Spirometry full details for md exam answer 15 marks
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SPIROMETRY — Complete MD Examination Answer (15 Marks)
Definition
Spirometry is the measurement of the volume of air inhaled or exhaled as a function of time. It is performed using a pneumotachometer that measures flow rates and integrates them to obtain volume, making it the most commonly used, standardised pulmonary function test.
— Murray & Nadel's Textbook of Respiratory Medicine
Lung Volumes & Capacities (Static)
Before interpreting spirometry, understanding the subdivisions of lung volume is essential.
Four Primary Lung Volumes (non-overlapping):
| Volume | Definition | Normal Value |
|---|---|---|
| Tidal Volume (TV) | Volume inhaled/exhaled during each quiet respiratory cycle | ~500 mL |
| Inspiratory Reserve Volume (IRV) | Maximal extra volume inspired from end of tidal inspiration | ~3000 mL |
| Expiratory Reserve Volume (ERV) | Maximal volume exhaled from end of tidal expiration | ~1200 mL |
| Residual Volume (RV) | Volume remaining after maximal exhalation (cannot be measured by spirometry alone) | ~1200 mL |
Four Lung Capacities (each = sum of ≥2 volumes):
| Capacity | Components | Normal Value |
|---|---|---|
| Total Lung Capacity (TLC) | TV + IRV + ERV + RV | ~6000 mL |
| Vital Capacity (VC) | TV + IRV + ERV | ~4800 mL |
| Inspiratory Capacity (IC) | TV + IRV | ~3500 mL |
| Functional Residual Capacity (FRC) | ERV + RV | ~2400 mL |
Note: RV, FRC, and TLC cannot be measured by spirometry alone — they require nitrogen washout, helium dilution, or body plethysmography.
Spirometric Measurements (Dynamic Volumes)
Key Parameters
| Term | Definition |
|---|---|
| FVC (Forced Vital Capacity) | Total volume exhaled forcefully from full inspiration |
| FEV₁ | Volume exhaled in the first second of a forced expiration |
| FEV₁/FVC ratio | Proportion of FVC expelled in 1 second (≥0.70 normal) |
| FEF₂₅₋₇₅% | Average mid-expiratory flow between 25–75% of FVC; reflects small airway function |
| PEFR | Peak expiratory flow rate |
| FEV₆ | Volume exhaled in 6 seconds; approximates FVC |
| MVV | Maximal voluntary ventilation — maximal air moved in 1 minute (~FEV₁ × 40) |
The Volume–Time Curve (FVC Curve)
The subject:
- Inhales maximally to TLC
- Exhales as forcefully and rapidly as possible
Volume (y-axis) is plotted against Time (x-axis).
Graph: Normal vs. Obstructive Pattern
| Normal | Obstructive | |
|---|---|---|
| FEV₁ | 4 L | 1.8 L |
| FVC | 5 L | 3.2 L |
| FEV₁/FVC | 0.80 | 0.56 |
In obstruction, the curve rises slowly and plateaus late; FEV₁ is disproportionately reduced.
Spirogram Patterns — Normal, Obstructive, Restrictive
(a) Volume–Time Spirograms
(i) Normal: Rapid rise, plateau at ~3.8–4 L by 2 seconds. FEV₁/FVC ≈ 80%
(ii) Obstructive (e.g., asthma, COPD): Slow rise, plateau reached late. FEV₁ markedly reduced. FEV₁/FVC < 70%. Post-bronchodilator improvement shown by shift from curve p → q.
(iii) Restrictive (e.g., fibrosis, pleural effusion): Rapid rise but low plateau (~2 L). FEV₁ reduced, but FEV₁/FVC normal or elevated (both volumes proportionally reduced).
(b) Changes in Lung Volumes
| Normal | Obstructive | Restrictive | |
|---|---|---|---|
| VC | Normal | ↓ (air trapping) | ↓↓ |
| TLC | Normal | ↑ (hyperinflation) | ↓↓ |
| RV | Normal | ↑↑ | ↓ |
| FEV₁ | Normal | ↓↓ | ↓ |
| FEV₁/FVC | ≥0.70 | < 0.70 | Normal/↑ |
The Flow–Volume Loop
Most informative for upper airway obstruction and central airway lesions. Plots flow (y-axis) vs. volume (x-axis) during a maximal inspiratory and expiratory manoeuvre.
