Park's Pediatric Cardiology for Practitioners, 6th Ed.

Chest Radiography

Chest radiography was an essential part of cardiac evaluation before echocardiography and Doppler studies became widely available to cardiologists. This simple test remains very useful to physicians who do not have access to echocardiography and Doppler studies. Cardiovascular abnormalities may be incidentally suspected by x-ray films that were obtained for other reasons. Chest Radiography also supplements information that is not gained by the echocardiography study, such as information on the lung parenchyma, airways, and vascular structures connected to the heart.

Chest radiographic films can provide the following information: heart size and silhouette; enlargement of specific cardiac chambers; pulmonary blood flow or pulmonary vascular markings; and other information regarding lung parenchyma, spine, bony thorax, abdominal situs, and so on. Posteroanterior and lateral views are routinely obtained.

Heart Size and Silhouette

Heart Size

Measurement of the cardiothoracic (CT) ratio is by far the simplest way to estimate the heart size in older children (Fig. 4-1). The CT ratio is obtained by relating the largest transverse diameter of the heart to the widest internal diameter of the chest:

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where A and B are maximal cardiac dimensions to the right and left of the midline, respectively, and C is the widest internal diameter of the chest. A CT ratio of more than 0.5 indicates cardiomegaly. However, the CT ratio cannot be used with accuracy in newborns and small infants, in whom a good inspiratory chest film is rarely obtained. In this situation, the degree of inadequate inspiration should be taken into consideration. Also, an estimation of the cardiac volume should be made by inspecting the posteroanterior and lateral views instead of the CT ratio.

To determine the presence or absence of cardiomegaly, the lateral view of the heart should also be inspected. For example, isolated right ventricular enlargement may not be obvious on a posteroanterior film but will be obvious on a lateral film. In a patient with a flat chest (or narrow anteroposterior diameter of the chest), a posteroanterior film may erroneously show cardiomegaly.

An enlarged heart on chest radiographs more reliably reflects a volume overload than a pressure overload. Electrocardiograms (ECGs) better represent a pressure overload than chest radiographic films.

Normal Cardiac Silhouette

The structures that form the cardiac borders in the posteroanterior projection of a chest roentgenogram are shown in Figure 4-2. The right cardiac silhouette is formed superiorly by the superior vena cava (SVC) and inferiorly by the right atrium (RA). The left cardiac border is formed from the top to the bottom by the aortic knob, the main pulmonary artery (PA), and the left ventricle (LV). The left atrial appendage (LAA) is located between the main PA and the LV and is not prominent in a normal heart. The right ventricle (RV) does not form the cardiac border in the posteroanterior view. The lateral projection of the cardiac silhouette is formed anteriorly by the RV and posteriorly by the left atrium (LA) above and the LV below. In a normal heart, the lower posterior cardiac border (i.e., LV) crosses the inferior vena cava (IVC) line above the diaphragm (see Fig. 4-2).

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FIGURE 4-1 Diagram showing how to measure the cardiothoracic (CT) ratio from the posteroanterior view of a chest radiographic film. The CT ratio is obtained by dividing the largest horizontal diameter of the heart (A + B) by the longest internal diameter of the chest (C).

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FIGURE 4-2 Posteroanterior and lateral projections of normal cardiac silhouette. Note that in the lateral projection, the right ventricle (RV) is contiguous with the lower third of the sternum and that the left ventricle (LV) normally crosses the posterior margin of the inferior vena cava (IVC) above the diaphragm. AO, aorta; LA, left atrium; LAA, left atrial appendage; LPA, left pulmonary artery; PA, pulmonary artery; RA, right atrium; RPA, right pulmonary artery; SVC, superior vena cava.

However, in a newborn, a typical normal cardiac silhouette is rarely seen because of the presence of a large thymus and because the films are often exposed during expiration. The thymus is situated in the superoanterior mediastinum. Therefore, the base of the heart may be widened, with resulting alteration in the normal silhouette in the posteroanterior view. In the lateral view, the retrosternal space, which is normally clear in older children, may be obliterated by the large thymus.

Abnormal Cardiac Silhouette

Although discerning individual chamber enlargement often helps diagnose an acyanotic heart defect, the overall shape of the heart sometimes provides important clues to the type of defect, particularly in dealing with cyanotic infants and children. A few examples follow, relating it to the status of pulmonary blood flow or pulmonary vascular markings.

