How PFTs are tested on Step 1

Questions give a set of numbers (FEV1, FVC, ratio, TLC, DLCO) or a flow-volume loop and ask for the disease, or describe a disease and ask which values change.

workspace_premium Read in order

1) FEV1/FVC — low means obstruction. 2) TLC — low means restriction. 3) DLCO — low means the alveolar–capillary membrane or pulmonary vessels are damaged.

Lung volumes

Four volumes combine into four capacities.

TLCTotal lung capacity = IRV + TV + ERV + RVVCVital capacity = IRV + TV + ERV (maximal exhaleafter maximal inhale)ICInspiratory capacity = IRV + TVFRCFunctional residual capacity = ERV + RV (end ofnormal exhale)RVResidual volume — cannot be exhaled; not measured byspirometry
Volumes and capacities. A capacity is the sum of two or more volumes.

Obstructive vs restrictive patterns

The ratio separates the two main patterns.

MeasureObstructiveIntrinsic restrictiveExtrinsic restrictive
FEV1↓↓↓↓
FVC↓ or normal↓↓
FEV1/FVC↓ (< 0.70)Normal or ↑Normal or ↑
TLC↑↓↓
RV↑ (air trapping)↓Variable
DLCO↓ in emphysema; normal or ↑ in asthma↓Normal
ExamplesAsthma, COPD, bronchiectasis, cystic fibrosisIPF, sarcoidosis, pneumoconioses, drug toxicityObesity, kyphoscoliosis, myasthenia, ALS, Guillain-Barré

In fibrosis, increased radial traction holds airways open, so the FEV1/FVC ratio may be supernormal.

DLCOLowEmphysema, ILD, PE,pulmonary HTN, anemiaNormalAsthma, chronic bronchitis,extrinsic restrictionHighAlveolar hemorrhage,polycythemia, asthma,obesity
DLCO. Correct DLCO for hemoglobin; anemia lowers it without lung disease.

Flow-volume loops and upper-airway obstruction

Plotting flow against volume shows where obstruction occurs and whether it varies with breathing.

Loop shapeScooped (concave) expiratory limbTall, narrow loop shifted rightFlattened inspiratory limb onlyFlattened expiratory limb onlyBoth limbs flattenedMeaningIntrathoracic small-airway obstruction(asthma, COPD)RestrictionVariable extrathoracic obstruction(vocal cord paralysis)Variable intrathoracic obstruction(tracheomalacia)Fixed upper airway obstruction (trachealstenosis)vs
Flow-volume loops. Extrathoracic airways collapse on inspiration; intrathoracic airways collapse on expiration.

Gas exchange and clinical application

The alveolar–arterial (A–a) oxygen gradient separates hypoventilation from lung disease. PAO₂ = FiO₂ (Patm − PH₂O) − PaCO₂/0.8.

Cause of hypoxemiaA–a gradientCorrects with O₂?
Hypoventilation (opioids, neuromuscular)NormalYes
High altitudeNormalYes
V/Q mismatch (PE, COPD, pneumonia)↑Yes
Diffusion limitation (ILD)↑Yes
Right-to-left shunt (ARDS, intracardiac shunt)↑No (poorly)

Shunt is the one cause that supplemental oxygen fails to correct, because shunted blood never meets ventilated alveoli.

functions Worked example — read the numbers

A 54-year-old with exertional dyspnea and fine bibasilar crackles has FEV1 1.9 L (62% predicted), FVC 2.2 L (58%), FEV1/FVC 0.86, TLC 64% predicted, and DLCO 41% predicted. What category of disease is present?

  1. FEV1/FVC is normal to high, so there is no obstruction.
  2. TLC is reduced, so the pattern is restrictive.
  3. DLCO is markedly reduced, pointing to a problem in the lung parenchyma rather than the chest wall or muscles.
  4. Fine "velcro" crackles fit fibrosis.

Answer. Intrinsic restrictive lung disease, most consistent with interstitial lung disease such as idiopathic pulmonary fibrosis.