Shape characteristics:
- Expiratory limb (upper): Rises rapidly to peak flow (effort-dependent initial portion), then descends linearly and effort-independently to RV
- Inspiratory limb (lower): Semicircular, entirely effort-dependent
| Pattern | Shape | Cause |
|---|---|---|
| A — Normal | Broad rounded expiratory, symmetric inspiratory | — |
| B — Airflow obstruction | Scooped/concave expiratory limb (↓ flow at low volumes) | COPD, emphysema, asthma |
| C — Fixed central obstruction | Both expiratory and inspiratory limbs flattened ("box-shaped") | Tracheal stenosis, goitre |
| D — Variable extrathoracic (upper airway) | Inspiratory limb flattened only | Vocal cord paralysis, tracheomalacia above thoracic inlet |
| E — Variable intrathoracic | Expiratory limb flattened only | Tracheomalacia below thoracic inlet |
Interpretation Algorithm
Step 1: Check FEV₁/FVC ratio
- < 0.70 (or < LLN) → Obstructive pattern → proceed to Step 2
- ≥ 0.70 → Go to Step 3
Step 2: (Obstructive) — Check FVC
- FVC normal → Simple obstruction
- FVC also low → Obstruction + air trapping or mixed pattern
Step 3: (Non-obstructive) — Check FVC
- FVC reduced (< 80% predicted), normal ratio → Restrictive pattern (confirm with TLC)
- Both normal → Normal spirometry
Step 4: Severity (GOLD classification for obstruction, post-bronchodilator FEV₁%)
| GOLD Grade | FEV₁ % predicted | Severity |
|---|---|---|
| 1 | ≥ 80% | Mild |
| 2 | 50–79% | Moderate |
| 3 | 30–49% | Severe |
| 4 | < 30% | Very severe |
Bronchodilator Reversibility Testing
- Administer short-acting bronchodilator (salbutamol 400 µg)
- Repeat spirometry after 15–20 minutes
- Positive reversibility = FEV₁ increase ≥ 12% AND ≥ 200 mL from baseline
- Suggests asthma (vs. fixed obstruction in COPD)
- Requires: LABAs withheld ≥12 h, SABAs withheld ≥6 h before test
Indications for Spirometry
- Diagnostic — suspected asthma, COPD, pulmonary fibrosis, occupational lung disease
- Monitoring — disease progression (e.g., serial FVC in IPF predicts survival)
- Pre-operative assessment — especially lung resection surgery
- Disability/legal evaluation — quantifying impairment
- Occupational screening — hazardous environments
- Assessing treatment response — bronchodilators, steroids
Acceptability & Reproducibility Criteria (ATS/ERS)
- Minimum 3 acceptable manoeuvres
- Best 2 FVC values within 150 mL of each other
- Best 2 FEV₁ values within 150 mL
- No cough, glottis closure, or early termination
- Initial 25–30% of curve is effort-dependent; remainder is effort-independent
Conditions Causing Each Pattern
| Obstructive | Restrictive |
|---|---|
| Asthma | Pulmonary fibrosis (IPF) |
| COPD / Emphysema | Sarcoidosis |
| Bronchiectasis | Pleural effusion |
| Cystic fibrosis | Kyphoscoliosis |
| Obliterative bronchiolitis | Neuromuscular disease |
| Pneumonectomy |
FEF₂₅₋₇₅% (Forced Mid-Expiratory Flow)
- Reflects small airway (peripheral airway) function
- More sensitive for early obstructive disease than FEV₁
- Limitation: Wide 95% CI in normal populations → poor specificity; high false-positive and false-negative rates
- Less reproducible than FEV₁
Maximal Voluntary Ventilation (MVV)
- Maximal volume breathed in 1 minute by voluntary effort
- Normal ≈ FEV₁ × 40 L/min
- Decreased in: airway obstruction, neuromuscular disease, deconditioning, morbid obesity
- Correlates with subjective dyspnoea and exercise tolerance
- Useful in pre-operative evaluation (lung resection)
Diffusing Capacity (DLCO) — Adjunct to Spirometry
Although not a spirometric parameter, it complements spirometry:
- Normal/elevated in asthma (↑ perfusion upper zones, large lung volumes)
- Reduced in emphysema (destroyed alveoli)
- Reduced in pulmonary fibrosis
- Unexpectedly elevated DLCO → suspect undiagnosed asthma
Summary Table
| Parameter | Obstructive | Restrictive | Mixed |
|---|---|---|---|
| FEV₁ | ↓↓ | ↓ | ↓↓ |
| FVC | Normal or ↓ | ↓↓ | ↓↓ |
| FEV₁/FVC | ↓ (<0.70) | Normal/↑ | ↓ |
| TLC | ↑ (hyperinflation) | ↓ | Variable |
| RV | ↑↑ | ↓ | ↑ |
| DLCO | ↓ (emphysema) / N (asthma) | ↓ | ↓ |
Sources: Murray & Nadel's Textbook of Respiratory Medicine; Harrison's Principles of Internal Medicine, 21e; Bailey & Love's Short Practice of Surgery, 28e; GOLD 2025 Report
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