1. A “boot-shaped” heart with decreased pulmonary blood flow is typical in infants with cyanotic tetralogy of Fallot (TOF). This is also seen in some infants with tricuspid atresia. Typical of both conditions is the presence of a hypoplastic main PA segment (Fig. 4-3A). ECGs are helpful in differentiating these two conditions. Whereas the ECG shows right axis deviation (RAD), right ventricular hypertrophy (RVH), and occasional right atrial hypertrophy (RAH) in TOF, it shows a “superior” QRS axis (i.e., left anterior hemiblock), RAH, and left ventricular hypertrophy (LVH) in tricuspid atresia.

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FIGURE 4-3 Abnormal cardiac silhouettes. A, “Boot-shaped” heart seen in cyanotic tetralogy of Fallot or tricuspid atresia. B, “Egg-shaped” heart seen in transposition of the great arteries. C, “Snowman” sign seen in total anomalous pulmonary venous return (supracardiac type).

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FIGURE 4-4 Schematic diagram showing roentgenographic findings of enlargement of the left atrium (LA) in the posteroanterior and lateral projections. Arrows show left mainstem bronchus elevation. In the posteroanterior view, “double density” and prominence of the left atrial append age (LAA) are also illustrated. The barium-filled esophagus (cross-hatched, vertical structure) is indented to the right. In the lateral view, posterior protrusion of the LA border is illustrated. The isolated enlargement of the LA shown here is only hypothetical, because it usually accompanies other changes. Other abbreviations are the same as those in Figure 4-2.

2. A narrow-waisted and “egg-shaped” heart with increased pulmonary blood flow in a cyanotic infant strongly suggests transposition of the great arteries (TGA). The narrow waist results from the absence of a large thymus and the abnormal relationship of the great arteries (see Fig. 4-3B).

3. The “snowman” sign with increased pulmonary blood flow is seen in infants with the supracardiac type of total anomalous pulmonary venous return (TAPVR). The left vertical vein, left innominate vein, and dilated SVC make up the snowman’s head (see Fig. 4-3C).

Evaluation of Cardiac Chambers and Great Arteries

Individual Chamber Enlargement

Identification of individual chamber enlargement is important in diagnosing a specific lesion, particularly when dealing with acyanotic heart defects. Although enlargement of a single chamber is discussed here, more than one chamber is usually involved.

Left Atrial Enlargement. An enlarged LA causes alterations not only of the cardiac silhouette but also of the various adjacent structures (Fig. 4-4). Mild LA enlargement is best appreciated in the lateral projection by the posterior protrusion of the LA border. Enlargement of the LA may produce “double density” on the posteroanterior view. With further enlargement, the LAA becomes prominent on the left cardiac border. The left mainstem bronchus is elevated. The barium-filled esophagus is indented to the right.

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FIGURE 4-5 Diagrammatic representation of changes seen in ventricular septal defect. Left ventricle (LV) enlargement in addition to enlargement of the left atrium (LA) and a prominent pulmonary artery (PA) segment. Other abbreviations are the same as those in Figure 4-2.

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FIGURE 4-6 Schematic diagrams of posteroanterior and lateral chest roentgenograms of atrial septal defect. There is enlargement of the right atrium (RA) and right ventricle (RV) and an increased pulmonary vascularity. Other abbreviations are the same as those in Figure 4-2.

Left Ventricular Enlargement. In the posteroanterior view, the apex of the heart is not only farther to the left but also downward. In the lateral view of LV enlargement, the lower posterior cardiac border is displaced farther posteriorly and meets the IVC line below the diaphragm level (Fig. 4-5).

Right Atrial Enlargement. RA enlargement is most obvious in the posteroanterior projection as an increased prominence of the lower right cardiac silhouette (Fig. 4-6). However, this is not an absolute finding because both false-positive and false-negative results are possible.

Right Ventricular Enlargement. Isolated right ventricular enlargement may not be obvious in the posteroanterior projection, and the normal CT ratio may be maintained because the RV does not make up the cardiac silhouette in the posteroanterior projection. RV enlargement is best recognized in the lateral view, in which it manifests itself by filling of the retrosternal space (see Fig. 4-6, lateral view).

Size of the Great Arteries

As in the enlargement of specific cardiac chambers, the size of the great arteries often helps make a specific diagnosis.

Prominent Main Pulmonary Artery Segment. The prominence of a normally placed PA in the posteroanterior view (Fig. 4-7A) results from one of the following:

1. Poststenotic dilatation (e.g., pulmonary valve stenosis)

2. Increased blood flow through the PA (e.g., atrial septal defect [ASD], ventricular septal defect [VSD])

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FIGURE 4-7 Abnormalities of the great arteries. A, Prominent main pulmonary artery (PA) segment. B, Concave PA segment resulting from hypoplasia. C, Dilatation of the aorta may be seen as a bulge on the right upper mediastinum by a dilated ascending aorta (AA) or as a prominence of the aortic knob (AK) on the left upper cardiac border.

