Atelectasis




Abstract


Atelectasis describes an incomplete expansion of lung tissue, which may be congenital or acquired. Congenital atelectasis is due to incomplete expansion of the lungs, including primary and secondary congenital atelectasis, whereas the most common causes of later atelectasis are loss of air in lung tissue previously expanded or collapse of a lung or lung tissue. Several pulmonary and extrapulmonary conditions may cause atelectasis. Occlusion of the bronchial lumen is the most common cause and may be due to airways inflammation and mucous plug, to increased viscosity of the bronchial mucus and to foreign bodies, among other causes. In occlusion of the bronchial lumen, the air of the occluded lung tissue has gradually been absorbed by the perfusing blood. Atelectasis may also occur during anesthesia. The middle right lobe is often the location of atelectasis caused by pulmonary infections as bronchiolitis or airways inflammation during asthma and is often called middle-lobe syndrome. Atelectasis may occur without clinical signs; plain chest radiography at end-inspiration is important for diagnosis. Multislice computed tomography may point to the diagnosis when plain x-rays do not suffice, and magnetic resonance imaging is well suited for the diagnosis and follow-up of atelectasis. Bronchography combined with diagnostic and therapeutic bronchoscopy may be diagnostic in selected cases. Chest physiotherapy is often the first-line treatment, but its efficacy is not yet proven. Middle-lobe atelectasis has been associated with positive bacterial cultures; in such cases antibiotic treatment may be tried. Bronchoalveolar saline lavage with or without direct instillation of dornase alfa has been tried on rare occasions, and lobectomy has been tried in cases of persistent right-middle-lobe and left-lower-lobe syndrome not responding to intensive medical therapy. Stent placement has been tried in carefully selected patients, with biodegradable stents offering a new alternative.




Keywords

atelectasis, congenital, acquired, bronchial lumen occlusion, lung tissue compression, atelectasis due to anesthesia, postoperative atelectasis, middle-lobe syndrome, foreign body, chest radiography, multislice computed tomography, magnetic resonance imaging, physiotherapy, antibiotic treatment, nebulized dornase alfa

 


The word atelectasis stems from two Greek words: ateles, meaning “imperfect,” and ektasiz, meaning “expansion.” Atelectasis thus means imperfect expansion, and the word is used to describe incomplete expansion of a lung or lung tissue. After inventing the stethoscope in 1816, René Laennec was the first to describe atelectasis as a finding during an autopsy in 1819. There are many causes of atelectasis, which is a common complication of both acute and chronic lung disease affecting patients of all ages. It may be congenital or acquired. Congenital atelectasis is usually due to incomplete expansion of the lungs, including primary and secondary congenital atelectasis. The most common cause of atelectasis is loss of air in lung tissue that was previously expanded, thus resulting in the collapse of a lung or lung tissue.




Etiology and Pathogenesis


Several mechanisms associated with a variety of pulmonary and extrapulmonary diseases may cause atelectasis. Pulmonary causes include obstruction of the bronchial lumen (the most common cause) and increased surface tension of the fluid lining the respiratory tract and alveoli. Extrapulmonary causes include compression of airways and lung tissue from outside the lung and weakness of respiratory muscles in neuromuscular disease. Obstruction of the bronchial lumen by mucus may have several causes, such as airway inflammation and impaired clearance of airway mucus caused by increased mucous viscosity (cystic fibrosis), reduced ciliary function, or a weak cough reflex secondary to neuromuscular disease. Box 70.1 shows the causes of atelectasis.



Box 70.1

Causes of Atelectasis


Intraluminal Causes





  • Airways inflammation with increased bronchial mucus and formation of mucus plug due to




    • Bronchial asthma



    • Respiratory tract infection




      • Bronchiolitis, Pneumonia




    • Bronchopulmonary dysplasia



    • Cystic fibrosis (increased viscosity of the mucus)



    • Primary and secondary ciliary dyskinesia (impaired mucociliary clearance)



    • Immunodeficiency



    • Tracheoesophageal fistula or esophageal atresia




  • Foreign body in the lower respiratory tract




    • Nuts, plastics, other foreign bodies, misplaced tracheal tube




Compression of the Airways





  • Lobar emphysema



  • Lymph node enlargement



  • Vascular ring



  • Complex congenital heart disease (e.g., enlargement of left atrium compressing left main bronchus)



Bronchial Wall Involvement





  • Airway stenosis




    • After aspiration or inhalation injury



    • After intubation



    • Complete cartilaginous rings




  • Bronchiectasis



  • Tracheobronchomalacia



  • Bronchial tumor



Compression of Lung Tissue





  • Pneumothorax



  • Congestive heart failure with cardiac enlargement



  • Hemothorax



  • Chylothorax



  • Lung tumor



Surfactant Dysfunction





  • Respiratory distress syndrome of the newborn



  • Adult respiratory distress syndrome



  • Other



Primary Atelectasis





  • Congenital malformation






Pathogenic Mechanisms


Congenital pulmonary airway malformations may prevent the normal aeration of parts of the lungs at birth due to lack of communication of the main bronchial tree with the affected parts of the lungs that have never been inflated, thus causing primary atelectasis. However, a secondary atelectasis may develop shortly after birth if a congenital malformation occludes or narrows the bronchial lumen, thus presenting as a differential diagnosis to primary atelectasis.


