What is Bibasilar Subsegmental Atelectasis?
Bibasilar subsegmental atelectasis is a collapsed and non-aerated region of lung tissue affecting small subsegmental airways at both lung bases, representing a manifestation of underlying disease rather than a disease itself. 1
Definition and Anatomic Distribution
Atelectasis describes a state of collapsed and non-aerated lung parenchyma that is otherwise structurally normal. 1 When described as "bibasilar subsegmental," this indicates:
- Bibasilar: Affecting both lung bases (bilateral lower lung zones) 2
- Subsegmental: Involving airways smaller than segmental bronchi, typically not visible on standard chest radiographs 2
- Size: Smaller than lobar or segmental atelectasis, often appearing as linear or platelike opacities 3
Mechanisms of Formation
Subsegmental atelectasis develops through several pathophysiologic mechanisms:
Primary Mechanisms
- Absorption atelectasis: Resorption of alveolar air distal to obstructing mucous plugs or secretions in small airways 3
- Compression: External pressure from pleural effusion, masses, or abdominal distention 1, 3
- Adhesive atelectasis: Surfactant deficiency or dysfunction increasing alveolar surface tension 1, 4
- Gravity-dependent collapse: Alterations in alveolar volume due to gravitational forces, particularly common at lung bases 3
Contributing Factors
- Impaired secretion clearance: Poor cough effectiveness, immobility, or postoperative state 5
- Reduced functional residual capacity: From supine positioning, obesity, or abdominal distention 6
- Inflammatory airway disease: Bronchiolitis or small airways inflammation causing mucous hypersecretion 2
Clinical Significance
Subsegmental atelectasis at the lung bases is often clinically insignificant but can indicate underlying pathology requiring investigation. 2, 1
When to Investigate Further
- Persistent or recurrent atelectasis: May indicate bronchiolitis, chronic aspiration, or obstructing lesions 2
- Associated symptoms: Cough, dyspnea, or fever warrant evaluation for infection or other causes 2
- Purulent secretions: Should prompt bronchoscopy to exclude suppurative airways disease 2
- Incomplete airflow reversal: Consider small airways disease if spirometry shows persistent obstruction 2
Radiographic Features
- Chest radiograph: May show linear or platelike opacities, though subsegmental atelectasis can be subtle or invisible 2, 3
- HRCT findings: Indirect signs include mosaic attenuation on expiratory imaging, though normal HRCT cannot exclude bronchiolar disease 2
- Direct signs: Crowded vessels, displaced fissures (when larger), though these are minimal in subsegmental disease 3
Common Clinical Contexts
Bibasilar subsegmental atelectasis frequently occurs in:
- Postoperative patients: Appears in up to 90% of anesthetized patients, often persisting for days 6
- Immobilized or hospitalized patients: Due to reduced lung expansion and secretion retention 5
- Bronchiolitis: Alternating regions of atelectasis and emphysema, particularly in chronic lung disease 2
- Pleural disease: Compressive atelectasis from effusions or pleural thickening 2, 7
Important Caveats
- Not a diagnosis: Atelectasis is a radiographic finding requiring identification of the underlying cause 1
- Differentiation from pneumonia: Cannot be distinguished from consolidation by imaging alone; requires clinical correlation with fever, productive cough, and microbiologic data 3
- Bronchoscopy indications: Reserved for persistent atelectasis with suspected obstruction, purulent secretions, or when common causes have been excluded 2
- HRCT limitations: Cannot visualize normal bronchioles (<2 mm), so clinically significant small airways disease may exist despite normal imaging 2