The major role of the respiratory system is the oxygenation of blood and the removal of the body’s waste products in the form of carbon dioxide. The respiratory system consists of two separate divisions, the upper tract located outside the thorax and lower tract found within the thoracic cavity (Figure 3-1). The upper respiratory system, which consists of the nasopharynx, oropharynx, and larynx, provides structure for the passage of air into the lower respiratory system. The lower respiratory system, which consists of the trachea, bronchi, and bronchioles, is composed of tubular structures responsible for conducting air from the upper respiratory structures. The smallest unit where gas exchange occurs consists of the terminal bronchiole, alveolar ducts, and alveolar sacs. With the use of the upper and lower respiratory structures, the air from outside the body enters the lungs. The single trachea branches out into two bronchi (one to each lung) at the carina (last segment of the trachea), which in turn branch out into progressively smaller bronchioles to produce a structure termed the bronchial tree because its appearance resembles an inverted tree. The tracheobronchial tree is lined with a mucous membrane (the respiratory epithelium) containing numerous hairlike projections called cilia. During inspiration, the air is moistened and warmed as it enters the lungs. The cilia act as miniature sweepers to prevent dust and foreign particles from reaching the lungs. When the ciliary blanket works correctly, the particles are moved away from the lungs to be coughed up or swallowed. Any damage to the respiratory epithelium and its cilia permits particles (entering with the inspired or inhaled air) to proliferate and produce a disease process.

Figure 3-1 Structure plan of the respiratory system. The inset shows the alveolar sacs where the interchange of oxygen and carbon dioxide takes place through the walls of the grapelike alveoli.

The vital gas exchange within the lung (called external respiration) takes place within the alveoli, extremely thin-walled sacs surrounded by blood capillaries, which represent the true parenchyma of the lung (see Figure 3-1 inset). Oxygen in the inhaled air diffuses from the alveoli into the blood capillaries, where it attaches to hemoglobin molecules in red blood cells and circulates to the various tissues of the body (called internal respiration). Carbon dioxide, a waste product of cellular metabolism, diffuses in the opposite direction, passing from the blood capillaries into the alveoli and then exiting the body during expiration (or exhalation). Because individual alveoli are extremely small, chest radiographs can demonstrate only a cluster of alveoli and their tiny terminal bronchioles, which are the basic anatomic units of the lung. A cluster of alveoli is termed the acinus.

Respiration is controlled by a center in the medulla at the base of the brain. The level of carbon dioxide in the blood regulates the respiratory center. Even a slight increase in the amount of carbon dioxide in the blood increases the rate and depth of breathing, such as when an individual exercises. The accumulation of waste gases that must be removed from the body (and the body’s need for additional oxygen) causes the respiratory center to stimulate the muscles of respiration—the diaphragm and the intercostal muscles between the ribs. Contraction of the muscles of respiration causes the volume of the chest cavity to increase. This decreases the pressure within the lungs and forces air to move into the lungs through the tracheobronchial tree. As the respiratory muscles relax, the volume of the chest cavity decreases, and air is forced out of the lungs. Special muscles of expiration (abdominal and internal intercostal muscles) may be needed for difficult breathing or in patients with decreased gas exchange, as occurs in emphysema.

Unlike most other organs, the lung has two different blood supplies. The pulmonary circulation is a low-pressure, low-resistance system through which oxygen enters and carbon dioxide exits the circulatory system. The bronchial circulation, which is a part of the high-pressure systemic circulation, supplies oxygenated blood to nourish (or support) the lung tissue itself.

A double-walled membrane consisting of two layers of pleura encases the lungs (Figure 3-2). The visceral pleura is the inner layer that adheres to the lung, whereas the parietal pleura lines the inner chest wall (the thoracic cavity). Between the two layers of pleura is a potential space (pleural space), which normally contains only a small amount of fluid to lubricate the surfaces to prevent friction as the lungs expand and contract. The airtight space between the lungs and the chest wall has a pressure a bit less than that in the lungs. This difference in pressure acts like a vacuum to prevent the lungs from collapsing. An inflammatory or neoplastic process that involves the pleura may produce fluid within the potential space (a pleural effusion).

