Chest

Diffuse Pulmonary Disease in the Newborn

Diffuse pulmonary disease causes respiratory distress much more commonly than surgical diseases, particularly in premature infants, who make up the majority of cases of respiratory distress in the newborn. A simple way to evaluate these patients and try to offer a limited differential diagnosis is to evaluate the lung volumes and to characterize the pulmonary opacities.

Lung volumes can be categorized as high, normal, or low. Normally, the apex of the dome of the diaphragm is expected to be at the level of approximately the 10th posterior rib. If present, lung opacity can be characterized as either streaky, perihilar (central) densities that have a linear quality or as diffuse, granular opacities that have an almost sand-like character. Classically, cases fall into one of the following two categories: (1) cases with high lung volumes and streaky perihilar densities and (2) cases with low lung volumes and granular opacities ( Table 3-1 ). This is more of a guideline rather than a rule because many neonates with diffuse pulmonary disease have normal lung volumes. The differential diagnosis for cases with high lung volumes and streaky perihilar densities includes meconium aspiration, transient tachypnea of the newborn (TTN), and neonatal pneumonia. Most of the neonates in this group are term. The differential for cases with low lung volumes and granular opacities includes surfactant deficiency and one specific type of infection—group B streptococcal (GBS) pneumonia. Most of these neonates are premature.

Meconium Aspiration Syndrome

Meconium aspiration syndrome results from intrapartum or intrauterine aspiration of meconium. It usually occurs secondary to stress, such as hypoxia, and more often occurs in term or postmature neonates. The aspirated meconium causes both obstruction of small airways secondary to its tenacious nature as well as chemical pneumonitis. The degree of respiratory failure can be severe and may also lead to pulmonary air leaks and persistent pulmonary hypertension. Radiographic findings include hyperinflation (high lung volumes), which may be asymmetric and patchy, and asymmetric lung densities that tend to have a ropy appearance and a perihilar distribution ( Fig. 3-1 ). Commonly, there are areas of hyperinflation alternating with areas of atelectasis. Pleural effusions can be present. Because of the small airway obstruction by the meconium, air-block complications are common, with pneumothorax occurring in 20%–40% of cases. Meconium aspiration syndrome is relatively common; it occurs in approximately 2%–10% of infants whose birth is associated with meconium-stained amniotic fluid. In the United States, a retrospective multicenter study reported that approximately 2% of infants had an admission diagnosis of meconium aspiration syndrome annually. Although meconium-stained amniotic fluid occurs in approximately 10%–15% of deliveries, meconium aspiration syndrome develops in less than 5% of them.

Transient Tachypnea of the Newborn

TTN is also referred to by a variety of other names, including wet lung disease and transient respiratory distress. It occurs secondary to delayed clearance of fetal lung fluid. Physiologically, the clearing of fetal lung fluid is facilitated by the “thoracic squeeze” during vaginal deliveries; therefore, most cases of TTN are related to cesarean section, in which the thoracic squeeze is bypassed. Other causes include maternal diabetes and use of maternal anesthesia or analgesia during delivery. The hallmark of TTN is a benign course. Respiratory distress develops by 6 h of age, peaks at 1 day of age, and is resolved by 2–3 days. There is a spectrum of radiographic findings similar to those seen with mild-to-severe pulmonary edema. There is a combination of airspace opacification, interstitial edema, prominent and indistinct pulmonary vasculature, fluid in the fissures, pleural effusion, and cardiomegaly ( Fig. 3-2 ). Lung volumes are normal to increased.

Neonatal Pneumonia

Neonatal pneumonia can be caused by a large number of infectious agents that can be acquired intrauterine, during birth, or soon after birth. With the exception of GBS pneumonia, which will be discussed separately, the radiographic appearance of neonatal pneumonia is that of patchy, asymmetric perihilar densities and hyperinflation ( Fig. 3-3 ). Pleural effusions may be present ( Fig. 3-4 ). Such cases of neonatal pneumonia may have a similar radiographic appearance to and be indistinguishable from meconium aspiration syndrome or TTN on imaging alone.

