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Pulmonary Function Tests (PFTs)

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1. Definition and Purpose

"Pulmonary function tests (PFTs) permit accurate, reproducible assessment of the functional state of the respiratory system. PFTs do not diagnose specific diseases. Different diseases cause different patterns of abnormalities in a battery of PFTs."

• Murray & Nadel's Textbook of Respiratory Medicine

• Quantify the severity of respiratory disease
• Detect disease early (e.g., subclinical COPD)
• Characterize natural history and response to treatment
• Assess operative risk and fitness for procedures (e.g., lung resection)

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2. Lung Volumes and Capacities

• Three volumes (TV, IRV, ERV) can be measured with a simple spirometer.
• RV and any capacity containing it (FRC, TLC) require body plethysmography, inert gas dilution, or nitrogen washout.

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3. Categories of PFTs

A. Spirometry

• The most useful, cost-effective, and commonly performed PFT.
• Measures volume of air inhaled/exhaled using a pneumotachometer.
• Key parameters:
  • FVC (Forced Vital Capacity): total air forcefully exhaled
  • FEV1 (Forced Expiratory Volume in 1 second): air expelled in the first second
  • FEV1/FVC ratio: distinguishes obstruction from restriction
  • PEF (Peak Expiratory Flow): max flow rate during forced exhalation
  • FEF 25-75%: mid-expiratory flow (reflects small airway disease)
• Effort dependent: the first 25-30% of maximal expiration requires patient cooperation and well-trained technicians.

B. Lung Volume Measurement

• Body plethysmography (preferred): uses Boyle's law (P1V1 = P2V2) in a sealed box
• Helium dilution / Nitrogen washout: dilution techniques for measuring FRC and TLC

C. Diffusing Capacity (DLCO / TLCO)

• Measures gas transfer across the alveolar-capillary membrane.
• Standard method: Single-breath DLCO - patient inhales 0.3% CO + inert tracer gas (helium, methane, or neon), holds breath for ~10 seconds, then exhales.
• Factors affecting DLCO (from Murray & Nadel):

D. Airway Resistance (Raw)

• Measured by body plethysmography; Raw = Palv / airflow
• Specific airway conductance (sGAW = GAW/TGV) is normalized for lung volume

E. Bronchodilator Reversibility Testing

• Albuterol (180 mcg) is inhaled; spirometry repeated after 10-20 minutes
• Significant reversibility: FEV1 or FVC improvement ≥12% AND ≥200 mL (ATS/ERS criteria)
• Helps distinguish asthma (reversible) from COPD (largely fixed obstruction)

F. Bronchial Provocation (Challenge) Tests

• Methacholine challenge: used when obstruction is suspected but baseline spirometry is normal
• Positive if PC20 (concentration causing 20% FEV1 fall) is low

G. Exercise Testing / 6-Minute Walk Test (6MWT)

• Patient walks a 30-m corridor for 6 minutes at comfortable pace
• Absolute contraindications: unstable angina, MI within 1 month
• Relative contraindications: resting HR >120, systolic BP >180, diastolic BP >100 mmHg
• Terminate if: chest pain, severe dyspnea, diaphoresis, profound desaturation

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4. Interpretation: The Three Patterns

A. Obstructive Pattern

B. Restrictive Pattern

1. Primary parenchymal - pneumonia, fibrosis, atelectasis, tumor
2. Surgical removal of lung tissue
3. Pleura/chest wall - pleural fibrosis, effusion, kyphoscoliosis, obesity
4. Neuromuscular disorders (spinal cord, peripheral nerve, NMJ, muscle)

C. Mixed Obstructive-Restrictive Pattern

• Low FEV1/VC AND reduced TLC
• Caused by pathologic processes affecting both airflow and lung volumes

D. DLCO Severity Classification (Fishman's)

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5. Reference Values and LLN

• Reference values account for height, age, sex, and ethnic background.
• The LLN (5th percentile / z-score < -1.645) is preferred over fixed cutoffs (e.g., FEV1 < 80% predicted).
• A study of 11,413 patients showed fixed cutoffs could misdiagnose >20% of patients.
• Z-scores (standard deviations from mean) avoid age/height bias inherent in percentage-of-predicted thresholds.
• Goldman-Cecil Medicine

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6. Severity Grading of Obstruction (GOLD-based)

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7. Specific PFT Findings by Disease

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8. Flow-Volume Loops

• Normal: characteristic bell shape on expiration, flat inspiratory limb
• Obstruction: concave (scooped) expiratory limb - flow drops rapidly
• Restriction: narrow tall loop with reduced volumes but preserved flow rates
• Variable extrathoracic obstruction (e.g., vocal cord dysfunction, tracheomalacia): flattening of the inspiratory limb
• Variable intrathoracic obstruction: flattening of the expiratory limb
• Fixed obstruction (tracheal stenosis): flattening of both limbs

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9. Quality Criteria

• At least 3 acceptable maneuvers with FVC and FEV1 values within 150 mL of each other
• The largest FVC and FEV1 from acceptable maneuvers are used
• Poor reproducibility with consistently subnormal peak flows raises concern for poor effort
• Consistently low values despite maximal effort may signal neuromuscular disease

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• Murray & Nadel's Textbook of Respiratory Medicine, 2-Volume - Chapters 31 & 32
• Fishman's Pulmonary Diseases and Disorders - Chapter 31
• Goldman-Cecil Medicine - Chapter 73
• Barash, Cullen, and Stoelting's Clinical Anesthesia - Chapter 38

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PFT Diagrams

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1. Lung Volumes and Capacities

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2. Volume-Time Spirograms (FVC Curve)

• Normal: FEV1 = 3.3 L, FVC = 4.0 L, FEV1/FVC = 83%
• Obstructive: FEV1 = 1.0 L, FVC = 2.0 L, FEV1/FVC = 50% (prolonged expiration)
• Restrictive: FEV1 = 1.8 L, FVC = 2.0 L, FEV1/FVC = 90% (short, steep, small volume)

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3. Flow-Volume Curve (Normal)

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4. Flow-Volume Loops: Disease Patterns

• Obstructive: shifted left (hyperinflation), concave/scooped expiratory limb, reduced peak flow
• Restrictive: narrow tall loop shifted right, reduced total volume, preserved flow rates
• Fixed obstruction: both inspiratory and expiratory limbs flattened ("box" shape)

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5. PFT Interpretation Algorithms

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6. DLCO Single-Breath Method

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Quick Reference Summary