Pulmonary Function Tests and Use of Spirometry in Clinical Medicine

PART A: PULMONARY FUNCTION TESTS (5 marks)

Definition and Overview

Classification of PFTs

Lung Volumes and Capacities

• Tidal Volume (VT): Volume inhaled/exhaled per breath cycle
• Inspiratory Reserve Volume (IRV): Additional volume beyond end of normal inspiration
• Expiratory Reserve Volume (ERV): Volume expelled beyond normal expiration
• Residual Volume (RV): Volume remaining after maximal exhalation (cannot be measured by spirometry alone)

• Total Lung Capacity (TLC): Volume at maximal inspiration = VT + IRV + ERV + RV
• Vital Capacity (VC): Maximal volume expelled without time constraint
• Inspiratory Capacity (IC): VT + IRV
• Functional Residual Capacity (FRC): ERV + RV (resting expiratory level)

Major PFT Patterns

• Decreased FEV1/FVC ratio (below LLN - lower limit of normal)
• Reduced FEV1; normal or reduced FVC
• Increased RV, RV/TLC ratio
• Concave (scooped-out) expiratory limb on flow-volume loop
• Examples: asthma, COPD, bronchiolitis, bronchiectasis, cystic fibrosis

• Reduced TLC (definitive criterion)
• Normal or elevated FEV1/FVC ratio
• Proportionate reduction in FEV1 and FVC
• Parenchymal causes: ILD, sarcoidosis, IPF; Extra-parenchymal: kyphoscoliosis, obesity, neuromuscular disease, pleural disease

• Reduced FEV1/FVC AND reduced TLC
• Seen in: bronchiectasis, cystic fibrosis, sarcoidosis, congestive heart failure

• Reduced FVC and FEV1 with normal FEV1/FVC and no documented low TLC

Diffusing Capacity (DLCO)

• Parenchymal diseases (IPF, emphysema)
• Pulmonary vascular disease
• Anemia Used to differentiate emphysema (low DLCO) from pure airway disease (normal DLCO).

Indications for PFTs (ATS Table 32.1)

PART B: SPIROMETRY IN CLINICAL MEDICINE (5 marks)

Principle and Technique

• At least 3 acceptable spirograms
• Two largest FVC values and two largest FEV1 values must be within 150 mL of each other
• Upper limit of 8 efforts if repeatability not met
• The first 25-30% of the maneuver is effort-dependent and requires patient cooperation and trained technician

• Age >6 years: Back-extrapolated volume ≤5% FVC or ≤0.1 L
• Age <6 years: Expiration may complete in <1 second; FEV0.75 preferred over FEV1
• Repeatability: FVC and FEV1 difference ≤100 mL or 5% (school-age); ≤100 mL or 10% (preschool)

Key Spirometric Parameters

Interpretation Algorithm

• If reduced (below LLN or <0.70) → Obstructive pattern
• If normal or elevated → proceed to Step 2

• If FVC reduced with normal ratio → possible restriction (confirm with lung volumes)
• If both FEV1/FVC and FVC are normal → normal spirometry

• Mild: ≥70%; Moderate: 60-69%; Moderately severe: 50-59%; Severe: 35-49%; Very severe: <35%

• FEV1 or FVC increases ≥12% AND ≥200 mL from baseline (ATS/ERS)
• In children: ≥12% increase in FEV1 is the standard threshold

Clinical Uses of Spirometry

• Confirms airflow obstruction with bronchodilator reversibility
• Monitors disease control and severity classification (GINA)
• FEV1 <80% predicted with reversibility is classic
• Note (Nelson 22e / Piyush Gupta): In children, exacerbations can occur even with normal FEV1; FEV1/FVC ratio may be more sensitive; normal spirometry does NOT exclude asthma
• PEFR monitoring at home for personal best baseline and action zones

• Post-bronchodilator FEV1/FVC <0.70 defines airflow limitation (GOLD criteria)
• Severity graded by FEV1 % predicted (GOLD I-IV)
• Serial spirometry tracks disease progression

• Mixed obstructive-restrictive pattern
• Serial FEV1 decline is the primary prognostic marker
• Guides timing of lung transplantation evaluation (FEV1 <30%)

• FEV1 and FVC used before lung resection surgery
• Predicted postoperative FEV1 (ppoFEV1) calculated to assess surgical risk

• Restrictive pattern (reduced FVC with normal/elevated FEV1/FVC)
• Combined with DLCO for staging and monitoring

• VC and MIP/MEP monitored serially in Guillain-Barré, SMA, Duchenne MD
• VC <15-20 mL/kg is threshold for elective intubation consideration

• Periodic spirometry surveillance in exposed workers (occupational asthma, silicosis, asbestosis)

• Flow-volume loop shows blunting/plateau of inspiratory limb
• Differentiates from asthma in children/adolescents (Piyush Gupta)

Pediatric-Specific Considerations (Nelson 22e / Piyush Gupta)

1. Age of reliable testing: Cooperative spirometry generally achievable from age 5-6 years; modified preschool protocols from age 3
2. FEV1/FVC ratio: Normal lower limit in young children is substantially higher than adults (>0.85 in children vs >0.70 in adults); applying adult cutoffs leads to under-diagnosis of obstruction
3. Reference values: GLI (Global Lung Function Initiative) 2012 reference equations recommended - they span age 3 to 95 years across multiple ethnicities
4. Impulse Oscillometry (IOS/FOT): Preferred for children <5 years and uncooperative patients; requires tidal breathing only; detects peripheral airway dysfunction even when spirometry is normal (PMID 42055592, 2026 systematic review confirms diagnostic utility in pediatric asthma)
5. FEV0.75: Used instead of FEV1 in children <6 years (expiration completes <1 second)
6. Bronchodilator testing: Standard in all children with suspected asthma; positive response ≥12% increase in FEV1
7. Bronchoprovocation: Methacholine or exercise challenge used to confirm bronchial hyperresponsiveness when spirometry is normal but asthma is suspected

Recent Updates (2024-2026)

• GOLD-GLI Joint Statement (2024-2025): Recommends using LLN (lower limit of normal, z-score <-1.645) rather than fixed ratio (FEV1/FVC <0.70) to define airflow obstruction - this is particularly relevant to avoid misclassification in young children and elderly
• Oscillometry for asthma diagnosis in children (Seneviratne et al., Eur Respir Rev 2026, PMID 42055592): Systematic review confirms impulse oscillometry has diagnostic utility for asthma in children, complementing spirometry especially in those who cannot perform forced maneuvers
• Lung volumes and oscillometry after preterm birth (Gibbons et al., Eur Respir Rev 2025, PMID 40436611): Preterm-born children show persistent reductions in lung volumes and gas transfer; oscillometry detects abnormalities not captured by spirometry alone
• SPIROMAC Trial (ongoing 2025): RCT evaluating spirometry-guided asthma management in children - preliminary evidence suggests clinical utility of regular spirometry in pediatric asthma care

Summary Table: Interpretation at a Glance

• PFTs do not diagnose diseases - they define functional patterns
• FEV1/FVC ratio is the first step; TLC confirms restriction
• In children, FEV1/FVC LLN is higher than adults (>0.85)
• Bronchodilator reversibility ≥12% + ≥200 mL confirms significant response
• DLCO is the most sensitive test for early parenchymal and vascular disease
• Serial spirometry (FEV1 decline) is the best prognostic marker in CF and COPD
• IOS/oscillometry is the preferred test in children <5 years