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SPIROMETRY — Complete MD Examination Answer (15 Marks)

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Definition

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Lung Volumes & Capacities (Static)

Note: RV, FRC, and TLC cannot be measured by spirometry alone — they require nitrogen washout, helium dilution, or body plethysmography.

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Spirometric Measurements (Dynamic Volumes)

Key Parameters

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The Volume–Time Curve (FVC Curve)

1. Inhales maximally to TLC
2. Exhales as forcefully and rapidly as possible

Graph: Normal vs. Obstructive Pattern

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Spirogram Patterns — Normal, Obstructive, Restrictive

(a) Volume–Time Spirograms

(b) Changes in Lung Volumes

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The Flow–Volume Loop

• Expiratory limb (upper): Rises rapidly to peak flow (effort-dependent initial portion), then descends linearly and effort-independently to RV
• Inspiratory limb (lower): Semicircular, entirely effort-dependent

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Interpretation Algorithm

Step 1: Check FEV₁/FVC ratio

• < 0.70 (or < LLN) → Obstructive pattern → proceed to Step 2
• ≥ 0.70 → Go to Step 3

Step 2: (Obstructive) — Check FVC

• FVC normal → Simple obstruction
• FVC also low → Obstruction + air trapping or mixed pattern

Step 3: (Non-obstructive) — Check FVC

• FVC reduced (< 80% predicted), normal ratio → Restrictive pattern (confirm with TLC)
• Both normal → Normal spirometry

Step 4: Severity (GOLD classification for obstruction, post-bronchodilator FEV₁%)

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Bronchodilator Reversibility Testing

• Administer short-acting bronchodilator (salbutamol 400 µg)
• Repeat spirometry after 15–20 minutes
• Positive reversibility = FEV₁ increase ≥ 12% AND ≥ 200 mL from baseline
• Suggests asthma (vs. fixed obstruction in COPD)
• Requires: LABAs withheld ≥12 h, SABAs withheld ≥6 h before test

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Indications for Spirometry

1. Diagnostic — suspected asthma, COPD, pulmonary fibrosis, occupational lung disease
2. Monitoring — disease progression (e.g., serial FVC in IPF predicts survival)
3. Pre-operative assessment — especially lung resection surgery
4. Disability/legal evaluation — quantifying impairment
5. Occupational screening — hazardous environments
6. Assessing treatment response — bronchodilators, steroids

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Acceptability & Reproducibility Criteria (ATS/ERS)

• Minimum 3 acceptable manoeuvres
• Best 2 FVC values within 150 mL of each other
• Best 2 FEV₁ values within 150 mL
• No cough, glottis closure, or early termination
• Initial 25–30% of curve is effort-dependent; remainder is effort-independent

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Conditions Causing Each Pattern

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FEF₂₅₋₇₅% (Forced Mid-Expiratory Flow)

• Reflects small airway (peripheral airway) function
• More sensitive for early obstructive disease than FEV₁
• Limitation: Wide 95% CI in normal populations → poor specificity; high false-positive and false-negative rates
• Less reproducible than FEV₁

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Maximal Voluntary Ventilation (MVV)

• Maximal volume breathed in 1 minute by voluntary effort
• Normal ≈ FEV₁ × 40 L/min
• Decreased in: airway obstruction, neuromuscular disease, deconditioning, morbid obesity
• Correlates with subjective dyspnoea and exercise tolerance
• Useful in pre-operative evaluation (lung resection)

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Diffusing Capacity (DLCO) — Adjunct to Spirometry

• Normal/elevated in asthma (↑ perfusion upper zones, large lung volumes)
• Reduced in emphysema (destroyed alveoli)
• Reduced in pulmonary fibrosis
• Unexpectedly elevated DLCO → suspect undiagnosed asthma

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Summary Table

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PHYSIOLOGY BEHIND SPIROMETRY — Exam Answer

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1. WHAT SPIROMETRY ACTUALLY MEASURES

• Spirometry measures airflow generated by the mechanical interaction between:
  • Respiratory muscles (force generators)
  • Lung elastic recoil (stored energy)
  • Airway resistance (opposition to flow)
• The instrument converts flow → integrates → volume over time

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2. FORCES DRIVING EXPIRATION

Two components of alveolar pressure (Palv):

Palv = Pleural pressure (Ppl) + Lung elastic recoil pressure

• At rest (no flow): Ppl is negative (~−5 cmH₂O), elastic recoil positive; net Palv = 0
• On forced expiration: expiratory muscles contract → Ppl becomes positive → Palv rises → air is driven out
• The driving pressure for expiratory flow = elastic recoil of the lung (not Ppl alone)