Secondary atelectasis is most often caused by collapse of normal lung tissue due to obstruction or alternatively due to compression of the bronchial lumen. The pores of Kohn form at 3–4 years of age and function as collateral communications between neighboring alveoli; they ensure a more even ventilation/perfusion ratio in the lung, thereby playing a role in preventing atelectasis. At more proximal levels no such collateral communications exist, and occlusion of the bronchial lumen initially leads to air trapping in lung tissue peripheral to the occluded bronchus. The trapped air is gradually absorbed, leading to atelectasis. Causes of obstruction or compression of the bronchial lumen are shown in Box 70.1 .


The solubility of the trapped gases determines their absorption rate. The absorption of atmospheric air will take place within hours, whereas oxygen is absorbed within minutes. Atelectasis therefore occurs more rapidly during ventilation with an increased inspired oxygen fraction and especially with 100% oxygen, compared with breathing normal air. This may partly explain the increased risk of atelectasis during anesthesia. The ventilation/perfusion ratio of the atelectatic tissue is regulated through the increased vascular resistance resulting from hypoxic vasoconstriction of the pulmonary vessels secondary to occlusion of the bronchial lumen.


Atelectasis may be caused by any process or procedure that occludes the bronchial lumen. Foreign bodies in the lower respiratory tract may lead to partial or complete occlusion of a bronchus, and complete occlusion will cause atelectasis. With an initial incomplete occlusion, the foreign body may cause inflammation of the mucous membranes, with resulting mucosal swelling and increased respiratory secretions causing complete obstruction of the bronchial lumen and the development of atelectasis.


Misplaced endotracheal intubation may cause total collapse of one lung when the distal part of the tracheal tube is located in a main bronchus (most often the right one).


Inflammatory processes within the bronchial tree are among the most common causes of obstruction of the bronchial lumen, including bronchial asthma (often eosinophilic inflammation) and acute bronchiolitis due to respiratory syncytial virus infections. In asthma and bronchiolitis, the right middle lobe and the lingula are the most common locations of atelectasis, so common that this is given the name of middle-lobe syndrome. The finding of positive bacterial cultures from the respiratory tract in children with asthma and bronchiolitis with atelectasis and middle-lobe syndrome has led to a suggested role for bacterial infection in long-standing atelectasis of asthma and bronchiolitis.


Airway inflammation due to asthma, bronchiolitis, and other respiratory infections may cause increased bronchial secretions, mucosal edema, bronchial smooth muscle contraction, and destruction of bronchial epithelium with reduced ciliary function, leading to the retention of mucus within the bronchial lumen. Destruction of the bronchial epithelium may alter the airway surface liquid, with an effect on surfactant function; it may thus enhance the tendency for bronchial collapse. In both bronchopulmonary dysplasia and respiratory distress syndrome of prematurity, abnormal surfactant function may contribute to the formation and persistence of atelectasis. Aspiration of meconium, acids, alkali and amniotic fluid also has this effect.


Many diseases increase the susceptibility of the respiratory tract to infection and lead to the accumulation of mucus, which predisposes to the development of atelectasis. These include immunodeficiency, primary ciliary dyskinesia (PCD), and cystic fibrosis.


Bronchial wall processes that narrow the bronchial lumen, including tracheobronchomalacia, vascular rings, tumors such as polyps, papillomas, and (rarely) bronchocentric carcinoma, may cause atelectasis in children. Bronchiectasis, usually caused by recurrent or long-standing airway inflammation, is often complicated by atelectasis.


Extrapulmonary processes may compress normal lung tissue and cause atelectasis without affecting the bronchi, as seen in some patients with congenital heart defects and also with pneumothorax or hemothorax. Rounded atelectasis, seen more often in adults than children, is mostly asymptomatic and associated with chronic pleural disease, lung fibrosis, or pleural effusions. It consists of infolding of atelectatic lung tissue with blood vessels, pleura, and sometimes bronchi.


Atelectasis is common in neuromuscular diseases. Muscular hypotonia impairs ventilation because of reduced movement of respiratory muscles and causes difficulty in clearing bronchial secretions, thus increasing the individual’s susceptibility to respiratory infections and atelectasis. Hypoventilation may also contribute to the development of atelectasis that is seen in children during anesthesia.