Surfactant-Deficient Disease

Surfactant-deficient disease (SDD), also known as respiratory distress syndrome (RDS) or hyaline membrane disease , is a common disorder, with approximately 24,000 new cases annually in the United States. The incidence of RDS increases with decreasing gestational age at birth. The risk of SDD increases with decreasing gestational age. In a study from the Child Health and Human Development (NICHD) Neonatal Research Network centers, approximately 98% of the neonates born at 24 weeks had RDS, while the incidence was 5% in those born at 34 weeks. SDD is also the most common cause of death in live newborns. SDD is related to the inability of premature type II pneumocytes to produce surfactant. Normally, surfactant coats the alveolar surfaces and decreases surface tension, which allows the alveoli to remain open. As a result of the lack of surfactant, there is alveolar collapse. The radiographic findings reflect these pathologic changes ( Fig. 3-5 ). Lung volumes are low. There are bilateral granular opacities that represent collapsed alveoli interspersed with open alveoli. Because the larger bronchi do not collapse, there are prominent air bronchograms. When the process is severe enough and the majority of alveoli are collapsed, there may be coalescence of the granular opacities, resulting in diffuse lung opacity. Imaging findings may not be prominent at birth, but a normal film at 6 h of age excludes the presence of SDD.

Surfactant Replacement Therapy

One of the therapies for SDD is surfactant administration. Surfactant can be administered via nebulized or aerosol forms. It is administered into the trachea via a catheter or an adapted endotracheal tube. The administration of surfactant in neonates with SDD is associated with decreased oxygen and ventilator setting requirements, decreased air-block complications, decreased incidence of intracranial hemorrhage and bronchopulmonary dysplasia (BPD), also known as chronic lung disease of prematurity, and decreased death rate. However, there is an associated increased risk for development of patent ductus arteriosus, pulmonary hemorrhage, as well as an acute desaturation episode in response to surfactant administration. Surfactant administration can be given on a rescue basis when premature neonates develop respiratory distress or can be given prophylactically in premature infants who are at risk. Prophylactic administration is commonly given immediately after birth and has become common practice. In response to surfactant administration, radiography may demonstrate complete, central, or asymmetric clearing of the findings of SDD ( Fig. 3-5 ). There is usually an increase in lung volumes. Neonates without radiographic findings of a response to surfactant have poorer prognoses than those who have radiographic evidence of a response. A pattern of alternating distended and collapsed acini may create a radiographic pattern of bubble-like lucencies that can mimic pulmonary interstitial emphysema (PIE). Knowledge of the timing of surfactant delivery is helpful to render accurate interpretation of chest radiographs taken in the neonatal intensive care unit (NICU).

Group B Streptococcal Pneumonia

GBS pneumonia is the most common type of pneumonia in neonates. The infection is acquired during birth, and approximately 10%–30% of women in labor are colonized by the organism. Premature infants are more commonly infected than are term infants. In contrast to the other types of neonatal pneumonia, the radiographic findings include bilateral granular opacities and low lung volumes ( Fig. 3-6 ), the identical findings in SDD. The presence of pleural fluid is a helpful differentiating factor because it is very uncommon in surfactant deficiency but has been reported in between 25% and 67% of cases of GBS pneumonia.

Persistent Pulmonary Hypertension of the Newborn

Persistent pulmonary hypertension of the newborn (PPHN), also known as persistent fetal circulation , is a term often used in the NICU and is addressed here because it can be a source of confusion. The high pulmonary vascular resistance that is normally present in the fetus typically decreases during the newborn period. In normal conditions, after clamping the cord and with the first breaths, there is a drastic drop in pulmonary vascular resistance permitting the circulation that in utero bypasses the lungs, to perfuse the lungs. If this drastic decrease in pulmonary vascular resistance does not take place, blood would continue to bypass the lungs and pulmonary pressures would remain abnormally high; the condition is termed persistent pulmonary hypertension . It is a physiologic finding rather than a specific disease. It can be a primary phenomenon or it can occur secondary to causes of hypoxia, such as meconium aspiration syndrome, neonatal pneumonia, or pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH). These patients are quite ill. The radiographic patterns are variable and are more often reflective of the underlying cause of hypoxia than the presence of persistent pulmonary hypertension. Occasionally, it may be seen in neonates with clear chest radiographs, and in these cases, it is referred as primary PPHN.