Figure 3-2 Lungs and pleura (transverse section). Note the parietal pleura, which lines the right and left pleural divisions of the thoracic cavity before folding inward near the bronchi to cover the lungs as the visceral pleura. The intrapleural space separates the parietal and visceral pleura. The heart, esophagus, and aorta are shown in the central mediastinum.

Internal devices

Endotracheal Tube

A chest radiograph should always be obtained immediately after endotracheal intubation to ensure proper positioning of the tube, because clinical evaluation (bilateral breath sounds, symmetric thoracic expansion, and palpation of the tube in the sternal notch) does not allow detection of the majority of malpositioned tubes. Daily radiographs are usually taken to ensure that the tube has not been inadvertently displaced by the weight of the respiratory apparatus, the patient’s coughing, or other unforeseen events. In addition, imaging permits prompt detection of complications of intubation and barotrauma (positive-pressure breathing), such as pneumothorax and pneumomediastinum.

The relationship between the tip of the tube and the carina (tracheal bifurcation) must be carefully assessed. When the head and neck are in a neutral position, the endotracheal tube tip ideally should be about 5 to 7 cm above the carina (Figure 3-3). With flexion and extension of the neck, the tip of the tube will move about 2 cm caudally and cranially, respectively.

Figure 3-3 Proper position of the endotracheal tube.

About 10% to 20% of endotracheal tubes require repositioning after insertion. A tube positioned too low usually extends into the right mainstem bronchus, where it eventually leads to atelectasis of the left lung (see Figure 3-76). A tube positioned excessively high or in the esophagus causes the inspired air to enter the stomach, causing severe gastric dilatation and a high likelihood of regurgitation of gastric contents and aspiration pneumonia.

Figure 3-76 Malpositioned endotracheal tube. Excessively low position of endotracheal tube in the bronchus intermedius causes collapse of right upper lobe and entire left lung.

Central Venous Catheters

Central venous catheters inserted into the subclavian vein or a more peripheral vein in the upper extremity are extremely useful for measurement of the central venous pressure (CVP) and for providing a conduit for the rapid infusion of fluid or chronic hyperalimentation. So that the CVP may be correctly measured, the catheter must be located within the true central venous system, beyond all the valves that interfere with direct transmission of right atrial pressure to the catheter. The optimal location is where the brachiocephalic veins join to form the superior vena cava (medial to the anterior border of the first rib on chest radiographs) or within the superior vena cava itself.

Because up to one third of CVP catheters are initially inserted incorrectly, the position of the catheter should be confirmed by a chest radiograph. The most common aberrant location of a CVP catheter is the internal jugular vein (Figure 3-4). CVP catheters that extend to the right atrium are associated with an increased risk of cardiac arrhythmias and even perforation. Extension of the catheter into the hepatic veins may result in the infusion of potentially toxic substances (some antibiotics and hypertonic alimentation solutions) directly into the liver. Even after successful placement, CVP catheters may change position as a result of patient motion or medical manipulation. Therefore, periodic radiographic confirmation of the catheter position is often recommended.

Figure 3-4 CVP catheter in the right internal jugular vein.

The anatomy of the subclavian region may lead to complications when a central catheter is introduced via the subclavian vein. Because the pleura covering the apex of the lung lies just deep to the subclavian vein, a pneumothorax may develop. This problem may be difficult to detect clinically, and thus a chest radiograph (if possible with the patient in an upright position and in expiration) should be obtained whenever insertion of a subclavian catheter has been attempted. Another complication is perivascular CVP catheter placement, which may result in ectopic infusion of fluid into the mediastinum or pleural space. This diagnosis should be suggested if there is rapid development of mediastinal widening or pleural effusion after CVP catheter insertion (Figure 3-5). Other complications include inadvertent puncture of the subclavian artery, air embolism, and injury to the phrenic nerve.

Figure 3-5 CVP catheter with its tip in the pleural space. A right subclavian catheter, which was introduced for total parenteral nutrition, perforated the superior vena cava and eroded into the right pleural space. Note the tip of the catheter projecting beyond the right border of the mediastinum (arrow). The direct infusion of parenteral fluid into the pleural space has led to a large right hydrothorax.

The peripherally inserted central catheter (PICC) has become the long-term venous access device used for home therapy and for patients undergoing chemotherapy (Figure 3-6).

Figure 3-6 PICC line placement. PICC line appears in the superior vena cava.

