Chest Radiography in Pediatric Cardiovascular Disease

Published on 27/02/2015 by admin

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Chapter 65

Chest Radiography in Pediatric Cardiovascular Disease

The role of chest radiography in the diagnosis and evaluation of congenital cardiovascular disease continues to evolve. At one time a major tool in the assessment of heart disease, radiography now occupies an ancillary role, with echocardiography serving as the major primary investigation after physical examination, especially in the neonatal period. However, the chest radiograph still may provide the first indication of unsuspected cardiovascular disease, and in infants and children with known cardiac disease, radiography offers an important overview of the heart and pulmonary circulation. Moreover, chest radiography is a vital tool in the early postoperative period and is useful in the follow-up of heart disease. These latter topics are beyond the scope of this chapter.

The major chest radiographic findings in patients with cardiac disease are cardiomegaly, pulmonary vascular changes (predominantly overcirculation or undercirculation), and signs of pulmonary venous hypertension and edema. However, several caveats need to be emphasized. First, children with relatively mild structural defects and even some children with severe or complex disease may have normal chest films. This situation is particularly true in newborns. In addition, the chest radiograph usually does not provide useful information about specific chamber size, hypertrophy, or intracardiac connections or malformations. Echocardiography, magnetic resonance imaging, computed tomography, or angiography are needed for precise evaluation of intracardiac structure and function. Furthermore, findings such as a boot-shaped or egg-shaped heart are nonspecific for tetralogy of Fallot or transposition of the great arteries. On the other hand, plain film findings may be specific for some extracardiac lesions, such as supracardiac total anomalous pulmonary venous return, aortic arch anomalies, pulmonary stenosis, and coarctation of the aorta.

A systematic approach to evaluation of the chest film consists of an assessment of heart size, shape, and position; pulmonary vasculature; the airway and mediastinum; visceral situs; and skeletal abnormalities. Applying such an approach often results in a diagnosis of a cardiovascular disease category such as a shunt or a right- or left-sided obstructive lesion, which in turn leads to a differential diagnosis and the identification of the likely etiology of nonspecific clinical findings, such as congestive heart failure or cyanosis (Box 65-1).

Box 65-1   Differential Diagnosis of Apparent Gross Cardiomegaly

Technique

As in all medical imaging, attention to detail is necessary to optimize the examination and its interpretation. Proper exposure, centering, collimation, patient positioning, and inspiration are necessary (e-Fig. 65-1).

Many films of infants are obtained using the anteroposterior projection and supine position. Because of the small size of the chest, this technique results in little magnification of the heart, as can be seen in larger children. Beam angulation also may affect the appearance of the heart and great vessels. With lordotic positioning, the heart may appear more globular, with an uplifted apex and accentuation of the pulmonary outflow tract; with reverse lordosis, much of the heart may be obscured by the hemidiaphragms. Oblique views are not useful for cardiac evaluation, and barium should be used only if a vascular ring or sling is suspected (and when such findings are likely, cross-sectional imaging should be considered for complete evaluation). Chest fluoroscopy is rarely used except to evaluate prosthetic valve function, the diaphragm, or airway dynamics.

Systematic Interpretation

Normal Anatomy and Physiology

One of the challenges in evaluating the chest radiograph of younger children is their variable anatomy and physiology. For example, the thymus is variable in size and position and may mimic cardiomegaly, abnormally positioned vessels, pericardial fluid, or a mediastinal mass (e-Fig. 65-2).

It is rare for the thymus to extend posteriorly. The thymus usually causes few problems in the interpretation of chest radiographs in children older than 6 years.

Newborn infants have physiologic pulmonary hypertension, and as a result, large shunt lesions do not appear until the pulmonary vascular resistance falls, which usually occurs by 4 to 6 weeks (Fig. 65-3). Similarly, newborn infants may not show the expected changes of severe pulmonary stenosis or atresia if the ductus arteriosus is patent.

The physiology of small airways in infants and young children (up to approximately 2 years of age) results in unique manifestations of pulmonary edema. Specifically, infants show hyperinflation as a response to interstitial edema, as would happen in the presence of airway inflammation with bronchiolitis (Figs. 65-3 and 65-4).

The hyperinflation occurs as an adaptive response to the interstitial edema to prevent small airway closure. In the absence of clinical signs of a respiratory infection, hyperinflation is an important sign of early pulmonary edema.

Heart Size

The size of the heart can be difficult to assess in the frontal projection of infants and young children because of the presence of the relatively large thymus and poor inspiration (see e-Figs. 65-1 and 65-2). Measurement of the cardiothoracic ratio is of little use. The lateral view provides a more reliable indication of true heart size by permitting an assessment of the anteroposterior dimension without interference from the thymus. However, the thymus does fill in the retrosternal space, obscuring the right ventricular outflow tract. In older children, the frontal radiograph is more useful, and the radiologist should evaluate both views to assess the three-dimensional volume of the heart. In a child with pectus excavatum, the heart may appear large in the frontal view but compressed in the lateral film. Marked cardiomegaly is seen in children with severe valve regurgitation, especially tricuspid valve disease (Ebstein anomaly), pericardial effusion, and cardiomyopathy, and it is rarely seen in children with cardiac tumors. Mediastinal masses may mimic cardiomegaly (e-Fig. 65-5).

