Spirometry Interpretation Physiology
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Direct answer
Forced expiratory volume in one second should be at least 70 per cent of forced vital capacity — an FEV1/FVC below 0.7 defines obstruction (GOLD criterion), while a proportionate reduction of both with a normal or raised ratio and a reduced total lung capacity defines restriction. Reference volumes for an adult male: tidal volume 500 mL, inspiratory reserve about 3000, expiratory reserve about 1100, vital capacity 4600-4800 mL; peak expiratory flow runs 400-600 L/min. Reversibility means a 12 per cent and 200 mL improvement in FEV1 after bronchodilator; GOLD grades COPD severity by FEV1 per cent predicted — mild 80 and above, moderate 50-79, severe 30-49, very severe below 30.
What you must remember
- Ratio rule: FEV1/FVC under 0.7 equals obstruction; both reduced with normal ratio points to restriction, which must be confirmed by a reduced TLC (spirometry alone cannot measure it).
- Obstructive signature: FEV1 reduced disproportionately, FVC normal or low, ratio low, residual volume and functional residual capacity raised (hyperinflation — RV is the key volume spirometry infers but cannot blow out).
- Restrictive signature: FEV1 and FVC reduced together, ratio preserved or high, TLC reduced; causes fill into parenchymal (fibrosis), pleural-chest wall (kyphoscoliosis, obesity) and neuromuscular (myasthenia, Guillain-Barre) groups.
- Reversibility criterion: FEV1 improving 12 per cent and at least 200 mL after 200-400 micrograms of salbutamol — separates asthma (fully reversible, often over 400 mL) from COPD (little or none).
- DLCO division: diffusion capacity low in emphysema and fibrosis, normal in asthma and neuromuscular restriction — the tie-breaker between obstructive mimics.
- Flow-volume loops: flattening of both limbs — fixed upper airway obstruction (tracheal stenosis); inspiratory limb flat alone — variable extrathoracic (vocal cord palsy); expiratory limb flat alone — variable intrathoracic (tracheomalacia, tumour); tall narrow loop — restriction; scooped expiratory limb — small airway obstruction.
- GOLD grades of COPD by FEV1 per cent predicted: 80+ mild, 50-79 moderate, 30-49 severe, under 30 very severe — staging after the 0.7 ratio establishes the diagnosis.
- Quality criteria: at least three acceptable manoeuvres, the two best FVC within 150 mL; Indian laboratories apply population-specific reference equations — predicted values differ substantially from Caucasian norms.
- PEFR home monitoring: morning dipping over 20 per cent diurnal variation supports asthma; PEFR 100-250 L/min in an attack flags severity, under 100 life-threatens.
- Mid-expiratory flow (FEF25-75): the earliest and most sensitive small-airway index, dipping before FEV1 in early obstruction.
A worked case: two breathless smokers
Two 55-year-old smokers report years of progressive breathlessness. The first spirometry: FEV1 1.6 L (48 per cent predicted), FVC 3.2 L, ratio 0.50, and total lung capacity 120 per cent predicted — obstruction with hyperinflation: COPD. The second: FEV1 1.9 L (57 per cent), FVC 3.0 L (65 per cent), ratio 0.63 — looks obstructive, but with a normal chest radiograph and dyspnoea out of proportion, body plethysmography shows TLC at 62 per cent: restriction, later confirmed as idiopathic pulmonary fibrosis. The lesson is procedural: when both volumes fall, never commit to obstruction without checking TLC — the ratio rises with age even in pure restriction, and 15 per cent of mixed patterns are misread by ratio alone. For the COPD patient, add DLCO: 55 per cent predicted signals emphysema; post-bronchodilator FEV1 unchanged confirms non-reversibility; and the grade (severe, 30-49) decides escalation to long-acting bronchodilators plus pulmonary rehabilitation.
Where students slip
Three recurring slips. Calling a low FVC with normal ratio "restriction" on spirometry alone — it is only a restrictive pattern until TLC proves it, since air trapping in obstruction drags FVC down as well. Second, forgetting that the 0.7 ratio is age-sensitive: an 80-year-old may be normal near 0.65, a 25-year-old abnormal at 0.75 — the lower limit of normal (LLN) exists for this reason even though GOLD keeps 0.7. Third, reading the flow-volume loop without stating which limb — examiners award the mark for "inspiratory limb flat — variable extrathoracic obstruction," not for "abnormal loop".
Frequently asked questions
What ratio defines airflow obstruction and what confirms restriction?
FEV1/FVC below 0.7 defines obstruction; restriction requires proportionate reduction of FEV1 and FVC with a normal or high ratio plus a reduced total lung capacity measured separately.
What bronchodilator response confirms reversibility?
A rise in FEV1 of at least 12 per cent and 200 mL from baseline within 15-30 minutes of inhaled salbutamol, the hallmark of asthma.
Why is total lung capacity needed to classify a restrictive defect?
Because spirometry cannot measure air that remains in the chest — a low FVC with normal ratio may reflect hyperinflation from obstruction rather than small lungs; TLC (plethysmography or helium dilution) settles it.
How does the flow-volume loop localise upper airway obstruction?
Fixed lesions flatten both limbs; variable extrathoracic lesions (vocal cord palsy) flatten inspiration; variable intrathoracic lesions (tracheomalacia, tumour) flatten expiration.
How does GOLD stage COPD severity?
By post-bronchodilator FEV1 per cent predicted once the 0.7 ratio establishes diagnosis: 80 and above mild, 50-79 moderate, 30-49 severe, below 30 very severe.