What is Atelectasis
Atelectasis is a state of collapsed and non-aerated lung parenchyma that occurs when alveoli lose their normal air content, resulting in reduced lung volume and impaired gas exchange. 1
Definition and Pathophysiology
Atelectasis represents a pathological condition rather than a disease itself, manifesting as a consequence of various underlying pulmonary and chest disorders. 1 Physiologically, it develops when collapsing forces—derived from positive pleural pressure and surface tension—overcome the expanding forces from alveolar pressure and parenchymal tethering. 2
Mechanisms of Development
Atelectasis occurs through three primary mechanisms:
- Airway obstruction (resorption atelectasis): Results from resorption of alveolar air distal to obstructing airway lesions, including mucus plugs or blood clots. 1, 3
- Compression: Caused by extrathoracic or intrathoracic processes (tension pneumothorax, space-occupying lesions, abdominal distention) or passive compression from simple pneumothorax and diaphragmatic dysfunction. 3
- Increased surface tension: Stems from surfactant deficiency (adhesive atelectasis) or surfactant dysfunction. 1, 3
Clinical Significance and Consequences
Perioperative Context
Atelectasis occurs in approximately 90% of patients undergoing general anesthesia, with 15-20% of lung tissue regularly collapsed at the base during uneventful anesthesia before any surgical intervention. 4 This perioperative atelectasis persists into the postoperative period and can last for several days. 5, 6
Physiologic Impact
The condition produces multiple adverse effects:
- Impaired oxygenation: Decreased lung compliance and gas exchange abnormalities. 6, 2
- Increased pulmonary vascular resistance: Compromising cardiovascular function. 6
- Local tissue responses: Including inflammation, immune dysfunction, and damage to the alveolar-capillary barrier with potential loss of lung fluid clearance and increased susceptibility to infection. 2
- Risk of pneumonia: Atelectatic areas serve as potential foci for infection. 4
Biomechanical Consequences
When mechanical ventilation is applied to heterogeneously aerated lungs (with atelectatic regions present), harmful biomechanical processes occur: concentration of mechanical forces, propagation of gas-liquid interfaces, and remote overdistension of non-collapsed regions. 2
Diagnostic Features
Radiographic Signs
Direct signs include:
- Crowded pulmonary vessels
- Crowded air bronchograms
- Displacement of interlobar fissures 3
Indirect signs include:
- Pulmonary opacification
- Elevation of the diaphragm
- Shift of trachea, heart, and mediastinum
- Displacement of the hilus
- Compensatory hyperexpansion of surrounding lung
- Approximation of ribs 3
Chest radiographs using both anterior-posterior and lateral projections are mandatory to document atelectasis, though differentiation from lobar consolidation can be challenging. 1
Clinical Presentations
Atelectasis can be classified descriptively as:
- Segmental, lobar, or whole lung
- Subsegmental
- Platelike, linear, or discoid
- Round
- Generalized or diffuse 3
Common clinical indications for evaluation include persistent atelectasis, unexplained respiratory distress, and recurrent atelectasis requiring therapeutic intervention such as mucus plug removal via flexible bronchoscopy. 7