What is atelectasis in the lungs?

What is Atelectasis?

Atelectasis is a state of collapsed and non-aerated lung tissue that is otherwise structurally normal, representing a pathological manifestation rather than a disease itself. 1

Definition and Basic Concept

Atelectasis refers to the collapse of lung parenchyma with loss of alveolar air, resulting in non-functional lung tissue that cannot participate in gas exchange. 1 This condition is present in approximately 90% of patients undergoing general anesthesia and represents one of the most commonly encountered abnormalities in chest radiology. 2, 3

Primary Mechanisms

Atelectasis develops through three main pathways:

1. Resorption (Absorption) Atelectasis

• Occurs when alveolar air is absorbed distal to an obstructing airway lesion 4
• Common causes include mucus plugs, foreign body aspiration, broncholithiasis (calcified lymph nodes eroding into airways), and tumors 5, 1
• High oxygen concentrations during anesthesia accelerate this process by replacing poorly absorbed nitrogen with rapidly absorbed oxygen 2, 6

2. Compressive Atelectasis

• Results from external compression of lung tissue by space-occupying processes 4
• Causes include pleural effusions (including malignant effusions), pneumothorax, chest wall masses, and abdominal distention 5, 4

3. Adhesive Atelectasis

• Develops from surfactant deficiency or dysfunction, increasing alveolar surface tension 1, 4
• Seen in neonatal respiratory distress syndrome and acute respiratory distress syndrome 5

Additional Mechanisms

• Cicatrization atelectasis: Results from pulmonary fibrosis with scarring and contraction 4
• Passive atelectasis: Caused by hypoventilation, diaphragmatic dysfunction, or simple pneumothorax 4
• Gravity-dependent atelectasis: Results from gravity-dependent alterations in alveolar volume, particularly common during anesthesia 4

Clinical Patterns and Presentations

Anatomic Distribution

• Segmental, lobar, or whole lung collapse: Typically from major airway obstruction 4
• Subsegmental atelectasis: Smaller areas of collapse 4
• Platelike (linear or discoid) atelectasis: Horizontal bands of collapse, can be associated with pulmonary embolism 5, 4
• Rounded atelectasis: A special form also called "folded lung" or Blesovsky's syndrome, most commonly associated with asbestos exposure and characterized by infolding of thickened visceral pleura 5

Radiographic Signs

Direct signs include crowded pulmonary vessels, crowded air bronchograms, and displacement of interlobar fissures. 4

Indirect signs include pulmonary opacification, elevation of the diaphragm, mediastinal shift, hilar displacement, compensatory hyperexpansion of surrounding lung, and rib approximation. 4

Clinical Significance

• Atelectasis impairs pulmonary gas exchange, resulting in decreased blood oxygenation through increased pulmonary shunt and ventilation-perfusion mismatch 2, 6
• The amount of atelectasis correlates directly with the degree of pulmonary shunt 2
• Atelectasis can persist for several days postoperatively and may serve as a focus for infection, contributing to pulmonary complications 3
• In obese patients, larger atelectatic areas develop compared to lean individuals 2
• Patients with COPD may paradoxically show less or even no atelectasis 2

Important Clinical Context

Atelectasis is a manifestation of underlying disease processes, not a disease entity itself. 1 The presence of dependent lung atelectasis on imaging (particularly on supine CT scans) may be mistaken for abnormal lung infiltration or may mask subtle abnormalities, which is why prone imaging is sometimes recommended. 7