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3. THE KEY CONCEPT: EQUAL PRESSURE POINT (EPP) — ★ High-yield ★

• As air flows from alveoli → mouth, airway pressure drops progressively due to frictional resistance
• At some point along the airway: intraluminal pressure = surrounding pleural pressure → this is the Equal Pressure Point (EPP)
• Downstream (mouthward) of EPP: Ppl exceeds intraluminal pressure → dynamic airway compression
• Result: Further increase in expiratory effort does NOT increase flow → flow becomes effort-independent

Key equation:

V̇max = Elastic recoil pressure (PL) ÷ Upstream resistance (Rus)

• Flow is determined only by lung recoil and upstream resistance, NOT by effort
• EPP moves upstream (toward alveoli) as effort increases → eventually fixed in lobar/segmental bronchi

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4. EFFORT-DEPENDENT vs. EFFORT-INDEPENDENT PORTIONS

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5. PRESSURE–VOLUME (P-V) CURVE — Basis of Lung Mechanics

• Lungs alone: tend to collapse inward at all volumes (elastic recoil inward)
• Chest wall alone: tends to spring outward below ~70% TLC
• FRC = equilibrium point where lung recoil inward = chest wall recoil outward (no muscle activity needed)
• TLC = point where inspiratory muscle force is fully opposed by combined recoil of lungs + chest wall
• RV = point where either chest wall rigidity OR airway closure prevents further exhalation

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6. WHY FEV₁/FVC IS THE KEY RATIO

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7. PHYSIOLOGY OF INDIVIDUAL PARAMETERS

FVC

• Determined by: TLC − RV
• Limited by: chest wall rigidity, airway closure, muscle weakness, lung stiffness

FEV₁

• Determined by: airway calibre + elastic recoil + effort (early phase)
• Most reproducible measure — standard deviation only ~200 mL in normal subjects

FEF₂₅₋₇₅%

• Reflects small airway (<2 mm) resistance
• Small airways contribute <20% of total airway resistance → disease here is "silent" until FEF₂₅₋₇₅% falls

PEFR (Peak Expiratory Flow Rate)

• Occurs within first 100–120 ms of forced expiration
• Entirely effort-dependent → useful for monitoring (asthma diary) but not diagnosis

MVV (Maximal Voluntary Ventilation)

• MVV ≈ FEV₁ × 40
• Reflects respiratory muscle endurance + coordination, not just airways

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8. PHYSIOLOGY OF OBSTRUCTIVE vs. RESTRICTIVE DEFECTS

Obstructive (e.g., asthma, COPD, emphysema)

• ↑ Airway resistance → EPP shifts far upstream → severe dynamic compression
• In emphysema: ↓ elastic recoil → Vmax = PL/Rus → both PL↓ and Rus↑ → severe flow limitation
• Air trapping: small airways close prematurely → RV↑, FRC↑, TLC↑
• Hyperinflation: diaphragm flattens → mechanical disadvantage → ↑ work of breathing

Restrictive (e.g., fibrosis, pleural disease, kyphoscoliosis)

• ↓ Lung compliance (stiff lungs) → reduced TLC, VC, FRC
• Elastic recoil is increased in fibrosis → flow rates preserved or even ↑
• FEV₁/FVC normal or elevated (stiff lung recoils quickly → empties fast proportionally)
• Work of breathing ↑ due to stiff lung (high elastic work)

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9. PHYSIOLOGY OF BRONCHODILATOR RESPONSE

• Bronchodilators (β₂ agonists) → ↑ intracellular cAMP → smooth muscle relaxation → ↑ airway calibre
• ↑ calibre → ↓ resistance → EPP moves downstream → ↓ dynamic compression → ↑ FEV₁ and FVC
• Positive response: FEV₁ ↑ ≥12% AND ≥200 mL → reversible obstruction (asthma > COPD)

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10. WHY RV CANNOT BE MEASURED BY SPIROMETRY — Physiology

• After maximal exhalation, air remains in lungs (RV) due to:
  1. Airway closure — small airways collapse at low lung volumes (closure volume > RV in elderly/smokers)
  2. Chest wall rigidity — in young individuals, chest wall springs back and limits further exhalation
• Since spirometry measures air moving in or out, trapped RV cannot be detected
• Measured by: Helium dilution, Nitrogen washout, or Body plethysmography (most accurate)

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QUICK PHYSIOLOGY SUMMARY TABLE

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