Atelectasis is defined as lung parenchyma not containing air. Atelectasis may be classified according to the location and pathogenesis.
Pathogenesis. If resorption of air from lung parenchyma takes place distal to an obstruction, the atelectasis is termed obstructive. The rate of absorption depends on the amount of collateral ventilation. The obstruction of a lobar bronchus is more likely to result in atelectasis than that of a segmental bronchus since lung segments have more collateral ventilation than lobes. Nonobstructive atelectasis may occur in pneumothorax, effusion, compression (tumor), loss of structural stability due to lack of surfactant (preterm birth, ARDS), and scarring with retraction. A special form is the rounded atelectasis, which is observed after asbestos exposure.
Obstructive atelectasis may be due to:
* Bronchial tumors (most commonly bronchial cancer [Figs. 13.1, 13.2], carcinoid tumor). The symptoms associated with atelectasis are generally not caused by the atelectasis but by the underlying disease such as bronchial cancer. They may consist of cough or hemoptysis.
* Obstruction of a bronchus by mucus, i. e., mucoid impaction.
* Obstruction of a bronchus by a foreign body. This should always be considered, in particular in children and elderly persons. As the aspirated material is often not radioopaque, the foreign body may not be detected in the chest radiograph. Foreign bodies are more often aspirated into the right- than into the left-sided bronchi.
Non-obstructive atelectasis is caused by:
* Loss of contact among visceral and parietal pleura, such as in pleural effusion or pneumothorax.
* Compression atelectasis is usually not a major diagnostic problem since the clinical presentation is dominated by underlying disease (pleural effusion, mass).
* Loss of alveolar stability. This may occur in premature neonates due to lack of surfactant or in ARDS.
* Scarring.
The most common atelectasis are plate-like atelectasis. They may occur as a consequence of
* reduced diaphragmatic motility, in particular after upper abdominal surgery
* pulmonary embolism.
     Rounded atelectasis are a special type of atelectasis with a rounded appearance, often in contact to the visecral pleura and with a tail. They may occur in subjects exposed to asbestos.
Left upper lobe atelectasis
Fig. 13.1 Left upper lobe atelectasis due to a central bronchial cancer. Elevated diaphragm on the left. 70-year-old man.
     a In the PA radiograph, the left cardiac border is not clearly delineated (positive silhouette sign).
     b In the lateral view, the lobar boundary is displaced ventrally. No air bronchogram.
• Occurs frequently in patients receiving mechanical ventilation with higher Fio2.
• Dependent regions of the lung are more prone to atelectasis: they are partially compressed, they are not as well ventilated, and there is no spontaneous drainage of secretions with gravity.
• Decreased or absent breath sounds
• Abnormal chest percussion
• Cough, dyspnea, decreased vocal fremitus and vocal resonance
• Diminished chest expansion, tachypnea, tachycardia
• Mechanical ventilation with higher Fio2
• Chronic bronchitis
• Cystic fibrosis
• Endobronchial neoplasms
• Foreign bodies
• Infections (e.g., TB, histoplasmosis)
• Extrinsic bronchial compression from neoplasms, aneurysms of ascending aorta, enlarged left atrium
• Sarcoidosis
• Silicosis
• Anterior chest wall injury, pneumothorax
• Alveolar injury (e.g., toxic fumes, aspiration of gastric contents)
• Pleural effusion, expanding bullae
• Chest wall deformity (e.g., scoliosis)
• Muscular weaknesses or abnormalities (e.g., neuromuscular disease)
• Mucus plugs from asthma, allergic bronchopulmonary aspergillosis, postoperative state
Physical examination allows detection of relatively large atelectases.
     Radiologically, the extent of atelectasis (e. g., upper lobe collapse) and the potential causes (lung cancer) are analyzed. The most important radiologic characteristics are:
* localized opacity
* displacement of a fissure
* volume loss.
Additional signs are:
* elevation of the diaphragm
* displacement of the mediastinum including the trachea
* displacement of the hilum
* reduced intercostal space
* absence of air bronchograms.
     Assessment of atelectasis requires detailed anatomic knowledge. Massive atelectasis appears as homogeneous opacities that can not be differentiated radiologically from other causes.
     Microatelectasis may cause ventilation-perfusion inhomogeneity with disturbance of gas exchange but may not be visible in the radiograph.
• Neoplasm
• Pneumonia
• Encapsulated pleural effusion
• Abnormalities of brachiocephalic vein and of the left pulmonary ligament
Atelectasis of the right upper lobe
Fig. 13.2 Atelectasis of the right upper lobe due to a central bronchial cancer. The fissure between the middle and upper lobe is displaced apically. No air bronchogramm, seen in a 69- year-old man.
Risc Factors for developing Atelectasis
• Chest x-ray
• CT scan and fiberoptic bronchoscopy (selected patients)
• Chest x-ray will confirm diagnosis.
• CT scan is useful in patients with suspected endobronchial neoplasm or extrinsic bronchial compression.
• Fiberoptic bronchoscopy (selected patients) is useful for removal of foreign body or evaluation of endobronchial and peribronchial lesions.

• Deep breathing, mobilization of the patient
• Incentive spirometry
• Tracheal suctioning
• Humidification
• Chest physiotherapy with percussion and postural drainage
• Positive-pressure breathing (CPAP by face mask, positive end-expiratory pressure [PEEP] for patients on mechanical ventilation)
• Use of mucolytic agents (e.g., acetylcysteine [Mucomyst])
• Recombinant human DNase (dornase alpha) in patients with cystic fibrosis
• Bronchodilator therapy in selected patients
Chest physiotherapy, humidification of inspired air, frequent nasotracheal suctioning
Prognosis varies with the underlying etiology.
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