Normal Anatomy and Artefacts

The anteroposterior (AP) diameter of the neonatal chest is almost as great as its transverse diameter, giving the chest a cylindrical configuration. The degree of rotation is best assessed by comparing the length of the anterior ribs visible on both sides. As newborn chest radiographs are taken in the AP plane, the normal cardiothoracic ratio can be as large as 60%.

The thymic size is variable and may alter with the degree of lung inflation. It may blend with the cardiac silhouette, it may have an undulating boarder due to underlying rib indentation (Fig. 76-1) or it may exhibit the classic ‘sail sign’ more commonly seen on the right side. It may involute rapidly with prenatal or postnatal stress, for example in severe illnesses such as hyaline membrane disease or infections, or following corticosteroid treatment.

The normal lung development is well described by Agrons et al.¹ During the embryonic phase of gestation (from 26 days to 6 weeks) the lung bud develops from the primitive foregut and divides to form the early tracheobronchial tree. During the pseudoglandular phase (6–16 weeks) there is airway development to the level of the terminal bronchioles, with a deficient number of alveolar saccules. Multiple alveolar ducts develop from the respiratory bronchioles during the cannicular or acinar phase (16–28 weeks). These ducts are lined by type II alveolar cells which can produce surfactant, and which differentiate into thin type I alveolar lining cells. At the end of this phase primitive alveoli form. Progressive thinning of the pulmonary interstitium allows gas exchange with approximation of the proliferating capillaries and the type I cells.

During the saccular phase (28–34 weeks) there is an increase in the number of terminal sacs, further thinning of the interstitium, continuing proliferation of the capillary bed and early development of the true alveoli.

The alveolar phase extends from approximately 36 weeks’ gestation until 18 month of age, with most alveoli formed at 5–6 months of age.

Idiopathic Respiratory Distress Syndrome

Idiopathic respiratory distress syndrome (IRDS) or hyaline membrane disease (HMD) mainly affects the premature infant less than 36 weeks’ gestational age. The primary problem in HMD is a deficiency of the lipoprotein pulmonary surfactant in association with structural immaturity of the lungs. The lipoproteins are produced in the type II pneumocytes, are concentrated in the cell lamellar bodies and then transported to the cell surface and expressed onto the alveolar luminal surface. These lipoproteins then combine with surface surfactant proteins (A, B, C, D), which are also produced by the type II pneumocytes to form tubular myelin. This is the principal contributor at the alveolar air–fluid interface which lowers alveolar surface tension and prevents acinar collapse on expiration.¹ Without this, there is alveolar collapse and, as a result, poor gas exchange, hypoxia, hypercarbia and acidosis. The alveolar ducts and terminal bronchioles are distended and lined by hyaline membranes which contain fibrin, cellular debris and fluid, thought to arise from a combination of ischaemia, barotrauma and the increased oxygen concentrations used in assisted ventilation.² Hyaline membrane formation can also occur in other neonatal chest conditions requiring ventilation.

Clinically these premature infants are usually symptomatic within minutes of birth with grunting, retractions, cyanosis and tachypnoea. Chest radiographic findings may be present shortly after birth but occasionally the maximum features may not be present until 6–24 hours of life.

Before the commencement of treatment, the typical radiographic features include underaeration of the lungs, fine granular opacification, which is diffuse and symmetrical, and air bronchograms (Fig. 76-2), due to collapsed alveoli interspersed with distended bronchioles and alveolar ducts. When there is less distension, the granularity is replaced by more generalised opacification or complete white-out of the lungs (Fig. 76-3). Atelectasis is the main cause of this opacification, but in the very premature infant in particular, oedema, haemorrhage and occasionally superimposed pneumonia contribute.

The clinical use of artificial surfactant, given as a liquid bolus through the endotracheal (ET) tube, has been a major therapeutic advance. It may not be evenly distributed throughout the lungs, leading to areas of atelectasis interspersed with areas of good aeration, and may produce radiographic findings similar to neonatal pneumonia or pulmonary interstitial emphysema (PIE) (Fig. 76-4). Correlation with the clinical picture is, therefore, very important.

