Spirometry
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Spirometry
Spirometry is the fundamental pulmonary function test (PFT) that measures the volume and flow of air during breathing maneuvers. It is performed using a pneumotachometer (measures flow, integrates to derive volume) or a volume-based spirometer.
Lung Volumes and Capacities
| Term | Definition |
|---|---|
| Tidal Volume (VT) | Volume inhaled/exhaled per normal breath |
| Inspiratory Reserve Volume (IRV) | Max extra volume inhaled from end-inspiration |
| Expiratory Reserve Volume (ERV) | Max extra volume exhaled from end-expiration |
| Residual Volume (RV) | Air remaining after maximal exhalation (not measurable by spirometry alone) |
| Vital Capacity (VC) | Max volume exhaled after maximal inspiration |
| Total Lung Capacity (TLC) | All gas in lungs at maximal inspiration (requires body plethysmography or gas dilution) |
| Functional Residual Capacity (FRC) | Volume at resting end-expiration |
| Inspiratory Capacity (IC) | Max volume inspired from resting expiratory level |
- Murray & Nadel's Textbook of Respiratory Medicine, Figure 31.2
Key Spirometric Parameters
The FVC Maneuver
The subject inhales maximally to TLC, then exhales as rapidly and forcefully as possible. This generates the volume-time curve (FVC curve) and the flow-volume loop.
| Parameter | Description | Normal |
|---|---|---|
| FVC (Forced Vital Capacity) | Total volume exhaled forcefully | Predicted by age, sex, height, ethnicity |
| FEV1 | Volume exhaled in the first 1 second | >80% predicted |
| FEV1/FVC ratio | Key obstructive index | ≥0.70 (GOLD) or above lower limit of normal (LLN) |
| FEV6 | Volume exhaled in 6 sec; approximates FVC | Valid substitute, easier for severe obstruction |
| FEF25-75% | Mean flow between 25-75% of FVC; reflects small airways | 60-140% predicted (high variability) |
| PEF | Peak expiratory flow | Effort-dependent; used in asthma monitoring |
The first 25-30% of the maximal expiratory maneuver is effort-dependent; the remainder is effort-independent and reflects intrinsic airway properties.
- Murray & Nadel's Textbook of Respiratory Medicine
Technique and Acceptability Criteria
- Patient inhales to TLC, seals lips around mouthpiece
- Forceful, maximal, continuous exhalation to end of breath
- A well-trained technician must communicate clearly - patient cooperation is critical
- Minimum 3 acceptable maneuvers required; two best FVC and FEV1 values should be within 150 mL
- Nose clip must be fitted to prevent air leak (air leak falsely lowers measurements)
- Incomplete exhalation gives falsely low FVC; poor initial effort gives falsely low FEV1
Common technical errors include:
-
Air leak from poorly fitting nose clip or mouthpiece - causes wandering baseline
-
Incomplete expiration - falsely low FVC, spuriously elevated FEF25-75%
-
Poor initial expiratory effort - falsely low FEV1 and FEF25-75%
-
Pfenninger and Fowler's Procedures for Primary Care
Pre-test Preparation
- Stop LABAs at least 12 hours before testing
- Stop short-acting bronchodilators at least 6 hours before testing
- Postbronchodilator spirometry values should be used for diagnosis (especially if obstruction is suspected)
Interpretation: The 3-Step Approach
Step 1: Inspect the flow-volume curve visually
Look for quality of effort and patterns (concavity in obstruction, reduced volumes in restriction, plateau in upper airway obstruction).
Step 2: Assess the FEV1/FVC ratio
- Low FEV1/FVC (<0.70 or <LLN) → suggests obstruction
- Normal FEV1/FVC with low FVC → suggests restriction (confirm with TLC)
Step 3: Use the full algorithm
- Murray & Nadel's Textbook of Respiratory Medicine, Figure 32.1
Patterns of Ventilatory Defect
Obstructive Pattern
- FEV1/FVC < 0.70 (GOLD) or < LLN (ATS/ERS)
- FEV1 reduced
- FVC may be normal or reduced (if air trapping or concurrent restriction)
- Flow-volume loop shows concave (scooped) expiratory curve; prolonged expiratory time
- Causes: Asthma, COPD, bronchiectasis, bronchiolitis
- In COPD, typical pattern is irreversible; in asthma, reversible (see below)
GOLD COPD Severity Staging (by FEV1 % predicted, post-bronchodilator):
| GOLD Stage | FEV1 % Predicted |
|---|---|
| GOLD 1 (Mild) | ≥80% |
| GOLD 2 (Moderate) | 50-79% |
| GOLD 3 (Severe) | 30-49% |
| GOLD 4 (Very Severe) | <30% |
Note: Current GOLD guidelines also incorporate symptoms (mMRC/CAT) and exacerbation history for comprehensive staging.