Neonatal Intensive Care Unit Support Apparatus

One of the primary roles of chest radiography in the NICU is to monitor support apparatus, including endotracheal tubes, enteric tubes, central venous lines, umbilical arterial and venous catheters, and extracorporeal membrane oxygenation (ECMO) catheters. The radiographic evaluation of many of these tubes is the same as that seen in adults and is not discussed here. When evaluating the positions of endotracheal tubes in premature neonates, it is important to consider that the length of the entire trachea may be only approximately 1 cm. Keeping the endotracheal tube in the exact center of such a small trachea is an impossible task for caregivers, and phone calls and reports suggesting that the tube needs to be moved 2 mm proximally may be more annoying than helpful. Direct phone communication may be more appropriately reserved for times when the tube is in a main bronchus or above the thoracic inlet. There is an increased propensity for inadvertent esophageal intubation to occur in neonates as compared with in adults. Although it would seem that esophageal intubation would be incredibly obvious clinically, this is not always the case. The authors have experienced cases in which a child has in retrospect been discovered to have been esophageally intubated for more than 24 h. Therefore, the radiologist may be the first to recognize esophageal intubation. Obviously, when the course of the endotracheal tube does not overlie the path of the trachea, the use of esophageal intubation is fairly obvious. Other findings of esophageal intubation include a combination of low lung volumes, gas within the esophagus, and gaseous distention of the bowel ( Fig. 3-7 ).

Umbilical Arterial and Venous Catheters

Umbilical lines are commonly used in the NICU. The pathway of the umbilical venous catheter is umbilical vein to left portal vein to ductus venosus, the most superior portion of the ductus venosus joins, or becomes part of, one of the hepatic veins as they enter the inferior vena cava ( Fig. 3-8 ). In contrast to umbilical arterial catheters, the course is in the superior direction from the level of the umbilicus. The ideal position of an umbilical venous catheter is with its tip at the junction of the right atrium and the inferior vena cava at the level of the hemidiaphragm. The umbilical venous catheter may occasionally deflect into the portal venous system rather than passing into the ductus venosus. Complications of such positioning can include hepatic hematoma or abscess.

Umbilical arterial catheters pass from the umbilicus inferiorly into the pelvis via the umbilical artery to the iliac artery. The catheters then turn cephalad within the aorta ( Fig. 3-9 ). These catheters can be associated with thrombosis of the aorta and its branches. Therefore, it is important to avoid positioning the catheter with the tip at the level of the branches of the aorta (celiac, superior mesenteric, and renal arteries) or at the origin of the great arteries (arch). There are two acceptable umbilical arterial catheter positions: high lines have their tips at the level of the descending thoracic aorta (T8–T10) and low lines have their tips below the level of L3. The catheter tip should not be positioned between T10 and L3 because of the risk for major arterial thrombosis. There is no clear consensus as to whether a high or low umbilical artery catheter line is better, and both positions are still currently used, although some authors advocate that the high position has fewer complications.

Peripherally Inserted Central Catheters in Children

One of the more common lines now seen in children, as in adults, is peripherally inserted central catheters (PICCs). In contrast to adults, in whom some of the PICCs can be as large as 6F, the PICC lines used in children, particularly infants, are often small in caliber (2F or 3F) so that they can be placed into their very small peripheral veins. These small-caliber PICCs can be very difficult to see on chest radiography, so some of them must be filled with contrast to be accurately visualized. The tip of the PICC line that enters the child from the upper extremity should be positioned with the tip in the midlevel of the superior vena cava or superior cavoatrial junction. It is essential that PICC lines not be left in place with the tip well into the right atrium. Particularly, with the small-caliber lines, the atrium can be lacerated, leading to pericardial tamponade, free hemorrhage, or death. Many such cases have been reported nationally. In addition, the PICC should not be too proximal in the superior vena cava because the distal portion of the line can flip from the superior vena cava into the contralateral brachiocephalic or jugular vein. At many children's hospitals, the PICC lines are inserted in a dedicated suite by a team of nurses, with supervision by pediatric interventional radiologists or other physicians. Ultrasound (US) is often used to guide vein cannulation, and certified child life specialists may coach many kids through the procedure without sedation. Fluoroscopy is used at the end of the procedure to adjust and document tip position in the cavoatrial junction.

