📊 Pulmonary Function Test Interpretation A Stepwise Guide for Medical Students · Spirometry, Volumes & DLCO

1. Why PFTs? Indications & Components

• Evaluate dyspnea, cough, or wheezing of unknown etiology.
• Diagnose and classify obstructive vs. restrictive lung disease.
• Assess severity and monitor disease progression (COPD, ILD).
• Preoperative risk assessment (especially thoracic surgery).
• Evaluate response to therapy (bronchodilators, antifibrotics).
• Disability / impairment evaluation.

Components Spirometry (FEV₁, FVC, FEV₁/FVC), Lung Volumes (TLC, RV, FRC), Diffusing Capacity (DLCO), Bronchodilator Response, Flow‑Volume Loops.

2. Spirometry: Core Measurements & Interpretation

Key Definitions

Parameter	Definition	Normal Value
FVC (Forced Vital Capacity)	Total volume exhaled forcefully after maximal inspiration	≥80% predicted
FEV₁ (Forced Expiratory Volume in 1 sec)	Volume exhaled in first second of FVC maneuver	≥80% predicted
FEV₁/FVC Ratio	Proportion of FVC exhaled in first second	≥0.70 (or >LLN)
FEF 25–75%	Average flow during middle half of FVC; reflects small airways	≥60% predicted
PEF (Peak Expiratory Flow)	Maximal expiratory flow; effort‑dependent	Wide variability

Step 1: Is it Obstructive or Restrictive?

Step 2: Severity Classification (Obstructive) — GOLD Criteria for COPD

GOLD Stage	FEV₁ (% predicted)	Severity
GOLD 1	≥80%	Mild
GOLD 2	50–79%	Moderate
GOLD 3	30–49%	Severe
GOLD 4	<30%	Very Severe

For Restrictive Disease: Severity based on TLC % predicted (mild 70–80%, moderate 60–69%, severe <60%).

3. Bronchodilator Response (Reversibility Testing)

• Perform spirometry before and 15–20 minutes after inhaled short‑acting bronchodilator (e.g., albuterol 400 μg).
• Significant response: ≥12% and ≥200 mL increase in FEV₁ or FVC.
• Positive response suggests asthma (though not diagnostic; some COPD patients also show reversibility).
• Absence of acute reversibility does not rule out asthma (may require longer treatment trial).

4. Lung Volumes: Confirming Restriction & Hyperinflation

Measured by body plethysmography (gold standard) or helium dilution / nitrogen washout. Spirometry cannot measure residual volume (RV).

Volume / Capacity	Definition	Obstructive	Restrictive
TLC (Total Lung Capacity)	Volume at maximal inspiration	↑ (hyperinflation)	↓ (<80% predicted)
RV (Residual Volume)	Volume after maximal exhalation	↑↑ (air trapping)	↓ or normal
FRC (Functional Residual Capacity)	Volume at end of quiet exhalation (RV + ERV)	↑	↓
RV/TLC Ratio	Fraction of TLC that is residual volume	↑ (>40%)	Normal or ↑
VC (Vital Capacity)	TLC − RV	↓ or normal	↓

5. Diffusing Capacity (DLCO)

• Measures gas transfer across alveolar‑capillary membrane using carbon monoxide (CO).
• Normal DLCO: ≥75% predicted (adjust for hemoglobin and carboxyhemoglobin).

DLCO	Mechanism	Clinical Examples
Decreased (↓)	↓ Surface area, ↑ membrane thickness, pulmonary vascular disease	Emphysema, IPF, ILD, PAH, Anemia (corrected value normal), Pneumonectomy
Increased (↑)	↑ Pulmonary blood volume, alveolar hemorrhage	Obesity, Asthma, Polycythemia, Early CHF, Alveolar hemorrhage (blood in alveoli binds CO), Left‑to‑right shunt
Normal	Isolated airway disease without parenchymal destruction	Chronic Bronchitis, Asthma (may be normal or ↑), Neuromuscular disease

6. Flow‑Volume Loops: Visual Patterns

The shape of the expiratory and inspiratory limbs provides critical diagnostic clues.