Catheter breakage and embolization can result from laceration of the catheter by the needle used to insert it, fracture at a point of stress, or detachment of the catheter from its hub. The catheter fragment may lodge in the vena cava, in the right side of the heart, or in branches of the pulmonary artery (Figure 3-7). Adverse results include thrombosis, infection, and perforation.

Figure 3-7 Broken CVP catheter. The sheared-off portion of the catheter (arrows) is located in the left lower lobe.

Swan-Ganz Catheter

The flow-directed Swan-Ganz catheter consists of a central channel for measuring pulmonary capillary wedge (PCW) pressure and a second, smaller channel connected to an inflatable balloon at the catheter tip. Cardiac output and CVP can also be measured using the Swan-Ganz catheter. It can be inserted at the bedside and floated to the pulmonary artery without the need for fluoroscopic monitoring.

Ideally the catheter is positioned so that it lies within the right or left main pulmonary artery. Inflating the balloon causes the catheter to float downstream into a wedge position; deflating the balloon permits the catheter to recoil into the central pulmonary artery. Unlike standard intravenous catheters, the Swan-Ganz catheter has a radiopaque strip down its center. Radiographically the tip of the tube is visualized within the borders of the mediastinum when properly placed; this would substantially decrease the likelihood of occlusion of the distal pulmonary vessel.

The most common complication associated with the use of a Swan-Ganz catheter is pulmonary infarction distal to the catheter tip. Infarction may result from occlusion of a pulmonary artery by the catheter itself (if it is wedged in a too peripheral vessel) or from clot formation in or about the catheter. Pulmonary infarction appears as a patchy air-space consolidation involving the area of the lung supplied by the pulmonary artery in which the catheter lies. The appropriate treatment is simply removal of the Swan-Ganz catheter; systemic heparinization is not required once this source of emboli or obstruction has been removed.

Transvenous Cardiac Pacemakers

Transvenous endocardiac pacing is the method of choice for maintaining cardiac rhythm in patients with heart block or bradyarrhythmias. Radiographic evaluation plays an important role in the initial placement of a pacemaker and in the detection of any subsequent complications. An overexposed image can demonstrate both the generator (for permanent pacemakers) and the course of the electrodes.

Ideally, the tip of the pacemaker should be positioned at the apex of the right ventricle. One common aberrant location is the coronary sinus. On a frontal radiograph, the tip often appears to be well positioned. A lateral projection is required to show that the tip is directly posterior in the coronary sinus, rather than in its proper position anterior in the right ventricle.

Although electrode fractures have become less common because of the development of new alloys, they are still a significant cause of pacing failure (Figure 3-8). The usual sites of fracture are near the pulse generator, at sharp bends in the wires, and at the point where the electrodes are inserted into the epicardium. Although most electrode fractures are easily detected on routine chest radiographs, some subtle fractures may be demonstrated only on oblique views or at fluoroscopy.

Figure 3-8 Pacemaker tip in coronary sinus. On the frontal projection, the tip of the electrode is angled slightly superiorly, traversing the heart in a higher plane than when it is located in the right ventricle.

Perforation of the myocardium by an intravenous electrode usually occurs at the time of insertion or during the first few days thereafter. Perforation should be suspected when the pacemaker fails to sense or elicit a ventricular response. Plain radiographs show the electrode tip lying outside the right ventricular cavity (Figure 3-9).

Summary of Findings for Internal Devices

Internal Device	Correct Placement*	Complications
Endotracheal tube	Tip of tube 5-7 cm above the carina	Low placement—atelectasis High placement—air entering the stomach
Central venous pressure catheters	Tip of catheter should be in the superior vena cava	Internal jugular vein placement Right atrium—possible arrhythmias or perforation Pneumothorax with placement Infusion of fluid into mediastinum or pleural space
Swan-Ganz catheters	Right or left main pulmonary artery seen radiographically within the borders of the mediastinum	Pulmonary infarction
Transvenous cardiac pacemakers	Overexpose to demonstrate the tip of the electrode at the apex of the right ventricle	Coronary sinus placement—needs a lateral chest image to distinguish Perforation at initial insertion

^*Placement determined by chest radiograph.