Extracorporeal Membrane Oxygenation

ECMO is a last resort therapy usually reserved for respiratory failure that has not responded to other treatments. ECMO is essentially a prolonged form of circulatory bypass of the lungs and is used only in patients who have reversible disease and a chance for survival. The majority of neonates who are treated with ECMO have respiratory failure as a result of meconium aspiration, persistent pulmonary hypertension (resulting from a variety of causes), severe congenital heart disease, or CDH. ECMO seems to be used less commonly now for diffuse pulmonary disease than it was in the 1990s.

There are two types of ECMO: arteriovenous and venovenous. In arteriovenous ECMO, the right common carotid artery and internal jugular veins are sacrificed. The arterial catheter is placed via the carotid and positioned with its tip overlying the aortic arch. The venous catheter is positioned with its tip over the right atrium ( Fig. 3-9 ). One of the main roles of a chest radiograph of children on ECMO is to detect any potential migration of the catheters. Careful comparison with previous studies to make sure that the catheters have not moved proximally or distally is critical. These patients have many bandages and other items covering the external portions of the catheters, so migration may be hard to detect on physical examination. In addition, anasarca is invariable during ECMO because the increasing soft tissue edema may result in accidental dislodgment and subsequent accidental decannulation. Note that there are various radiographic appearances of the ECMO catheters. Some catheters end where the radiopaque portion of the tube ends, and others have a radiolucent portion with a small metallic marker at the tip ( Fig. 3-9 ). It is common to see white-out of the lungs soon after a patient is placed on ECMO as a result of sudden decrease in airway pressure, decreased ventilator settings, and third-space shifting of fluid ( Fig. 3-9 ). Patients on ECMO are anticoagulated and are therefore at risk for hemorrhage. They are often monitored by head US to exclude development of intracranial hemorrhage.

Types of Ventilation: High-Frequency Oscillator versus Conventional Ventilation

High-frequency oscillators are commonly used to treat neonates in the NICU. In contrast to conventional ventilation, high-frequency oscillators use supraphysiologic rates of ventilation with very low tidal volumes. Conventional ventilation has been likened to delivering a cupful of air approximately 20 times a minute. In contrast, high-frequency oscillation is like delivering a thimbleful of air approximately 1000 times per minute. The air is vibrated in and out of the lung, and appropriate gas exchange occurs at significantly lower peak inspiratory pressures than those required with conventional ventilation. The mechanism of how oscillators work is poorly understood. In conventional ventilation, the diaphragm moves up and down, whereas during high-frequency ventilation the diaphragm stays “parked” at a certain anatomic level. This level can be adjusted by changing the mean airway pressure of the oscillator. Caregivers usually like to maintain the diaphragm at approximately the level of the 10th posterior ribs. In general, the radiographic appearance of neonatal pulmonary diseases is not affected by whether the patient is being ventilated by conventional or high-frequency ventilation.

Complications in the Neonatal Intensive Care Unit

As in adult intensive care units, major complications detected by chest radiographs include those related to air leak complications, lobar collapse, or acute diffuse pulmonary consolidation. Another type of complication seen in neonates is the development of chronic lung disease of prematurity. Imaging findings of lobar collapse and air-block complications, such as pneumothorax, pneumomediastinum, and pneumopericardium, are similar in neonates and adults and will not be discussed here. One type of air-block complication that is unique to neonates is pulmonary insterstitial emphysema (PIE).