Pulmonary Vasculature

Chest radiography provides a window into the pulmonary circulation, which is the main area in which chest radiography supplements the information gained from echocardiography. The pulmonary vasculature may show evidence of increased flow, decreased flow, normal flow, or pulmonary venous hypertension. The pulmonary flow may be asymmetrical in the setting of tetralogy of Fallot, pulmonary atresia, or other rare lesions. The assessment of the pulmonary vasculature is both important and difficult. Poor-quality films that are rotated or obtained during expiration are difficult to interpret (see e-Fig. 65-1). Many radiographs in younger children are obtained in the supine position, and therefore flow is symmetrical from base to apex.

Increased Pulmonary Vascularity

The size of the pulmonary vessels is noticeably larger only when the amount of flow doubles (thus when the pulmonary/systemic ratio is 2 : 1) (e-Figs. 65-6 and 65-7).

Because of this phenomenon, smaller shunts are not detectable. Detecting moderate increases in flow requires considerable experience. One useful sign is to compare the end-on pulmonary artery to the adjacent bronchus. An arterial dimension greater than that of the bronchus is suggestive of increased flow. However, a slightly larger dimension can be seen normally. The larger vessels extend more peripherally into the lung. Increases in pulmonary blood flow should be accompanied by a proportional increase in heart size because of volume loading.

Pulmonary Venous Hypertension and Pulmonary Edema

Early pulmonary venous hypertension often manifests as hyperinflation in infants younger than 2 years (see Fig. 65-4 and e-Fig. 65-6). As interstitial fluid accumulates, the perihilar bronchi and vessels become poorly defined. Septal (i.e., Kerley) lines are uncommon in children. Eventually, frank alveolar pulmonary edema is seen. In the presence of severe heart failure, it can be difficult to determine whether the failure is caused by pulmonary venous hypertension alone or is associated with an underlying large left-to-right shunt (e-Fig. 65-9) (Box 65-3). Pulmonary edema can obscure vessel detail, and left heart failure can distend the vessels. The detection of heart disease can be difficult in the child who presents with an acute viral respiratory illness because the inflammation can produce ill-defined vascular markings and hyperinflation similar to the findings in congestive heart failure. In a supine patient, pleural fluid will layer posteriorly. Unlike in children with large shunts, with left-sided obstruction or pump failure, the heart often is disproportionate to the vascularity (e-Fig. 65-9, A)

Box 65-3   Differential Diagnosis for the Radiographic Appearance of Pulmonary Venous Congestion

Asymmetrical Pulmonary Flow

The chest film must be carefully evaluated for asymmetrical flow, which can occur as a result of pulmonary arterial stenosis or hypoplasia, pulmonary venous obstruction, or disturbances in ventilation with secondary vasoconstriction, as well as postoperatively. Care must be taken to avoid misinterpretation of the rotated radiograph (see e-Fig. 65-1). In children with decreased pulmonary blood flow as a result of tetralogy of Fallot or pulmonary atresia, stenosis of the pulmonary artery (usually the left at the site of ductal insertion) is a common problem (e-Fig. 65-11).

The left pulmonary artery can even become isolated, and the lung will fail to grow. Early recognition of this complication can allow repair and lead to normal lung growth.

Airway and Mediastinum

The chest film is evaluated for the presence of the thymus, a mediastinal mass, the side of the aortic arch, and the presence of a vascular ring. The size and position of the trachea is an important indicator of arch abnormalities. A careful search should be made on both frontal and lateral films for displacement or narrowing of the trachea (e-Fig. 65-12).

Mechanical displacement or obstruction of large airways occurs in anomalies of the aortic arch, pulmonary arteries, and more severe forms of cardiomegaly (see e-Figs. 65-5, 65-6, 65-7, 65-9, 65-10, and 65-12). The position and contour of the descending aorta should be carefully examined. In coarctation of the aorta, the only sign in younger children may be a leftward convexity to the descending aorta (e-Fig. 65-13). Such a contour is seen often in older adults as a result of age-related ectasia, but it is abnormal in children.

Situs

The chest film assessment is incomplete unless abnormalities of abdominal and thoracic situs (the anatomic location of organs that are asymmetrically positioned in the body) are sought and the cardiac position relative to visceral situs is determined. Dextrocardia in situs solitus is strongly associated with complex cardiac abnormalities. Abnormal situs can be subtle, with a normal-appearing liver and stomach. Airway anatomy and lung morphology are valuable indicators of visceral situs. Symmetrical bronchi are seen in almost all patients with right isomerism (asplenia) (e-Fig. 65-14) and in 68% of patients with left isomerism (polysplenia). Splenic dysfunction and intestinal malrotation occur in these children.

Bony Abnormalities

A complete evaluation of the chest radiograph must include the bony thorax. Few skeletal abnormalities are strongly associated with congenital heart disease, but abnormalities of the spine (e.g., scoliosis, segmentation anomalies, and rib anomalies) and sternum (e.g., an abnormal number of ossification centers and caudal deficiency) occur. Rib notching is rarely seen in younger children with aortic coarctation, but it may be present in adolescents and teenagers in whom intercostal artery collaterals have developed (e-Fig. 65-15). Bony changes are common after a thoracotomy.

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