Obstructive vs Restrictive Lung Disease
Uses the FEV1/FVC ratio to split obstructive disease (air trapping → low ratio, high lung volumes) from restrictive disease (small, stiff lungs → normal/high ratio, low TLC), then uses DLCO and the A–a gradient to sort the subtypes — the single highest-yield pulmonary PFT concept on Step 1.
The one number that splits them: FEV1/FVC
Pulmonary function tests sort chronic lung disease into two physiologic patterns, and the single most important value is the FEV1/FVC ratio.
- Obstructive disease = trouble getting air OUT. Airflow limitation drops FEV1 more than FVC, so the FEV1/FVC ratio falls (< 0.70). Air can't fully escape, so it gets trapped and lung volumes rise (↑ RV, ↑ TLC = hyperinflation).
- Restrictive disease = trouble getting air IN. Small, stiff lungs (or a chest wall/neuromuscular problem) lower FVC and FEV1 together, so the FEV1/FVC ratio is normal or high (≥ 0.70, often > 0.80) and the defining feature is a reduced TLC.
Everything else on the PFT flows from this: obstructed lungs are too full and empty too slowly; restricted lungs are too small and empty just fine.
- FEV1/FVC is THE discriminator: obstructive < 0.70; restrictive normal or increased.
- In obstruction both FEV1 and FVC may fall, but FEV1 falls more, so the ratio drops.
- Obstructive lungs are hyperinflated: ↑ TLC, ↑ RV, ↑ RV/TLC (air trapping), ↑ FRC.
- Restrictive lungs are small: the defining abnormality is a ↓ TLC — spirometry alone can only suggest restriction; you confirm it by measuring TLC (body plethysmography).
- DLCO decodes the subtype: ↓ in emphysema and in interstitial fibrosis; normal or ↑ in asthma; normal in chronic bronchitis and in chest-wall/neuromuscular restriction.
- A–a gradient sorts restriction: intrinsic (parenchymal — IPF, ARDS) widens the A–a gradient and lowers DLCO; extrinsic (chest-wall/neuromuscular) leaves both normal — pure hypoventilation.
- Asthma = reversible obstruction: FEV1 improves ≥ 12% AND ≥ 200 mL after bronchodilator. COPD is largely irreversible.
- If spirometry is normal but asthma is suspected, do a methacholine (bronchoprovocation) challenge.
Obstructive vs Restrictive at a Glance
| Feature | Obstructive | Restrictive |
|---|---|---|
| FEV1 | ↓↓ | ↓ |
| FVC | Normal or ↓ | ↓↓ |
| FEV1/FVC | ↓ (< 0.70) | Normal or ↑ (≥ 0.70, often > 0.80) |
| TLC | ↑ (hyperinflation) | ↓ (defining feature) |
| RV | ↑ (air trapping) | ↓ |
| Flow–volume loop | Scooped/concave; shifted to higher volumes | Tall & narrow; shifted to lower volumes |
| DLCO | ↓ emphysema · nl/↑ asthma · nl chronic bronchitis | ↓ intrinsic · normal extrinsic |
| Classic causes | COPD, asthma, bronchiectasis, cystic fibrosis | IPF, pneumoconioses, sarcoidosis, kyphoscoliosis, obesity, neuromuscular disease |
Flow–volume loops & confirming restriction
The flow–volume loop makes the patterns visual.
- Obstructive: a scooped-out, concave expiratory limb (dynamic airway collapse), with the whole loop shifted toward higher volumes because of air trapping.
- Restrictive: a tall, narrow loop shifted toward lower volumes, with a normal or even steep expiratory slope — small lungs that empty quickly.
Classic trap: a low FVC with a normal FEV1/FVC ratio suggests restriction, but a low FVC can also occur in severe obstruction with air trapping. That's why a genuinely reduced TLC (measured by plethysmography or gas dilution) is required to confirm restriction — you cannot diagnose it from spirometry alone.
Two flavors of restriction: intrinsic vs extrinsic
| Feature | Intrinsic (parenchymal / interstitial) | Extrinsic (chest wall · pleura · neuromuscular) |
|---|---|---|
| Lung tissue | Stiff, scarred, inflamed | Normal lung — just can't be expanded |
| DLCO | ↓ | Normal |
| A–a gradient | ↑ (diffusion defect / V–Q mismatch) | Normal (pure hypoventilation) |
| Examples | IPF, asbestosis & other pneumoconioses, sarcoidosis, ARDS, drug-induced (bleomycin, amiodarone, methotrexate) | Obesity, kyphoscoliosis, ankylosing spondylitis, myasthenia gravis, Guillain–Barré, ALS |
Obstructive vignettes
- Long-time smoker, barrel chest, pursed-lip breathing, decreased breath sounds, CXR with hyperinflation and flattened diaphragms → emphysema ("pink puffer"). PFT: ↓ FEV1/FVC, ↑ TLC, ↓ DLCO.
- Productive cough ≥ 3 months/yr for ≥ 2 years, cyanotic and edematous → chronic bronchitis ("blue bloater"); DLCO normal.
- Episodic wheeze, nocturnal cough, allergen/exercise triggers, reversible obstruction → asthma; DLCO normal/↑.
Restrictive vignette
- Progressive exertional dyspnea, dry cough, fine end-inspiratory "Velcro" crackles, clubbing, honeycombing on high-resolution CT → idiopathic pulmonary fibrosis. PFT: ↑ FEV1/FVC, ↓ TLC, ↓ DLCO.
Next-best-step logic
- Spirometry first → read the FEV1/FVC ratio.
- Obstruction? Give a bronchodilator to test reversibility (asthma vs COPD).
- Suspected restriction? Order full lung volumes (TLC) to confirm, then use DLCO and the A–a gradient to separate intrinsic (both abnormal) from extrinsic (both normal) causes.
PAINT — causes of restrictive lung disease:
- P — Pleural (effusion, thickening, fibrothorax)
- A — Alveolar (pulmonary edema, ARDS, alveolar hemorrhage)
- I — Interstitial (IPF, pneumoconioses, sarcoidosis, drug-induced fibrosis)
- N — Neuromuscular (myasthenia gravis, Guillain–Barré, ALS, phrenic nerve palsy)
- T — Thoracic wall / cage (kyphoscoliosis, obesity, ankylosing spondylitis)
Sort them by DLCO: only the parenchymal (intrinsic) causes — Alveolar and Interstitial — damage lung tissue, so DLCO is usually low (classic exception: diffuse alveolar hemorrhage raises DLCO, because intra-alveolar blood binds the inhaled CO). The extrinsic causes — Pleural, Neuromuscular, and Thoracic-cage — leave the lung parenchyma normal, so DLCO stays normal.
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