What are the causes of atelectasis?

Causes of Atelectasis

Atelectasis is primarily caused by three major mechanisms: airway obstruction, compression of lung parenchyma, and increased surface tension in alveoli and bronchioles. 1

Obstructive Causes

• Airway obstruction - Most common mechanism leading to resorption atelectasis:
  • Mucus plugging - Common in postoperative patients, chronic bronchitis, asthma, and cystic fibrosis 2
  • Foreign body aspiration - Particularly common in children; presents with abrupt onset of cough 3
  • Broncholiths - Calcified peribronchial lymph nodes that encroach on airways, usually from fungal or mycobacterial granulomatous lymphadenitis 3
  • Endobronchial tumors - Can cause chronic persistent cough and recurrent pneumonia 3
  • Blood clots - May occur after hemoptysis or trauma 2

Compressive Causes

• External compression of lung parenchyma:
  • Pleural effusion - Significant if >300-400 mL, causing passive atelectasis 2, 4
  • Pneumothorax - Both simple (passive atelectasis) and tension (compressive atelectasis) 5
  • Space-occupying lesions - Tumors, enlarged heart, or mediastinal masses 5
  • Abdominal distention - Elevated diaphragm pushing against lower lung fields 5
  • Chest wall abnormalities - Severe kyphoscoliosis or obesity 3

Surface Tension Related Causes

• Surfactant deficiency or dysfunction - Leading to adhesive atelectasis:
  • Acute respiratory distress syndrome (ARDS) 2
  • Neonatal respiratory distress syndrome in premature infants 3
  • Oxygen toxicity - High concentrations of oxygen can damage surfactant 2
  • Mechanical ventilation - Especially with high tidal volumes 3

Special Clinical Scenarios

Perioperative Atelectasis

• General anesthesia - Occurs in 90% of patients undergoing general anesthesia due to:
  • Muscle paralysis causing diaphragmatic dysfunction
  • High fraction of inspired oxygen (FiO₂) promoting absorption atelectasis
  • Supine positioning leading to dependent atelectasis 6

Rounded Atelectasis

• Folded lung syndrome (Blesovsky's syndrome) - Presents radiographically as a mass that may be mistaken for a tumor
• Most commonly associated with asbestos exposure
• Results from infolding of thickened visceral pleura with collapse of intervening lung parenchyma
• Characterized by the pathognomonic "comet sign" on HRCT 3

Chronic Lung Disease of Infancy

• Bronchopulmonary dysplasia - Characterized by regions of atelectasis alternating with emphysema
• Associated with mechanical ventilation, oxygen toxicity, and inflammation
• Maternal chorioamnionitis may play a key role in development 3

Post-obstructive Pulmonary Edema

• Can develop following relief of severe airway obstruction (e.g., laryngospasm)
• Negative intrathoracic pressure created by forceful inspiratory efforts against an obstructed airway
• More common in young muscular adults (male:female ratio 4:1)
• Presents with dyspnea, agitation, cough, pink frothy sputum, and low oxygen saturations 3

Radiographic Signs

The direct signs of atelectasis include:

• Crowded pulmonary vessels
• Crowded air bronchograms
• Displacement of interlobar fissures

Indirect signs include:

• Pulmonary opacification
• Elevation of the diaphragm
• Shift of the trachea, heart, and mediastinum
• Displacement of the hilus
• Compensatory hyperexpansion of surrounding lung
• Approximation of ribs 5

Clinical Implications

Atelectasis can lead to significant clinical consequences including:

• Impaired oxygenation and ventilation-perfusion mismatch
• Decreased lung compliance
• Increased pulmonary vascular resistance
• Risk of secondary infection and pneumonia
• Prolonged mechanical ventilation and hospital stay 7

Understanding the underlying mechanism of atelectasis is crucial for appropriate management and prevention of complications that can impact patient morbidity and mortality.