Congenital/hereditary diseases

Cystic Fibrosis

Cystic fibrosis (mucoviscidosis) is a hereditary disease characterized by the secretion of excessively viscous mucus by all the exocrine glands; it is caused by a defective gene in the middle of chromosome 7. Cystic fibrosis is the most common clinically important genetic disorder among white children. This disorder also affects the pancreas and digestive system. However, 90% of the morbidity and mortality related to cystic fibrosis occurs as a result of respiratory involvement.

Figure 3-9 Fracture of a cardiac pacemaker wire (arrow).

In the lungs, thick mucus secreted by mucosa in the trachea and bronchi blocks the air passages. The thick mucus is the result of an imbalance of sodium and chloride production and reabsorption. These mucous plugs lead to focal areas of lung collapse. Recurrent pulmonary infections are common because bacteria that are normally carried away by mucosal secretions adhere to the sticky mucus produced in this condition. Because of the recurring nature of the disease, by age 10 years many children have widespread bronchiectasis with the formation of large cysts and abscesses. In the pancreas, blockage of the ducts by mucous plugs prevents pancreatic enzymes from entering the duodenum. This process impairs the digestion of fat, resulting in failure of the child to gain weight and the production of large, bulky, foul-smelling stools. In about 10% of newborns with cystic fibrosis, the thick mucus causes obstruction of the small bowel (meconium ileus) (Figure 3-10). Bowel perforation with subsequent fatal peritonitis may occur.

Figure 3-10 Meconium ileus in cystic fibrosis. Massive small bowel distention with profound soap-bubble effect of gas mixed with meconium.

Involvement of the sweat glands in cystic fibrosis causes the affected child to perspire excessively. The perspiration excess leads to a loss of large amounts of salt (sodium, potassium, and chloride), two to three times the normal amount. Patients are therefore extremely susceptible to heat exhaustion in hot weather. The presence of excessive chloride on the skin is the basis for the “sweat test,” a simple and reliable test for cystic fibrosis.

Radiographic Appearance

Radiographically, cystic fibrosis causes generalized irregular thickening of linear markings throughout the lungs that, when combined with the almost invariable hyperinflation, produces an appearance similar to that of severe chronic lung disease in adults (Figure 3-11). Computed tomography (CT) screening to detect structural lung damage and assess disease progression is becoming more accepted by clinicians. There is a concern regarding the radiation dose if CT were used on a routine basis (yearly), especially because people with cystic fibrosis are now living into their 30s.

Figure 3-11 Cystic fibrosis. Multiple small cysts superimposed on diffuse, coarse, reticular pattern.

Treatment

Patient well-being depends on the use of prophylactic antibiotics, chest physiotherapy (percussions), and improved airflow. Prophylactic antibiotics reduce the risk of lung infections that may cause permanent lung damage or bronchiectasis. Chest physiotherapy (hand tapping against the chest) prevents lungs from filling with viscous mucus by keeping the mucus moving. Improved airflow depends on the use of bronchodilators. Currently to help control pulmonary infections, administered by inhalation is recombinant human deoxyribonuclease (DNase), an enzyme that digests the DNA of bacterial and inflammatory cells in the lungs. The newest research trials focus on methods to control the production and reabsorption of sodium and chloride. In the future, gene therapy will be a viable alternative for patients with cystic fibrosis.

Hyaline Membrane Disease

Hyaline membrane disease, also known as idiopathic respiratory distress syndrome (IRDS), is one of the most common causes of respiratory distress in the newborn. It occurs primarily in premature infants, especially those who have diabetic mothers or who have been delivered by cesarean section. Hypoxia and increasing respiratory distress may not be immediately evident at birth but almost always appear within 6 hours of delivery.

The progressive underaeration of the lungs in hyaline membrane disease results from a lack of surfactant and immature lungs. Surfactant consists of a mixture of lipids, proteins, and carbohydrates that creates a high surface tension, requiring less force to inflate and maintain the alveoli. Normally the alveolar cell walls produce lipoprotein, which maintains the surface tension within the alveoli. This tension permits the alveoli to remain inflated so that atelectasis does not occur. The disease process results from surfactant deficiency caused by cell immaturity or birth trauma.

Radiographic Appearance

In addition to pronounced underaeration, the radiographic hallmark of hyaline membrane disease is a finely granular appearance of the pulmonary parenchyma (Figure 3-12). A peripherally extending air bronchogram develops because the small airways dilate and stand out clearly against the atelectasis in the surrounding lung.

