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Pulmonary Function Tests (PFT) — Complete 15-Mark Answer

Definition & Overview

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I. Lung Volumes and Capacities

Key Point: TV, VC, and IRV/ERV can be measured by spirometry alone. FRC, RV, and TLC require helium dilution, nitrogen washout, or body plethysmography. — Murray & Nadel's Textbook of Respiratory Medicine

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II. Methods of Measuring Lung Volumes

1. Body Plethysmography (Gold Standard)

• Patient sits inside an airtight constant-volume chamber ("body box")
• Makes inspiratory efforts against a closed shutter
• Applies Boyle's Law (P₁V₁ = P₂V₂) to calculate FRC/RV
• Advantage: Measures all lung gas including trapped gas
• Disadvantage: May overestimate in severe obstruction (abdominal gas compression)

2. Helium Dilution

• Patient breathes known concentration of helium at FRC
• Helium mixes with lung gas and is diluted
• Formula: C₁ × V₁ = C₂ × (V₁ + FRC)
• Disadvantage: Underestimates in severe airway obstruction (trapped gas not measured)

3. Nitrogen Washout

• Patient breathes 100% oxygen; nitrogen "washes out"
• Expired nitrogen volume measured until concentration approaches zero
• Based on conservation of mass principle

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III. Spirometry

Key Spirometric Measurements:

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IV. Interpretation of Spirometry

Step-by-Step Approach (ATS/ERS):

1. Visual inspection of flow-volume curve — quality check and large airway lesions
2. FEV₁/FVC ratio → If low → obstructive pattern; if normal → normal or restrictive
3. FVC → If low with normal ratio → may be restrictive (needs TLC confirmation)
4. TLC → Low TLC confirms restriction; normal TLC + low FVC = nonspecific pattern

Patterns of Disease:

"The major diffuse obstructive disorders are emphysema, chronic bronchitis, bronchiectasis, and asthma. An FEV₁/FVC ratio of less than 0.7 generally indicates the presence of obstructive disease." — Robbins & Kumar Basic Pathology

"In diffuse restrictive diseases, FVC is reduced and the expiratory flow rate is normal or reduced proportionately. Hence, the ratio of FEV₁ to FVC is near normal." — Robbins Basic Pathology

Grading of Severity (ATS/ERS — based on FEV₁ % predicted):

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V. Flow-Volume Loop Analysis

Characteristic Patterns:

• Normal: High peak flow, smooth descending expiratory limb
• Obstructive (COPD/asthma): Reduced PEFR, scooped-out (concave) descending limb — hallmark of small airway collapse
• Restrictive: Narrow loop (reduced volumes), steep and tall but with normal shape proportionally
• Fixed large airway obstruction (e.g., tracheal stenosis, tracheal tumor): Plateaus on BOTH inspiratory and expiratory limbs (flow limited in both directions)
• Variable extrathoracic obstruction (e.g., vocal cord paralysis): Plateau only on inspiratory limb (negative intratracheal pressure during inspiration collapses the lesion)
• Variable intrathoracic obstruction (e.g., tracheomalacia): Plateau only on expiratory limb

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VI. Diffusing Capacity for Carbon Monoxide (DLCO / Transfer Factor TLco)

Principle:

Method (Single-breath technique):

1. Patient exhales to RV
2. Inhales mixture of 0.3% CO + 10% He (tracer) to TLC
3. Holds breath 10 seconds
4. Exhales; alveolar sample analyzed
5. DLCO = CO uptake rate / driving pressure

Clinical Significance:

"DLCO is reduced in patients with interstitial lung diseases and emphysema. Values below 60% of predicted have been associated with increased risk of postoperative pulmonary complications." — Miller's Anesthesia, 10e

"In IPF, the decrease in DLCO results from both a loss of the pulmonary capillary volume and the presence of ventilation-perfusion abnormalities." — Murray & Nadel's Respiratory Medicine

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VII. Bronchodilator Reversibility Testing

