Expansion
Obstructive lowers the ratio; restrictive lowers both values proportionately
Expansion
| Obstructive | Restrictive | |
|---|---|---|
| FEV1 | Markedly reduced | Reduced |
| FVC | Normal or reduced | Reduced |
| FEV1/FVC | Below 0.7 | Normal or increased |
| Total lung capacity | Normal or raised | Reduced |
| Residual volume | Raised | Reduced |
Obstructive: asthma, chronic obstructive pulmonary disease, bronchiectasis, bronchiolitis. Air enters but leaves slowly, so FEV1 falls disproportionately and gas is trapped.
Restrictive: pulmonary fibrosis, chest wall deformity, neuromuscular weakness, obesity, pleural disease. The lungs are small but empty normally, so the ratio is preserved.
Reversibility after a bronchodilator, an improvement in FEV1 of 12 per cent and 200 ml or more, favours asthma over chronic obstructive pulmonary disease.
Note that FVC can fall in severe obstruction from gas trapping, so a low FVC alone does not prove restriction; total lung capacity is needed to confirm it.