Pattern	Appearance	Clinical Significance
Normal	Rapid rise to peak, linear descent	–
Obstructive	Scooped (concave) expiratory limb; may see reduced peak flow	COPD, Asthma (dynamic airway collapse)
Restrictive	Tall, narrow loop; reduced volumes but preserved flow relative to volume	ILD, Obesity, Chest wall restriction
Fixed Upper Airway Obstruction	Flattening of both inspiratory and expiratory limbs (box shape)	Tracheal stenosis, Goiter, Tumor
Variable Extrathoracic Obstruction	Flattening of inspiratory limb only	Vocal cord paralysis / dysfunction, Retrosternal goiter
Variable Intrathoracic Obstruction	Flattening of expiratory limb only	Tracheomalacia, Distal tracheal tumor

7. Classic PFT Patterns by Disease

Condition	FEV₁/FVC	FEV₁	FVC	TLC	RV	DLCO
Asthma	↓ (or normal between exacerbations)	↓	↓ or N	N or ↑	N or ↑	N or ↑
Chronic Bronchitis	↓	↓	↓ or N	N	N or ↑	N or ↓
Emphysema	↓	↓↓	↓	↑↑	↑↑	↓↓
Idiopathic Pulmonary Fibrosis (IPF)	N or ↑	↓	↓↓	↓↓	↓ or N	↓↓
Obesity	N	N or ↓	↓	↓	N	N
Neuromuscular Disease	N	↓	↓↓	↓	↑ (weak expiration)	N
Pulmonary Hypertension (PAH)	N or ↓ (mild)	N or ↓	N or ↓	N	N	↓ (disproportionate to volumes)
Bronchiectasis	↓ (often)	↓	↓ or N	N or ↑	↑	N or ↓

8. Additional PFT Components & Provocation Testing

Methacholine Challenge (Bronchoprovocation)

• Indicated when asthma is suspected but spirometry is normal.
• Patient inhales increasing doses of methacholine (muscarinic agonist) → induces bronchoconstriction.
• Positive test: ≥20% fall in FEV₁ from baseline at or before the maximum dose (PC₂₀).
• Contraindications: Severe airflow obstruction (FEV₁ <50%), uncontrolled hypertension, recent MI/stroke, pregnancy.

Maximal Voluntary Ventilation (MVV)

• Patient breathes as hard and fast as possible for 12–15 seconds. Estimates ventilatory reserve.
• Reduced in neuromuscular disease, poor effort, severe obstruction.
• Used in preoperative evaluation for lung resection (predicted postoperative FEV₁ and DLCO).

Respiratory Muscle Strength

• MIP (Maximal Inspiratory Pressure): Measures inspiratory muscle strength (diaphragm). Normal < -60 cm H₂O (more negative is stronger).
• MEP (Maximal Expiratory Pressure): Measures expiratory muscle strength (abdominals). Normal > +100 cm H₂O.
• Reduced in neuromuscular disorders (ALS, myasthenia gravis, Guillain‑Barré).

9. Stepwise PFT Interpretation Algorithm

1. Check Quality: Acceptable and repeatable? (3 acceptable maneuvers, 2 reproducible).
2. Look at FEV₁/FVC Ratio:
   • <0.70 (or <LLN) → Obstructive pattern. Go to Step 3.
   • ≥0.70 → Normal or Restrictive. Go to Step 4.
3. If Obstructive:
   • Assess severity by FEV₁ % predicted (GOLD 1–4).
   • Check bronchodilator response (≥12% & 200 mL = significant reversibility).
   • Check TLC: ↑ = hyperinflation; normal = simple obstruction.
   • Check DLCO: ↓ = emphysema or parenchymal destruction; N/↑ = asthma / chronic bronchitis.
4. If Normal Ratio (FEV₁/FVC ≥0.70):
   • Check FVC: Normal → Normal PFTs (consider methacholine challenge if high suspicion).
   • FVC <80% → Possible restriction. Must measure TLC.
   • TLC <80% → Confirmed Restriction. TLC normal → Non‑restrictive pattern (e.g., poor effort, early obstruction with air trapping).
5. If Restrictive: Check DLCO. ↓ DLCO → parenchymal disease (ILD). Normal DLCO → extrapulmonary restriction (obesity, neuromuscular, pleural).

10. Clinical Case Applications

11. Predicted Values & Lower Limit of Normal (LLN)

• PFT values are compared to predicted values based on age, sex, height, and ethnicity (e.g., NHANES III, GLI).
• Lower Limit of Normal (LLN): 5th percentile of healthy population. More accurate than fixed ratio (0.70) for diagnosing obstruction in elderly or very young.
• Fixed Ratio (FEV₁/FVC <0.70): Simple but may overdiagnose COPD in elderly (normal aging reduces ratio) and underdiagnose in young adults.
• Always interpret PFTs in clinical context; borderline values require clinical correlation.

12. Quick Reference: PFT Patterns Summary

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