Figure 3-12 Hyaline membrane disease. Diffuse granular pattern of pulmonary parenchyma associated with air bronchograms (arrows).

Treatment

New treatment advances for this disease include the use of an artificial surfactant, which offers the best therapy to reduce morbidity and mortality from this disease process. The artificial surfactant is administered into the airways via a saline solution.

The treatment of hyaline membrane disease includes the use of positive-pressure ventilators that pump air (often with high concentrations of oxygen) into the lungs through an endotracheal tube. The positive-pressure ventilator ensures satisfactory levels of tissue oxygenation. The high ventilator pressure may cause leakage of air from overinflated alveoli or small terminal bronchioles, leading to interstitial emphysema, pneumothorax, and pneumopericardium, all of which further decrease the expansion of the lungs.

Summary of Findings for Congenital/Hereditary Disorders

Inflammatory disorders of the upper respiratory system

Croup

Croup is primarily a viral infection of young children that produces inflammatory obstructive swelling localized to the subglottic portion of the trachea. The edema causes inspiratory stridor or a barking cough, depending on the degree of laryngeal obstruction.

Inflammatory disorders of the lower respiratory system

Pneumonia

Acute pneumonia is an inflammation of the lung that can be caused by a variety of organisms, most commonly bacteria and viruses. Regardless of the cause, pneumonias tend to produce one of three basic radiographic patterns.

Alveolar Pneumonia

Alveolar, or air-space, pneumonia, exemplified by pneumococcal pneumonia, is produced by an organism that causes an inflammatory exudate that replaces air in the alveoli, so that the affected part of the lung is no longer air containing but rather appears solid, or radiopaque (Figure 3-15). The inflammation spreads from one alveolus to the next by way of communicating channels, and it may involve pulmonary segments or an entire lobe (lobar pneumonia).

Figure 3-15 Alveolar pneumonia (pneumococcal). Homogeneous consolidation of right upper lobe and medial and posterior segments of right lower lobe. Note associated air bronchograms (arrows).

Radiographic Appearance: Consolidation of the lung parenchyma with little or no involvement of the airways produces the characteristic air bronchogram sign (Figure 3-16). The sharp contrast between air within the bronchial tree and the surrounding airless lung parenchyma permits the normally invisible bronchial air column to be seen radiographically. The appearance of an air bronchogram requires the presence of air within the bronchial tree, which suggests that the bronchus is not completely occluded at its origin. Presence of an air bronchogram excludes the diagnosis of a pleural or mediastinal lesion because there are no bronchi in these regions. Because air in the alveoli is replaced by an equal or almost equal quantity of inflammatory exudate and because the airways leading to the affected portions of the lung remain open, there is no evidence of volume loss in alveolar pneumonia.

Figure 3-16 Air bronchogram sign in pneumonia. Frontal chest radiograph demonstrates air within intrapulmonary bronchi in patient with diffuse alveolar pneumonia of left lung.

Bronchopneumonia

Bronchopneumonia, typified by staphylococcal infection, is primarily an inflammation that originates in the bronchi or the bronchiolar mucosa and spreads to adjacent alveoli. Because alveolar spread of the infection in the peripheral air spaces is minimal, the inflammation tends to produce small patches of consolidation. Bronchial inflammation causing airway obstruction leads to atelectasis with loss of lung volume.

Radiographic Appearance: The small patches of consolidation may be seen radiographically as opacifications that are scattered throughout the lungs, but are separated by an abundance of air-containing lung tissue (Figure 3-17); air bronchogram is absent. If consolidation causes obstructed airways, atelectasis is evident.

Figure 3-17 Bronchopneumonia (staphylococcal). Ill-defined consolidation at right base.

Interstitial Pneumonia

Interstitial pneumonia is most commonly produced by viral and mycoplasmal infections. In this type of pneumonia, the inflammatory process predominantly involves the walls and lining of the alveoli and the interstitial supporting structures of the lung, the alveoli septa.

Radiographic Appearance: The interstitial dispersal of the infection produces a linear or reticular pattern (Figure 3-18). When seen on end, the thickened interstitium may appear as multiple small nodular densities. Left untreated, interstitial pneumonia may cause “honeycomb lung,” which is demonstrated on CT as cystlike spaces and dense fibrotic walls (Figure 3-19).