the aorta or an interrupted aortic arch is present. FUTURE DIRECTIONS Refinements in pediatric echocardiographic techniques produce a highly accurate picture of 439

12 ECHOCARDIOGRAPHY IN Figure 44-7 Hypoplastic Left Heart Right atrium Left atrium Left ventricle Right ventricle 440

13 ECHOCARDIOGRAPHY IN Figure 44-8 Truncus Arteriosus Aorta Pulmonary artery Right ventricle Common truncal valve Left ventricle 441

14 ECHOCARDIOGRAPHY IN the anatomy and the evolving physiology of congenital cardiac lesions. For many straightforward lesions, invasive studies may be eliminated entirely. In some lesions, such as atrial septal defect, coarctation of the aorta, and patent ductus arteriosus, more often than not surgery is performed on the basis of a noninvasive perioperative evaluation. A variety of complex lesions, such as AVSDs, single ventricle, and complex conotruncal anomalies, have also been surgically palliated or repaired without cardiac catheterization, although the approaches used vary locally by the experience of the diagnostic and surgical team with each abnormality. Even for complex lesions requiring cardiac catheterization, advances in echocardiography have reduced the number of cardiac catheterizations needed in the lifetime of a single patient. As the use of echocardiography has become dominant in the preoperative evaluation of cardiac lesions, cardiac catheterization has taken on a more therapeutic role, being used in the closure of patent ductus arteriosus and septal defects, the occlusion of vascular structures, and the relief of outflow obstructions (see chapter 45). These advances are closely interrelated with the evolution of cardiac surgery toward the repair of increasingly complex lesions at increasingly earlier ages (chapter 46). REFERENCES George B, Disessa TG, Williams RG, Friedman WF, Laks H. Coarctation repair without catheterization in infants. Am Heart J 1987;114: Leung MP, Mok CK, Hui PW. Echocardiographic assessment of neonates with pulmonary atresia and intact ventricular septum. J Am Coll Cardiol 1988;12: Murphy DJ, Ludomirsky A, Huhta JC. Continuous wave Doppler in children with ventricular septal defect: Noninvasive estimation of intraventricular pressure gradient. Am J Cardiol 1986;57: Pasquini L, Sanders SP, Parness IA, Colan SD. Diagnosis of coronary artery anatomy by two-dimensional echocardiography in patients with transposition of the great arteries. Circulation 1987;75: Sanders SP, Bierman FZ, Williams RG. Conotruncal malformation: Diagnosis in infancy using subxiphoid two dimensional echocardiography. Am J Cardiol 1982;50: Shimazaki Y, Maehara T, Blackstone EH, Kirklin JW, Bargeron LM. The structure of the pulmonary circulation in tetralogy of Fallot with pulmonary atresia. J Thorac Cardiovasc Surg 1988;95: Smallhorn JF, Freedom RM. Pulsed Doppler echocardiography in the pre-operative evaluation of total anomalous pulmonary venous connection. J Am Coll Cardiol 1986;8: Snider AR, Serwer GA, Ritter SB. Echocardiography in Pediatric Heart Disease. St. Louis: Mosby; 1997: