On chest X-ray, the most common site of lobar collapse in adults is which of the following?

Explanation

## Atelectasis (Collapse): Most Common Site in Adults **Key Point:** Left lower lobe collapse is the most common site of lobar atelectasis in adults, particularly in post-operative and critically ill patients, due to anatomical and physiological factors favouring left-sided dependent collapse. ### Why Left Lower Lobe is Most Common | Factor | Left Lower Lobe | Right Lower Lobe | |--------|-----------------|------------------| | Bronchial angle | More acute, longer path | More vertical, shorter | | Cardiac compression | Heart lies on left side, compresses LLL | Less compression | | Gravity dependence | Posterior/basal — most dependent | Also dependent but less compressed | | Post-operative risk | Highest overall | High but less than LLL | | Frequency of collapse | **Most common (~40–50%)** | Second most common (~25–35%) | **High-Yield:** The left lower lobe bronchus is longer and more acutely angled than the right, and the overlying heart compresses the left lower lobe, impairing mucociliary clearance and predisposing to mucus plugging and collapse. This is the predominant reason LLL collapse is most frequent in adults (Felson's Principles of Chest Roentgenology; Grainger & Allison's Diagnostic Radiology). ### Radiological Features of Left Lower Lobe Collapse 1. **Volume loss signs** - Mediastinal shift toward left - Elevation of left hemidiaphragm - Approximation of left-sided ribs 2. **Silhouette sign** - Loss of left hemidiaphragm outline (posterior segment collapses behind heart) - Preserved left heart border (distinguishes from lingular/LUL collapse) 3. **Opacity pattern** - Triangular retrocardiac opacity (sail sign) - Visible on lateral film as posterior wedge - No air bronchograms (collapsed airways) ### Relative Frequency of Lobar Collapse in Adults | Lobe | Approximate Frequency | |------|----------------------| | **Left lower lobe** | **~40–50% (most common)** | | Right lower lobe | ~25–35% | | Right upper lobe | ~10–15% | | Left upper lobe | ~5–10% | **Mnemonic:** **LLL LEADS** — Left Lower Lobe is the Leading (most common) site of lobar collapse in adults. ### Clinical Causes of Left Lower Lobe Collapse 1. **Post-operative** (most common overall) - Mucus plugging after general anaesthesia - Reduced diaphragmatic excursion after abdominal/cardiac surgery 2. **Cardiac compression** - Cardiomegaly further compresses LLL bronchus 3. **Obstruction** - Endobronchial tumour (left lower lobe bronchus) - Aspiration (less common on left due to bronchial angle) 4. **Pleural effusion / Compression** - Left-sided effusion compressing LLL **Clinical Pearl:** In a post-operative patient (especially after cardiac or upper abdominal surgery) with a new retrocardiac triangular opacity and loss of the left hemidiaphragm outline, left lower lobe collapse from mucus plugging is the first diagnosis to consider. Aggressive pulmonary hygiene (incentive spirometry, chest physiotherapy, early ambulation) is the key preventive measure. ### Distinction: Consolidation vs Collapse | Feature | Consolidation | Collapse | |---------|---------------|----------| | Volume | Preserved | Decreased | | Air bronchograms | Present | Absent | | Mediastinal shift | Absent | Present (toward affected side) | | Silhouette sign | May be present | Usually present | | Opacity pattern | Homogeneous with air bronchograms | Wedge or triangular | | Most common lobe | Any (depends on etiology) | **Left lower lobe** | [cite: Felson's Principles of Chest Roentgenology, 4th ed., Ch 4; Grainger & Allison's Diagnostic Radiology, 6th ed.]