Restrictive Pattern
- FEV1/FVC normal (both FEV1 and FVC reduced proportionally)
- FVC <80% predicted with normal ratio - suggestive; confirmed by low TLC on body plethysmography
- FEV1/FVC may actually be >70% (e.g., 90% in fibrosing alveolitis)
- Causes: Pulmonary fibrosis, pleural disease, chest wall deformity, neuromuscular disease, obesity
Example values (Bailey & Love):
- Normal: FEV1 3.1 L, FVC 3.8 L, FEV1/FVC 82%
- Obstructive (asthma): FEV1 1.4 L, FVC 3.5 L, FEV1/FVC 40% → post-bronchodilator: FEV1 2.5 L, FVC 3.5 L, FEV1/FVC 71%
- Restrictive (fibrosing alveolitis): FEV1 1.8 L, FVC 2.0 L, FEV1/FVC 90%
Mixed Pattern
- Low FEV1/FVC + low TLC
- Both obstruction and restriction co-exist
Non-Specific Pattern
- Normal FEV1/FVC with low FEV1 or low FVC, but TLC is normal
- Does not fit classical obstructive or restrictive criteria
Bronchodilator Reversibility Testing
After baseline spirometry, administer a short-acting bronchodilator (typically salbutamol 400 mcg) and repeat after 15-20 minutes.
Positive bronchodilator response (ATS/ERS criteria):
- Increase in FEV1 of >12% AND >200 mL from baseline
- Also: increase in FVC of >20% and >200 mL
This indicates reversible airflow obstruction and is suggestive (not diagnostic) of asthma. In asthma, the FEV1/FVC is typically <70%; in severe asthma with significant air trapping, the FVC may fall and paradoxically increase the FEV1/FVC ratio.
- Murray & Nadel's Textbook of Respiratory Medicine
Spirometry in Specific Diseases
Asthma (FEV1-based severity, NAEP 2007)
| FEV1 % Predicted | Severity |
|---|---|
| ≥80% | Mild (if symptomatic) |
| 60-79% | Moderate |
| ≤60% | Severe |
COPD
- Diagnosis requires postbronchodilator FEV1/FVC < 0.70
- Flow-volume loop: concave expiratory limb, reduced flow at all volumes
- Volume-time curve: prolonged expiratory time
Additional Related Tests
| Test | What it Measures | Clinical Use |
|---|---|---|
| DLCO (diffusing capacity for CO) | Gas transfer across alveolar-capillary membrane | Low in emphysema, ILD; normal/high in asthma |
| TLC (total lung capacity) | All lung gas at maximal inflation (body plethysmography) | Confirms restriction (low TLC) or hyperinflation (high TLC) |
| RV/TLC ratio | Air trapping index | Elevated in obstruction |
| MVV | Maximal voluntary ventilation | Can be estimated as FEV1 × 35-40; estimates exercise ventilatory reserve |
Indications for Spirometry
- Diagnosis of obstructive or restrictive lung disease (asthma, COPD, ILD, NMD)
- Monitoring disease progression or treatment response
- Occupational surveillance - workers in hazardous environments
- Preoperative assessment - particularly before lung resection surgery
- Disability evaluation
Note: Spirometry alone cannot establish a specific disease diagnosis - it characterizes the pattern and severity of ventilatory defect. Most diseases cause a predictable type of defect, and the test is sufficiently reproducible to follow disease course over time.
- Murray & Nadel's Textbook of Respiratory Medicine; Bailey & Love's Short Practice of Surgery; Pfenninger & Fowler's Procedures for Primary Care
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