Pulmonary Function Testing
On this page
Direct answer
Separating obstructive from restrictive physiology is the core job of pulmonary function testing — spirometry, lung volume measurement and diffusing capacity together. Obstruction is a post-bronchodilator FEV1/FVC ratio below 0.7, with reversibility defined as a 12 per cent and 200 mL FEV1 gain after a bronchodilator; restriction is a reduced total lung capacity, suspected when FVC falls with a normal ratio but confirmed only by volume measurement. The diffusing capacity and the flow-volume loop then localise the disease within each pattern.
What you must remember
- Obstruction: FEV1/FVC below 0.7 (or below the lower limit of normal) after bronchodilator, with FEV1 per cent predicted grading severity; causes span asthma, chronic obstructive pulmonary disease, bronchiectasis and bronchiolitis.
- Reversibility: an increase in FEV1 of 12 per cent and 200 mL or more after 200 to 400 micrograms of salbutamol supports asthma; its absence does not exclude asthma between attacks, where bronchoprovocation (a 20 per cent FEV1 fall with methacholine) or peak-flow variability becomes the tool.
- Restriction: reduced total lung capacity on body plethysmography or helium dilution, with proportionately reduced FVC and vital capacity and a normal or raised ratio — causes include interstitial lung disease, neuromuscular weakness, chest wall disease and obesity, where only mild restriction with a reduced expiratory reserve volume is expected.
- Diffusing capacity for carbon monoxide: reduced in emphysema, fibrosis, pulmonary embolism, pulmonary hypertension and anaemia; increased in asthma, alveolar haemorrhage, polycythaemia, left-to-right shunts, exercise and altitude; correct for haemoglobin.
- Flow-volume loops: a flat plateau on both inspiration and expiration indicates fixed upper airway obstruction (tracheal stenosis, goitre); flattening confined to inspiration suggests variable extrathoracic obstruction (vocal cord palsy); flattening confined to expiration suggests variable intrathoracic obstruction (tracheomalacia, central tumour); a scooped expiratory contour belongs to diffuse obstruction.
- Serial peak expiratory flow supports occupational asthma diagnosis and control (morning dipping), and fractional exhaled nitric oxide quantifies type 2 inflammation.
- The six-minute walk test adds capacity and desaturation data for interstitial and pulmonary vascular disease.
Reading one report, top to bottom
Take a report in fixed order. First the ratio: post-bronchodilator FEV1/FVC below 0.7 means obstruction, graded by FEV1 per cent predicted. Next the bronchodilator response: a 12 per cent and 200 mL rise supports asthma, but its absence between attacks excludes nothing, moving the toolkit to bronchoprovocation (20 per cent FEV1 fall with methacholine) or peak-flow variability — the occupational asthma question is answered by serial peak flow at and away from work. Then the volumes: obstruction with air trapping also masquerades as low FVC, which is why the volume step exists. Then the diffusing capacity, corrected for haemoglobin: reduced in emphysema, fibrosis, embolism, pulmonary hypertension and anaemia, increased in asthma, alveolar haemorrhage, polycythaemia and shunts — the combination logic landing fibrosis against emphysema. Finally the flow-volume loop localises central lesions: flat on both limbs for fixed upper airway obstruction; flat on inspiration alone for variable extrathoracic (vocal cord palsy); flat on expiration alone for variable intrathoracic (tracheomalacia, central tumour); scooped expiration for diffuse obstruction.
Where students slip
A low FVC with a normal ratio is labelled restriction without the total lung capacity — but obstruction with air trapping, muscle weakness and mixed patterns all present that way, so the pattern is only suggestive until volumes confirm. The 12 per cent and 200 mL pair is quoted as one half or the other, and the exam supplies an FEV1 gain that satisfies only one to catch it. Diffusing-capacity interpretation forgets the haemoglobin correction and the paradoxically high values in alveolar haemorrhage and asthma. And the loop question — extrathoracic versus intrathoracic variable obstruction, which limb is flattened — is the single most-lost mark in this topic: inspiratory flattening for cords, expiratory for the central tumour, both for the fixed stenosis.
Frequently asked questions
What defines obstructive physiology on spirometry?
A post-bronchodilator FEV1/FVC ratio below 0.7, with severity graded by FEV1 per cent predicted.
What counts as significant bronchodilator reversibility?
An FEV1 rise of at least 12 per cent and 200 mL after an inhaled bronchodilator.
How is restriction confirmed?
By reduced total lung capacity on body plethysmography or gas dilution — a low FVC with normal ratio only suggests it.
When is the diffusing capacity increased?
In asthma, alveolar haemorrhage, polycythaemia, left-to-right shunts, exercise and altitude — remembering haemoglobin correction.
What does a plateau of both loop limbs indicate?
Fixed upper airway obstruction, such as tracheal stenosis or a large goitre.
Which loop pattern fits vocal cord palsy?
Flattening of the inspiratory limb alone — variable extrathoracic obstruction.