Clinical Manifestations


The symptoms and signs of atelectasis will depend on whether single or multiple lobes are involved, the underlying cause, and the age of the patient. In full-term newborn infants with respiratory distress, the presence of atelectasis when accompanied by situs inversus and the need for prolonged supplemental oxygen is highly predictive of a PCD diagnosis (see Chapter 71 ). In this instance, atelectasis caused by mucous plugging secondary to poor mucociliary clearance is most prevalent in the upper (75%) and middle (25%) lobes. Infants with bronchiolitis who develop lobar atelectasis are more likely to have severe disease and require admission to the intensive care unit, while exhaustion and sudden severe deterioration may indicate the development of massive atelectasis affecting a whole lung. Preterm infants with bronchiolitis are at higher risk of developing atelectasis, and younger children in general are more at risk of developing atelectasis than older children and adults owing to less well developed collateral ventilation effected by the pores of Kohn and the canals of Lambert. In children with asthma, acute deterioration of symptom control may be due to the development of atelectasis affecting the right middle lobe, so-called right-middle-lobe syndrome ( Fig. 70.1 ). Similarly, an exacerbation of symptoms in children with cystic fibrosis or PCD may be due to the development of atelectasis secondary to bacterial infection in the lower airways, causing increased production of mucus and cellular debris with subsequent mucus plugging.




Fig. 70.1


Three-year-old girl with repeated episodes of atelectasis of the middle lobe. Chest radiograph. (A) Frontal projection shows slight blurring of right heart contour. (B) Lateral projection shows triangular opacity with apex toward the hilum.


An episode of sudden paroxysmal coughing with or without subsequent respiratory distress in a previously healthy young child accompanied by x-ray features of atelectasis may indicate the need for bronchoscopy to exclude an inhaled foreign body. Atelectasis may not cause detectable abnormalities on clinical examination; thus the diagnosis must be made radiologically. But generally clinical signs relate to the size of the atelectasis. There may be impaired oxygen saturation, decreased expansion of the chest on the affected side, dullness to percussion, and diminished or absent breath sounds. If the atelectasis is partial or airway obstruction is not complete, crackles may be heard during inspiration and expiration. In some cases of significant or even whole-lung atelectasis, oxygen saturation may be normal, since alveolar hypoxia can induce reflex vasoconstriction and thus minimize ventilation/perfusion mismatch. Paradoxically, intubation and mechanical ventilation of such patients with supplemental oxygen may cause a temporary deterioration in oxygen saturation due the abolition of the protective vasoconstrictive reflex, thus inducing intrapulmonary shunting and the perfusion of unventilated, atelectatic lung tissue that does not take part in gas exchange. Lung function in patients with atelectasis may be normal if the atelectasis is small or demonstrate a restrictive pattern with a reduced FEV 1 and FVC, and a normal FEV 1 /FVC.




Diagnosis


The prompt diagnosis of atelectasis in children is important, since early detection and subsequent treatment may lead to an improved outcome. Diagnosis is aided by an understanding of situations in which atelectasis is more likely to occur and the underlying pathophysiologic mechanisms. For example, postoperative atelectasis is not uncommon, particularly in children undergoing cardiac surgery. However, since atelectasis is not always detectable clinically, it may be discovered unexpectedly, for example, during routine chest x-ray in a child with a chronic underlying condition such as asthma or PCD. Flexible bronchoscopy has a role in the diagnosis of atelectasis when there is suspected airway obstruction due to, for example, foreign-body inhalation, mucous plugging, endobronchial tuberculosis, airway malacia, external compression from a vascular ring, enlarged lymph nodes, or an enlarged heart. It may also be therapeutic in cases of intraluminal obstruction.


The most frequently used modality for the diagnosis of atelectasis is chest radiography. Frontal projection is always included, but sometimes lateral views are better suited, as in atelectasis of the right middle lobe and the lower lobes. Oblique views may be of particular value in segmental atelectasis. Sometimes fluoroscopy is also used to delineate difficult locations of increased opacification. Fluoroscopy may also be used to diagnose air trapping and mediastinal shift when a foreign body is suspected. In older children, however, an x-ray at end-inspiration followed by another at end-expiration will suffice. Because atelectasis results in volume reduction of the affected part of the lung, this may lead to general signs such as elevation of the diaphragm and narrowing of ipsilateral intercostal spaces ( Fig. 70.2 ). Shift of the mediastinum and tracheal contours toward the affected side is quite common, but these general signs may be absent if emphysema develops in the ipsilateral lung or if the atelectasis occurs together with ipsilateral pleural effusion. Atelectasis of the right upper lobe is seen as a combination of increased opacity and volume reduction leading to elevation of the interlobar fissure ( Fig. 70.3 ). A summary of major findings on chest radiography related to extent and location of the atelectasis is presented in Table 70.1 .


Jul 3, 2019 | Posted by in RESPIRATORY | Comments Off on Atelectasis
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