3. Increased pressure in the PA (e.g., pulmonary hypertension)

4. Occasional normal finding in adolescents, especially girls

Hypoplasia of the Pulmonary Artery. A concave main PA segment with a resulting “boot-shaped” heart is seen in TOF and tricuspid atresia (see Fig. 4-7B); obviously, malposition of the PA must be ruled out.

Dilatation of the Aorta. An enlarged ascending aorta may be observed in the frontal projection as a rightward bulge of the right upper mediastinum, but a mild degree of enlargement may easily escape detection. Aortic enlargement is seen in TOF and aortic stenosis (as poststenotic dilatation) and less often in patent ductus arteriosus (PDA), coarctation of the aorta (COA), Marfan’s syndrome, or systemic hypertension. When the ascending aorta and aortic arch are enlarged, the aortic knob may become prominent on the posteroanterior view (see Fig. 4-7C).

Pulmonary Vascular Markings

One of the major gorals of radiologic examination is assessment of the pulmonary vasculature. Although many textbooks explain how to detect increased pulmonary blood flow, this is one of the more difficult aspects of interpreting chest radiographs of cardiac patients. There is no substitute for the experience gained by looking at many chest radiographs with normal and abnormal pulmonary blood flow.

Increased Pulmonary Blood Flow

Increased pulmonary vascularity is present when the right and left PAs appear enlarged and extend into the lateral third of the lung field, where they are not usually present; there is increased vascularity to the lung apices where the vessels are normally collapsed; and the external diameter of the right PA visible in the right hilus is wider than the internal diameter of the trachea.

Increased pulmonary blood flow in an acyanotic child represents ASD, VSD, PDA, endocardial cushion defect, partial anomalous pulmonary venous return (PAPVR), or any combination of these. In a cyanotic infant, increased pulmonary vascular markings may indicate TGA, TAPVR, hypoplastic left heart syndrome, persistent truncus arteriosus, or a single ventricle.

Decreased Pulmonary Blood Flow

Decreased pulmonary blood flow is suspected when the hilum appears small, the remaining lung fields appear black, and the vessels appear small and thin. Ischemic lung fields are seen in cyanotic heart diseases with decreased pulmonary blood flow such as critical stenosis or atresia of the pulmonary or tricuspid valves, including TOF.

Pulmonary Venous Congestion

Pulmonary venous congestion is characterized by a hazy and indistinct margin of the pulmonary vasculature. This is caused by pulmonary venous hypertension secondary to LV failure or obstruction to pulmonary venous drainage (e.g., mitral stenosis, TAPVR, cor triatriatum). Kerley’s B lines are short, transverse strips of increased density best seen in the costophrenic sulci. This is caused by engorged lymphatics and interstitial edema of the interlobular septa secondary to pulmonary venous congestion.

Normal Pulmonary Vasculature

Pulmonary vascularity is normal in patients with obstructive lesions such as pulmonary stenosis or aortic stenosis. Unless the stenosis is extremely severe, pulmonary vascularity remains normal in pulmonary stenosis. Patients with small left-to-right shunt lesions also show normal pulmonary vascular markings.

Systematic Approach

The interpretation of chest radiographs should include a systematic routine to avoid overlooking important anatomic changes relevant to cardiac diagnosis.

Location of the Liver and Stomach Gas Bubble

The cardiac apex should be on the same side as the stomach or opposite the hepatic shadow. When there is heterotaxia, with the apex on the right and the stomach on the left (or vice versa), the likelihood of a serious heart defect is great. An even more ominous situation exists with a “midline” liver, associated with asplenia (Ivemark’s) syndrome or polysplenia syndrome (Fig. 4-8). These infants usually have complex cyanotic heart defects that are difficult to correct.

Skeletal Aspect of Chest Radiographic Film

Pectus excavatum may flatten the heart in the anteroposterior dimension and cause a compensatory increase in its transverse diameter, creating the false impression of cardiomegaly. Thoracic scoliosis and vertebral abnormalities are frequent in cardiac patients. Rib notching is a specific finding of COA in an older child (usually older than 5 years) and is usually found between the fourth and eighth ribs (Fig. 4-9).

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FIGURE 4-8 A chest radiographic film of the chest and upper abdomen of a newborn infant with polysplenia syndrome. Note a symmetrical liver (“midline liver”), a stomach bubble in the midline, dextrocardia, and increased pulmonary vascularity.

