What is Bibasilar Atelectasis?
Bibasilar atelectasis is a state of collapsed and non-aerated lung tissue affecting both lung bases (bilateral lower lobes), representing a pathological condition rather than a disease itself. 1
Definition and Anatomical Location
Atelectasis describes collapsed lung parenchyma that is otherwise structurally normal but non-aerated 1. When occurring "bibasilar," this specifically affects the basilar (lower) segments of both lungs simultaneously. This is an extremely common finding, particularly in hospitalized and post-operative patients 2.
Primary Mechanisms
Bibasilar atelectasis develops through several distinct pathophysiological mechanisms:
Compression Atelectasis
- Pleural effusions are a common cause of compressive atelectasis at the lung bases, where fluid accumulates preferentially due to gravity 3
- External masses or space-occupying lesions can compress lung tissue 3
- Abdominal distention pushes the diaphragm upward, compressing basilar lung segments 4
Absorption (Resorption) Atelectasis
- Occurs when alveolar air is resorbed distal to obstructed airways 4
- Foreign body aspiration and broncholithiasis (calcified lymph nodes eroding into airways) can cause obstructive patterns 3
Gravity-Dependent Atelectasis
- Results from gravity-dependent alterations in alveolar volume, particularly affecting dependent lung regions 4
- Up to 15-20% of lung tissue at the bases regularly collapses during anesthesia before any surgery begins 5
- Approximately 90% of anesthetized patients develop some degree of atelectasis 5
Adhesive Atelectasis
- Stems from surfactant deficiency or dysfunction, increasing surface tension in alveoli 1, 4
- Can occur in neonatal respiratory distress syndrome 3
Passive Atelectasis
- Caused by hypoventilation, diaphragmatic dysfunction, or simple pneumothorax 4
Clinical Significance and Risk Factors
The bibasilar distribution is particularly common because:
- Basilar lung regions are most susceptible to compression from pleural effusions, abdominal processes, and positional effects 3
- Loss of functional residual capacity (FRC) and muscle tone during anesthesia or critical illness preferentially affects dependent lung zones 5
- COPD and other chronic pulmonary conditions increase atelectasis risk 3
Radiographic Appearance
Direct signs on imaging include:
- Crowded pulmonary vessels in the affected areas 4
- Crowded air bronchograms 4
- Displacement of interlobar fissures 4
Indirect signs include:
- Pulmonary opacification at the bases 4
- Elevation of the hemidiaphragms 4
- Compensatory hyperexpansion of upper lobes 4
Special consideration: Rounded atelectasis (also called folded lung) can occur following pleuritis, particularly from asbestos exposure, and may mimic a mass lesion with a characteristic "comet sign" on CT 3
Clinical Consequences
Bibasilar atelectasis can persist for several days postoperatively and may serve as a focus for infection, contributing to pulmonary complications 5. The severity of consequences depends on extent, mechanism, patient vulnerability, abruptness of onset, and compensatory reserves 2.
Common Clinical Pitfalls
- Do not confuse atelectasis with lobar consolidation/pneumonia - this is a frequent diagnostic dilemma 1. The diagnosis of atelectatic pneumonia requires clinical signs/symptoms plus identification of pathogenic bacteria, not just radiographic atelectasis alone 4
- Platelike atelectasis can be a non-specific finding in pulmonary embolism and should not be dismissed 3
- Rounded atelectasis may be mistaken for tumor; look for the pathognomonic comet sign on high-resolution CT 3