• Spirometry performed before and 15–20 minutes after short-acting bronchodilator (salbutamol 400 µg)
• Significant reversibility: ≥12% AND ≥200 mL increase in FEV₁ or FVC
• Positive in: Asthma (hallmark), partially reversible in COPD
• ATS/ERS recommendation: Post-bronchodilator values should be used for spirometry interpretation

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VIII. Obstructive vs. Restrictive — Specific Disease Correlations

Obstructive Diseases:

• Asthma: ↓FEV₁/FVC, normal DLCO, reversible obstruction, air trapping (↑RV)
• COPD/Emphysema: ↓FEV₁/FVC, ↑TLC, ↑RV, ↓↓DLCO (emphysema), irreversible
• Chronic Bronchitis: ↓FEV₁/FVC, normal DLCO
• Bronchiectasis: ↓FEV₁/FVC, variable DLCO

Restrictive Diseases:

• Pulmonary fibrosis (IPF): ↓TLC, ↓FVC, normal FEV₁/FVC, ↓↓DLCO
• Sarcoidosis, asbestosis: Restrictive ± mixed, ↓DLCO
• Neuromuscular disease (GBS, MG): ↓TLC, ↓MIP/MEP, normal DLCO
• Chest wall disorders (kyphoscoliosis, obesity): ↓TLC, normal DLCO
• ARDS: Classic acute restrictive pattern

Mixed Pattern:

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IX. Arterial Blood Gas (ABG) Correlation

• Type 1 Respiratory Failure (PaO₂ <60 mmHg, normal PaCO₂): V/Q mismatch — IPF, PE
• Type 2 Respiratory Failure (PaO₂ <60 + PaCO₂ >45 mmHg): Ventilatory pump failure — COPD, neuromuscular disease
• Respiratory Alkalosis + Hypoxemia: Early IPF (hyperventilation to compensate)

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X. Exercise Testing (CPET — Cardiopulmonary Exercise Test)

• Measures VO₂ max — gold standard for cardiopulmonary reserve
• Identifies cardiovascular vs. ventilatory vs. peripheral limitation
• 6-Minute Walk Test (6MWT): Simpler; correlates with DLCO and VO₂ max in IPF; strong predictor of mortality

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XI. Preoperative Assessment (Pulmonary Risk)

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Summary Algorithm (ATS/ERS):

Spirometry
     │
     ├── FEV₁/FVC < LLN → OBSTRUCTIVE
     │        │
     │        ├── DLCO normal → Asthma / Chronic Bronchitis
     │        └── DLCO ↓ → Emphysema
     │
     └── FEV₁/FVC ≥ LLN
              │
              ├── FVC normal → NORMAL (no restriction)
              └── FVC ↓ → Measure TLC
                       │
                       ├── TLC ↓ → RESTRICTIVE
                       │      ├── DLCO ↓ → Parenchymal (IPF, fibrosis)
                       │      └── DLCO normal → Chest wall / NM disease
                       ├── TLC normal → Nonspecific pattern
                       └── TLC ↑ + RV ↑ → Air trapping (pseudo-restriction)

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Key Exam Points to Remember

1. FEV₁/FVC <0.70 = obstructive; TLC <80% = restrictive (gold standard for restriction)
2. DLCO ↓ in emphysema (structural loss) but normal in asthma and chronic bronchitis
3. Helium dilution underestimates in severe obstruction (trapped gas); body plethysmography is gold standard
4. Flow-volume loop: Scooped expiratory = obstructive; plateaus on both limbs = fixed large airway obstruction; narrow loop = restrictive
5. Bronchodilator reversibility ≥12% + ≥200 mL = significant (asthma vs. COPD distinction)
6. Post-bronchodilator spirometry is used for definitive classification
7. FVC ↓ with preserved FEV₁/FVC ratio in IPF — the restrictive signature
8. DLCO corrected for Hb — reduce by 7% per g/dL Hb below 14.6 g/dL (anemia artificially lowers DLCO)

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