Identification of the Aorta

1. Identification of the descending aorta along the left margin of the spine usually indicates a left aortic arch; identification along the right margin of the spine indicates a right aortic arch. The right aortic arch is frequently associated with TOF or persistent truncus arteriosus. Some of the patients with right aortic arch may have vascular ring (see Chapter 16).

2. When the descending aorta is not directly visible, the position of the trachea and esophagus may help locate the descending aorta. If the trachea and esophagus are located slightly to the right of the midline, the aorta usually descends normally on the left (i.e., left aortic arch). In the right aortic arch, the trachea and esophagus are shifted to the left.

3. In a heavily exposed film, the precoarctation and postcoarctation dilatation of the aorta may be seen as a “figure of 3.” This may be confirmed by a barium esophagogram with an E-shaped indentation (Fig. 4-10).

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FIGURE 4-9 Rib notching (arrows) in an 11-year-old girl with coarctation of the aorta. (From Caffey J: Pediatric X-ray Diagnosis, 7th ed. Chicago, Mosby, 1978.)

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FIGURE 4-10 A, The figure-of-3 configuration indicates the site of coarctation with the large proximal segment of aorta or prominent left subclavian artery above and the poststenotic dilatation of the descending aorta below it. B, Barium esophagogram reveals the E-shaped indentation or reversed figure-of-3 configuration. (From Caffey J: Pediatric X-ray Diagnosis, 7th ed. Chicago, Mosby, 1978.)

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FIGURE 4-11 Roentgenogram showing the typical “sail sign” on the right mediastinal border.

Upper Mediastinum

1. The thymus is prominent in healthy infants and may give a false impression of cardiomegaly. It may give the classic “sail sign” (Fig. 4-11). The thymus often has a wavy border because this structure becomes indented by the ribs. On the lateral view, the thymus occupies the superoanterior mediastinum, obscuring the upper retrosternal space.

2. The thymus shrinks in cyanotic infants and infants under severe stress from congestive heart failure. In TGA, the mediastinal shadow is narrow (“narrow waist”), partly because of the shrinkage of the thymus gland. Infants with DiGeorge syndrome have an absent thymic shadow and a high incidence of aortic arch anomalies.

3. “Snowman figure” (or figure-of-8 configuration) is seen in infants, who are usually older than 4 months, with anomalous pulmonary venous return draining into the SVC via the left SVC (vertical vein) and the left innominate vein (see Fig. 4-3C).

Pulmonary Parenchyma

1. Pneumonia is a common complication in patients with increased pulmonary blood flow, such as those with a large PDA or VSD.

2. A long-standing density, particularly in the lower left lung field, suggests pulmonary sequestration. In this condition, there is an aberrant, nonfunctioning pulmonary tissue, which does not connect with the bronchial tree and derives its blood supply from the descending aorta. The venous drainage is usually to the pulmonary venous system.

3. A vertical vascular shadow along the lower right cardiac border may suggest PAPVR from the lower lobe and sometimes the middle lobe of the right lung, called scimitar syndrome. Its pulmonary venous drainage is usually to the inferior vena cava either just above or below the diaphragm. This syndrome is often associated with other anomalies, including hypoplasia of the right lung and right pulmonary artery, sequestration of right lung tissue receiving arterial supply from the aorta, and ASD.

PART 2

Special Tools in Evaluation of Cardiac Patients

OUTLINE

Introduction

5. Noninvasive Imaging Tools

6. Other Noninvasive Investigation Tools

7. Invasive Procedures

Special Tools in Evaluation of Cardiac Patients

A number of special tools are available to cardiologists in the evaluation of pediatric cardiac patients. Some tools are readily available and frequently used, but others are more specialized and are available only at tertiary centers. Echocardiography is the mainstay of noninvasive imaging tools and is available to almost every cardiologist. This tool usually provides the final diagnosis for most pediatric cardiac problems and is discussed in depth. Magnetic resonance imaging and computed tomography are other noninvasive imaging tools that have gained a supplemental role in cardiac evaluation. The discussion of these tools will be limited to the pros and cons of the techniques. Other noninvasive tools frequently used by cardiologists include exercise stress test and ambulatory electrocardiography (e.g., Holter monitor). These tests are discussed in depth. Cardiac catheterization and angiocardiography are invasive tests that usually provide conclusive anatomic and physiologic information and the final diagnosis. Although catheter intervention procedures are not diagnostic, Part 2 discusses it briefly because they are usually performed with cardiac catheterization. Electrophysiologic evaluation is not included in the discussion because they are too specialized and are only rarely performed by specially trained electrophysiologists.