Pulmonary Interstitial Emphysema

In patients with severe surfactant deficiency, ventilatory support can result in marked increases in alveolar pressure, leading to perforation of alveoli. The air that escapes into the adjacent interstitium and lymphatics is referred to as PIE. PIE appears on radiographs as bubble like or linear, nonbranching lucencies, and can be focal or diffuse ( Fig. 3-10 ). The involved lung is usually noncompliant and seen to have a static volume on multiple consecutive chest radiographs. The finding is typically transient. The importance of detecting PIE is that it serves as a warning sign for other impending air-block complications, such as pneumothorax, and its presence can influence caregivers in decisions, such as switching from conventional to high-frequency ventilation.

It can be difficult to differentiate diffuse PIE from the bubble-like lucencies that are associated with developing BPD. When encountering this scenario, the patient's age can help to determine which is more likely. Most cases of PIE occur in the first week of life, prior to when most cases of BPD develop. In patients older than 4 weeks, BPD is more likely. In addition, in patients who have undergone a series of daily films, PIE may be noted to occur abruptly, whereas BPD tends to occur gradually. As previously mentioned, SDD partially treated by surfactant replacement can cause a pattern of lucencies that may mimic PIE as well.

Rarely, PIE can persist and develop into an expansive, multicystic mass. The air cysts can become large enough to cause mediastinal shift and compromise pulmonary function. Often the diagnosis is indicated by sequential radiography showing evolution of the cystic mass from original findings typical of PIE. In unclear cases, computed tomography (CT) demonstrates that the air cysts are in the interstitial space by showing the broncho-vascular bundles positioned within the center of the air cysts. The broncho-vascular bundles appear as linear or nodular densities in the center of the lucent cysts.

Causes of Acute Diffuse Pulmonary Consolidation

Acute diffuse pulmonary consolidation is nonspecific in neonates, as it is in adults, and can represent blood, pus, or water. In the neonate, the specific considerations include edema, which may be secondary to the development of patent ductus arteriosus ( Fig. 3-11 ); pulmonary hemorrhage, to which surfactant therapy predisposes; worsening surfactant deficiency (during the first several days of life but not later); or developing neonatal pneumonia ( Table 3-2 ). Diffuse microatelectasis is another possibility because neonates have the propensity to artifactually demonstrate diffuse lung opacity on low lung volume radiographs obtained with expiratory technique ( Fig. 3-12 ); this should not be mistaken for another cause of consolidation. Such radiographs showing low lung volumes offer little information concerning the pulmonary status of the patient and should be repeated when clinically indicated.

Bronchopulmonary Dysplasia

BPD, or chronic lung disease of prematurity , is a common complication, seen in premature infants and is associated with significant morbidity rates. BPD typically occurs in neonates treated with oxygen and positive pressure ventilation for respiratory failure, usually SDD. BPD is uncommon in children born at greater than 32 weeks of gestational age, but it occurs in more than 50% of premature infants born at less than 1000 g. BPD is the most common chronic lung disease of infancy.

The definition of BPD has evolved over time; currently, one of the most accepted definition is that of oxygen dependence for at least 28 days after birth, whereas the severity is graded in proportion to the respiratory support required at term. The most recent revision (2019), based on a prospective NICHD (National Institute of Child Health and Human Development) network study, again includes the presence of persistent parenchymal lung disease confirmed by radiography and the need of some form of respiratory support at 36 weeks of age.

BPD is related to injury to the lungs that is thought to result from some combination of mechanical ventilation and oxygen toxicity. Although four discrete and orderly stages of the development of BPD were originally described, they are not seen commonly and are probably not important to know. BPD typically occurs in a premature infant who requires prolonged ventilator support. At approximately the end of the second week of life, persistent hazy density appears throughout the lungs. Over the next weeks to months, a combination of coarse lung markings, bubble-like lucencies, and asymmetric aeration can develop ( Fig. 3-13 ). Eventually, focal lucencies, coarse reticular densities, and band-like opacities develop. In childhood survivors of BPD, many of these radiographic findings decrease in prominence over the years and only hyperaeration may persist. The radiographic findings may completely resolve. Clinically, many children with severe BPD during infancy may eventually improve to normal pulmonary function or may only have minor persistent problems such as exercise intolerance, predisposition to infection, or asthma.