In general, infants greater than 27 weeks’ gestation respond best to surfactant therapy. In the very premature infant, less than 27 weeks’ gestation, the lungs become clear following surfactant administration, but they are still immature with fewer alveoli than normal. This results in inadequate gas exchange, leads to prolonged ventilation, hazy lung opacification and occasionally a picture similar to that seen in bronchopulmonary dysplasia (Fig. 76-5).

The chest radiograph may demonstrate sudden cardiac enlargement, left atrial enlargement causing elevation of the left main bronchus and varying degrees of pulmonary oedema (Fig. 76-6). There is an increasing use of prophylactic continuous positive airway pressure (CPAP) ventilation in infants suspected of developing IRDS, which helps reduce the incidence of complications in these infants. High-frequency ventilation is also used to reduce the incidence of barotrauma, particularly in the very premature infant. In these infants the radiographs do not differ significantly from those infants receiving conventional ventilation. The chest radiograph is used to assess the degree of lung inflation. The dome of the diaphragm should project at the level of the 8th–10th posterior ribs if the mean airway pressure is appropriately adjusted.

The use of positive pressure ventilation in the newborn is the most common cause of pneumothorax, pneumomediastinum, pulmonary interstitial emphysema (Fig. 76-7) and pneumopericardium (Fig. 76-8). These complications have become much less common in infants who have been treated with surfactant and high-frequency ventilation. Areas of atelectasis can occur in surfactant deficiency and are frequently due to poor clearance of secretions (Fig. 76-9). Premature infants are at an increased risk of pneumonia, which may coexist with IRDS. Pulmonary haemorrhage resulting in airspace opacification may also be a superimposed problem, and is usually due to severe hypoxia and capillary damage (Fig. 76-10). Bronchopulmonary dysplasia (BPD) or chronic lung disease is a significant long-term complication of IRDS. Because of the many advances in neonatal care, its incidence and severity have reduced significantly in infants born at 28 weeks’ gestation or older. The unchanged overall incidence is due to the increased survival of the infants of extreme prematurity as they require more prolonged ventilation. Air leaks, patent ductus arteriosus and infection are contributing factors as they also prolong ventilation. A higher incidence of BPD has been demonstrated in infants with previous culture-proven Ureaplasma urealyticum pneumonitis.³

The four classic stages of BPD described by Northway⁴ are now very rarely seen. Nowadays the most common radiographic appearance is diffuse interstitial shadowing with mild-to-moderate hyperinflation of gradual onset (Fig. 76-11). A new type of BPD was described by Jobe in 1999⁵ in immature infants with minimal lung disease at birth, and who become symptomatic during the first week of life. This entity seems inseparable from the condition described previously as Wilson–Mikity syndrome.

Typically the infants have mild-to-moderate respiratory distress without cyanosis in the first couple of hours. The process resolves rapidly with almost complete resolution in 48 hours. Treatment consists of supportive oxygen and maintenance of body temperature. Radiographically, the most common appearances are mild overinflation, prominent blood vessels, perihilar interstitial shadowing and fluid in the transverse fissure with occasional small pleural effusions (Fig. 76-12). There may be mild associated cardiomegaly. The appearances may be asymmetrical with right-sided predominance, which remains unexplained. The features may simulate meconium aspiration syndrome and congenital neonatal pneumonia, particularly when severe.

The definition of meconium aspiration syndrome is an infant born through meconium-stained amniotic fluid where the symptoms cannot be otherwise explained.⁶ It is thought that fetal hypoxia causes fetal intestinal hyperperistalsis and passage of meconium, which is aspirated by a gasping fetus. It is most common in infants who are post-mature. It is diagnosed by the presence of meconium below the level of the vocal cords.

The radiographic features may, in part, be due to the inhalation of meconium itself in utero or during birth. It is a thick viscous substance and may lead to areas of atelectasis and overinflation. It may migrate to the distal airways, causing complete or partial obstruction and lead to a ‘ball-valve’ effect. It may also cause a chemical pneumonitis (Fig. 76-13). There may be associated alterations in the pulmonary vasculature, leading to pulmonary arterial hypertension. Air leaks are common and small associated pleural effusions may be seen.

