Write a short note on pulmonary function tests.
Purpose
PFTs objectively measure lung volumes, airflow and gas exchange — to diagnose and classify disease (obstructive versus restrictive), assess severity and progression, evaluate treatment response, and provide pre-operative and disability assessment.
Lung Volumes
| Term | Definition |
|---|---|
| Tidal volume | Volume of a normal quiet breath (~500 mL) |
| RESIDUAL VOLUME (RV) | Volume remaining after maximal expiration — CANNOT be measured by spirometry |
| Vital capacity (VC) | Maximal volume expired after maximal inspiration |
| Functional residual capacity (FRC) | Volume at the end of a normal expiration |
| Total lung capacity (TLC) | VC + RV — requires body plethysmography, helium dilution or nitrogen washout |
Key point: RV, FRC and TLC cannot be measured by spirometry, which measures only what can be exhaled.
Spirometry
Measures FVC, FEV1, the FEV1/FVC ratio, PEF and FEF 25–75% (small airway function, which falls early).
| Pattern | FEV1 | FVC | FEV1/FVC | TLC |
|---|---|---|---|---|
| OBSTRUCTIVE | ↓↓ | Normal or ↓ | ↓ (< 0.70) | Normal or ↑ (air trapping) |
| RESTRICTIVE | ↓ | ↓↓ | Normal or ↑ | ↓↓ |
| Mixed | ↓↓ | ↓↓ | ↓ | ↓ |
OBSTRUCTIVE causes: COPD, asthma, bronchiectasis, cystic fibrosis, bronchiolitis obliterans, upper airway obstruction.
RESTRICTIVE causes:
- Intrapulmonary: ILD and pulmonary fibrosis, sarcoidosis, pneumoconiosis, post-tuberculous fibrosis (important in India), ARDS, pneumonectomy
- Extrapulmonary: pleural — effusion, thickening; chest wall — kyphoscoliosis, ankylosing spondylitis, obesity; neuromuscular — Guillain–Barré syndrome, myasthenia, muscular dystrophy, motor neurone disease, phrenic palsy; ascites, pregnancy
Reversibility
Repeat 15–20 minutes after a bronchodilator. An increase in FEV1 of ≥ 12% AND ≥ 200 mL supports ASTHMA. COPD shows little reversibility and the ratio remains below 0.70 after bronchodilation — the diagnostic criterion.
Flow-volume Loops
| Pattern | Appearance | Causes |
|---|---|---|
| Obstructive | "Scooped out" concave expiratory limb | COPD, asthma |
| Restrictive | Narrow tall loop with preserved shape | ILD, chest wall, neuromuscular |
| FIXED upper airway obstruction | Flattening of BOTH limbs | Tracheal stenosis, goitre, tracheal tumour |
| VARIABLE EXTRAthoracic | Flattening of the INSPIRATORY limb | Vocal cord palsy, laryngeal tumour |
| VARIABLE INTRAthoracic | Flattening of the EXPIRATORY limb | Tracheomalacia, lower tracheal tumour |
Diffusing Capacity (dlco)
Reflects the surface area and thickness of the alveolar-capillary membrane and capillary blood volume.
| DLCO REDUCED | DLCO INCREASED |
|---|---|
| EMPHYSEMA | Alveolar HAEMORRHAGE (Goodpasture's, vasculitis) — the classic cause |
| ILD / pulmonary fibrosis | Polycythaemia |
| Pulmonary vascular disease — PE, pulmonary hypertension | Left-to-right shunt |
| Anaemia (corrected for haemoglobin) | Asthma (mildly); exercise |
Clinically valuable: DLCO distinguishes EMPHYSEMA (reduced) from ASTHMA (normal or raised), and ILD (reduced) from chest wall or neuromuscular restriction (normal). An isolated fall in DLCO with normal spirometry suggests pulmonary vascular disease.
Other Tests
Arterial blood gas; six-minute walk test (functional capacity and exertional desaturation); bronchial provocation for suspected asthma with normal spirometry; MIP/MEP and supine FVC for neuromuscular weakness (a supine fall > 20% indicates diaphragmatic weakness); cardiopulmonary exercise testing.
Practical Points
Results are effort-dependent — proper coaching and three acceptable reproducible manoeuvres are required. Interpret against Indian reference equations — Caucasian-derived predicted values overestimate Indian lung volumes by 10–15% and produce spurious "restriction". Contraindications: recent myocardial infarction, unstable angina, recent pneumothorax, recent eye or abdominal surgery, and haemoptysis of unknown cause.
The FEV1/FVC ratio tells you the PATTERN; the FVC and TLC tell you the SEVERITY; the DLCO tells you WHERE the problem is. Restriction with a low DLCO means parenchymal disease (ILD); restriction with a normal DLCO means extrapulmonary restriction (obesity, pleural, chest wall or neuromuscular). That single measurement redirects the whole investigation.
All questions in Respiratory System
- Q1Discuss the aetiology, pathogenesis, clinical features, investigations and management of chronic obstructive pulmonary disease. ★
- Q2Discuss the aetiopathogenesis, clinical features, diagnosis and management of pulmonary tuberculosis. ★
- Q3Discuss the aetiopathogenesis, clinical features, investigations and management of bronchial asthma, including acute severe asthma. ★
- Q4Discuss the classification, aetiology, clinical features, severity assessment and management of pneumonia. ★
- Q5Discuss the definition, causes, pathophysiology, clinical features and management of acute respiratory distress syndrome. ★
- Q6Discuss the risk factors, clinical features, investigations and management of pulmonary thromboembolism. ★
- SN1Write a short note on pneumothorax and hydropneumothorax.
- SN2Write a short note on the clinical approach to acute dyspnoea.
- SN3Write a short note on pulmonary function tests.
- SN4Write a short note on respiratory failure.
- SN5Write a short note on pleural effusion.
- SN6Write a short note on eosinophilic lung disease — ABPA, hypersensitivity pneumonitis and tropical pulmonary eosinophilia.
- SN7Write a short note on bronchiectasis.
- SN8Write a short note on lung abscess.
- SN9Write a short note on haemoptysis.
- SN10Write a short note on interstitial lung disease.
- SN11Write a short note on the important clinical signs in respiratory disease.
- SN12Write a short note on bronchogenic carcinoma.