Atelectasis is a loss of lung volume that may be caused by a variety of ventilation disorders, for instance, bronchial injury or an obstructive mass such as a tumor. It may be categorized as obstructive, nonobstructive, postoperative, or rounded. Clinical features depend on the severity and extent of atelectasis, ranging from no symptoms to respiratory distress. Physical examination reveals a dull note on percussion and diminished breathing sounds over the affected area. On x-ray, the atelectatic section of the lung appears condensed and, due to decreased lung volume, may extend to the surrounding tissue. This effect can lead to an elevated diaphragm and mediastinal shift to the affected side. Treatment depends on the underlying cause. Complications of atelectasis include pneumonia or, depending on the extent of disease, respiratory failure.
- Obstructive atelectasis (most common): airway obstruction (e.g., by a foreign body, mucus plug, malignancy) → nonventilated alveoli → reabsorption of gas in the poststenotic space → lung collapse
- Compression atelectasis: external space-occupying lesion (e.g., pleural effusion) that compresses the lung → forcefully pushes air out of the alveoli
- Adhesive atelectasis: surfactant deficiency or dysfunction → increases surface tension of alveoli → instability and collapse (e.g., acute respiratory distress syndrome (ARDS) in adults, respiratory distress syndrome in premature infants)
- Cicatrization (contraction) atelectasis: parenchymal scarring that leads to contraction of the lung (due to chronic destructive lung processes such as tuberculosis, sarcoidosis, and fibrosis, e.g., secondary to radiation therapy)
- Relaxation atelectasis: loss of contact between parietal and visceral tissue (e.g., pneumothorax, )
- Replacement atelectasis: All the alveoli in an entire lobe are replaced by tumor (e.g., bronchioloalveolar cell carcinoma) → loss of volume → lung collapse
- Postoperative atelectasis: one of the most common post-operative complications (especially after chest or abdominal surgery); often occurs within 72 hours of surgery
- Rounded atelectasis: folding of the atelectatic lung tissue (with fibrous bands and adhesions) to the pleura (e.g., asbestosis)
- Symptoms depend on the acuity and extent of atelectasis.
- Dull percussion note, diminished breath sounds, and decreased fremitus over the affected lung
- Possibly tracheal deviation towards the side of lesion
- Arterial blood gas analysis: hypoxemia, potentially low PaCO2, and respiratory alkalosis
Chest x-ray and CT: evidence of lobar collapse 
- Direct signs: displacement of fissures and homogeneous opacification of the collapsed lobe
- Indirect signs
- Bronchoscopy (diagnostic and therapeutic): A biopsy may be performed if the etiology is uncertain despite imaging (e.g., to exclude malignancy) and mucus plugs can be removed. 
- Adequate analgesia
- Early mobilization
- Lung expansion maneuvers that increase positive end-expiratory pressure (PEEP)
- Treatment of underlying condition
The risk of atelectasis after surgery can be avoided by prescribing opioids in doses that are sufficient for pain relief, as well as encouraging the use of incentive spirometry. At the same time, opioids should be used with caution due to their suppression on coughing. Smoking should be avoided 6–8 weeks prior to surgery.
We list the most important complications. The selection is not exhaustive.