Approximately 30% of infants will require mechanical ventilation. The mortality rate has been improved by the use of inhaled nitric oxide, to treat severe pulmonary hypertension and also by extracorporeal membrane oxygenation (ECMO), which is used only in those infants where the conventional treatments have failed. The ECMO technique can be used either with the veno-arterial method, where one catheter is placed in the internal jugular vein and one in the carotid artery, or the veno-venous method, where a double lumen catheter is placed in the internal jugular vein, superior vena cava or right atrium (Fig. 76-14). The circulation bypasses the lungs, which are minimally inflated, and allows physiologic levels of oxygen saturation.

The most common features seen on the chest radiograph in term infants who present with severe acute symptoms in the first 24–48 h are coarse bilateral asymmetrical alveolar opacification with or without associated interstitial change (Fig. 76-15). In these infants the radiographic changes may mimic meconium aspiration syndrome or severe transient tachypnoea. The presence of pleural effusions, pulmonary hyperinflation and mild cardiomegaly may not be helpful in differentiating pneumonia from these other conditions.

In the premature infant there maybe diffuse fine granular opacification, similar to the appearances seen in IRDS.⁷ Some infants may have both IRDS and group B streptococcus pneumonia. Chlamydial infection classically presents first with conjunctivitis at 1–2 weeks after birth and the lung infection does not usually become evident until 4–12 weeks of age. Typically the radiograph demonstrates interstitial opacification with some hyperinflation.

The association of Ureaplasma urealyticum with neonatal pneumonia is increasingly recognised. These infants have a mild early course and develop features of BPD at an earlier age than would be expected in a premature infant.⁸

Air Leaks

Spontaneous pneumothorax and pneumomediastinum causes respiratory distress in the newborn infant. Many are transient and do not require intervention. A pneumothorax may be radiographically subtle in sick infants as supine radiographs are usually performed and free air accumulates over the lung surface, producing a hyperlucent lung and increased sharpness of the mediastinum (Figs. 76-7 and 76-14). A pneumomediastinum usually outlines the thymus (Fig. 76-16) and when there is a pneumopericardium the air surrounds the heart (Fig. 76-8).

In infants who do not have hydrops, the most common cause of a congenital pleural effusion is chylothorax. The cause is unknown, and late maturation of the thoracic duct has been suggested as an aetiology. The abnormality is usually detected on antenatal ultrasound (US) and in utero drainage may be performed to prevent pulmonary hypoplasia. Postnatally, the chest radiograph demonstrates the pleural effusions (Fig. 76-17). Aspirated fluid will have a high lymphocyte count but will not have a milky appearance until such time as the infant is fed with fat. Resolution is usually complete but often after multiple aspirations.

Disorders of surfactant deficiency due to a genetic abnormality in the surfactant protein B (SpB)⁹ and C (SpC)¹⁰ and the ATP-binding cassette transporter protein A3 (ABCA3) can lead to interstitial lung disease. Inherited mutations in the SpB and ABCA3 are autosomal recessive and may present immediately after birth with respiratory symptoms. Mutations in the SpC are autosomal dominant and may present later in infancy. The chest radiograph may show diffuse hazy opacification initially, with the later development of interstitial shadowing which may be progressive (Fig. 76-18A). Computed tomography (CT) demonstrates diffuse ground-glass opacification with septal thickening¹¹ and cystic change (Figs. 76-18B and C).

The umbilical arterial line courses inferiorly in the umbilical artery, into the internal and common iliac arteries and then into the aorta. The tip should be positioned to avoid the origins of the major vessels, which are usually between T6 and T9 (Fig. 76-19) or in some institutions inferior to L3 vertebral bodies.

The umbilical venous line courses superiorly towards the liver. It enters the left portal vein, through the ductusvenosus and into the inferior vena cava (IVC). The ideal position is at the junction of the IVC and the right atrium (Fig. 76-19).

The correct position of central venous lines or peripherally inserted central catheters (PICC) is controversial. The position of PICC line tips inserted through the upper limbs is usually in the superior vena cava. The tips of those inserted through the lower limbs are usually positioned at the junction of the IVC and the right atrium. US may be particularly helpful in assessing a catheter’s position and injection of very small amounts of intravenous water-soluble, low osmolar contrast medium may also be useful in checking the position of the tip.

Nasogastric tube tip positions should always be reported on, in order to avoid misplacement of nasogastric feeds.