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ABCDE chest X-ray interpretation airway breathing circulation diaphragm everything else
Here is a comprehensive guide to CXR interpretation using the ABCDE systematic approach:
CXR Interpretation: The ABCDE Approach
Before You Start — Check Image Quality (RIPE)
Before applying ABCDE, always assess technical quality:
| Letter | Meaning | What to Check |
|---|---|---|
| R | Rotation | Distance from spinous processes to medial clavicular heads should be equal bilaterally (≤2–3 mm difference is acceptable) |
| I | Inspiration | Count posterior ribs at spine — minimum 9 ribs visible for adequate inspiration. Inadequate inspiration falsely increases opacity. |
| P | Penetration/Exposure | Vertebral bodies should be just visible behind the heart. Overexposure "burns out" lung parenchyma; underexposure causes everything to look white. |
| E | Extras / View | Note projection (PA vs AP), date, patient name, laterality markers |
AP vs PA: Cardiomegaly criteria differ — AP magnifies the heart, so the 50% rule applies only to PA films.
A — Airway
- Trachea: Should be midline. A deviated trachea suggests tension pneumothorax (away from side), collapse/fibrosis (toward side), or mediastinal mass.
- Follow trachea to the carina (usually at the level of T4–T5/angle of Louis).
- Normal carina angle: <70° (widening >70° suggests left atrial enlargement).
- Trace to right and left main bronchi.
- Check for foreign bodies, intubation tube position (ETT tip should be 2–4 cm above carina).
B — Breathing (Lung Fields)
Compare each zone symmetrically (upper, mid, lower zones):
| Feature | Normal | Abnormal Finding |
|---|---|---|
| Lung markings | Fine white lines (pulmonary vessels) visible to periphery | Absent markings → pneumothorax |
| Opacification | Black (air-filled) | White opacity → consolidation, collapse, effusion, tumour |
| Distribution | Symmetric | Unilateral or asymmetric opacity |
Key patterns to recognise:
- Consolidation: Air bronchogram present, lobar/segmental, non-shifting
- Collapse/Atelectasis: Shift of fissures, tracheal deviation toward affected side
- Pleural effusion: Blunting of costophrenic angles, meniscus sign, homogeneous density
- Pneumothorax: Absent lung markings, visible pleural line, hyperlucency
- Pulmonary oedema: Bilateral perihilar ("bat-wing") hazy opacities, upper lobe diversion, Kerley B lines
- Pneumonia: Consolidation with air bronchograms, lobar or patchy
- Hilar abnormalities: In 70% of normal patients, the left hilum is higher than the right. Bilateral hilar enlargement → sarcoidosis, lymphoma, primary TB
C — Circulation (Cardiac & Vessels)
Heart size:
- Cardiothoracic ratio (CTR) >50% on PA = cardiomegaly
- On lateral: LV enlargement casts a shadow >2 cm posterior to the IVC shadow
- Enlarged pulmonary artery segment → extra hump on the left heart border
Heart borders:
- Right border: SVC (upper), right atrium (lower)
- Left border: Aortic knuckle → pulmonary trunk → left atrial appendage → left ventricle
- Silhouette sign: loss of a border indicates adjacent consolidation/collapse
Mediastinum:
- Width >8 cm at the aortic arch is considered wide (aortic dissection, haematoma, lymphoma, thyroid/thymic mass)
- Alternatively: >25% of thoracic diameter at the carina level
D — Diaphragm
- Right hemidiaphragm is normally 2–20 mm higher than the left (due to the liver beneath).
- The stomach bubble sits under the left hemidiaphragm.
- Free air under the diaphragm = surgical emergency (bowel perforation) — the diaphragm becomes visible as a separate white line lifted above the liver.
- Blunted costophrenic angles: ≥200–300 mL of fluid needed to blunt the lateral CP angle on PA; ≥75 mL to blunt the posterior angle on lateral.
- Causes of elevated hemidiaphragm: atelectasis, paralysis, pleural effusion, post-lobectomy, phrenic nerve palsy, hepatomegaly.
E — Everything Else
Bones:
- Ribs: fractures (look for cortical breaks), lytic lesions, metastases
- Clavicles, scapulae, shoulder joints
- Spine: vertebral body heights, sclerotic/lytic lesions, scoliosis
Soft tissues:
- Subcutaneous emphysema
- Mastectomy (absent breast shadow = asymmetric soft tissue)
- Neck soft tissue masses
Devices / Lines:
| Device | Correct Position |
|---|---|
| ETT | 2–4 cm above carina |
| Central venous catheter | Tip at SVC–RA junction |
| Nasogastric tube | Below left hemidiaphragm, crosses midline |
| Chest drain | At apex (pneumothorax) or base (effusion) |
| Pacemaker leads | RV apex (ventricular lead), RA (atrial lead) |
Quick Summary Table
| Letter | Structure | Key Questions |
|---|---|---|
| A | Airway | Trachea midline? Carina angle? Tube positions? |
| B | Breathing | Both lung fields clear? Consolidation, effusion, pneumothorax? |
| C | Circulation | CTR <50%? Mediastinum width normal? Heart borders intact? |
| D | Diaphragm | Right > left? CP angles sharp? Free air? |
| E | Everything else | Bones intact? Devices in correct position? Soft tissue abnormalities? |
Pro Tips
- Always compare with old films when available — even subtle change in size or opacity is significant.
- After a focused finding, do a final sweep of the entire film — don't anchor on the obvious finding and miss a second pathology.
- PA film is standard for outpatients; AP (portable) is used for inpatients — remember AP magnifies the heart and moves the scapulae over the lung fields.
- Some pathologies (pulmonary oedema, sarcoidosis) are bilateral — don't be reassured by "symmetric" changes.
Sources: Pfenninger & Fowler's Procedures for Primary Care, 3rd Ed. | Nursing Times systematic ABCDE CXR approach (Rodrigues et al., 2014)
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