Respiratory Function Tests

RFTs Overview - Lung Checkup Basics

Respiratory Function Tests (RFTs) are essential non-invasive tools for evaluating lung health and function.

• Purpose of RFTs:
  • Diagnosis (e.g., asthma, COPD).
  • Monitoring disease progression & therapy.
  • Pre-operative risk assessment.
• Classification (Key Types):
  • Spirometry: measures airflow.
  • Lung Volumes: measures air capacity (e.g., TLC, RV).
  • Diffusion Capacity (DLCO): assesses gas exchange.
  • ABGs: evaluate $O_2$/$CO_2$ levels.
  • Exercise Tests (CPET): assess exercise capacity.

⭐ RFTs are non-invasive tests crucial for differentiating obstructive vs. restrictive lung diseases.

Spirometry - Airflow Adventures

Key Spirometry Measures:

• FVC (Forced Vital Capacity): Max air exhaled post max inspiration.
• FEV1 (Forced Expiratory Volume in 1s): Air exhaled in 1st second.
• FEV1/FVC Ratio: Key. Normal >0.7-0.8. $FEV_1/FVC < \textbf{0.7}$ (or 70%) = Obstruction.
• PEFR (Peak Expiratory Flow Rate): Max expiratory speed.
• FEF25-75%: Mid-flow; small airways.

Flow-Volume Loops:

• 📌 Obstructive: 'Scooped-out' appearance. Restrictive: Narrow, tall "witch's hat".

Interpreting Patterns: Severity of obstruction is graded by FEV1 %predicted (e.g., Mild >70%, Moderate 60-69%, Severe 35-49%).

Spirometry Interpretation Algorithm:

Lung Volumes & Capacities - Air Space Sizing

• Lung Volumes (Static):
  • Tidal Volume (TV): Air per quiet breath; ~500 mL.
  • Inspiratory Reserve Volume (IRV): Max extra air inhaled after normal inspiration; ~3000 mL.
  • Expiratory Reserve Volume (ERV): Max extra air exhaled after normal expiration; ~1100 mL.
  • Residual Volume (RV): Air left in lungs after max exhalation; ~1200 mL. (Not measured by spirometry)
• Lung Capacities (Combinations of ≥2 volumes):
  • Inspiratory Capacity (IC): $IC = TV + IRV$; ~3500 mL.
  • Functional Residual Capacity (FRC): $FRC = ERV + RV$; ~2300 mL. (Lung volume at rest)
  • Vital Capacity (VC): $VC = IRV + TV + ERV$; ~4600 mL.
  • Total Lung Capacity (TLC): $TLC = RV + ERV + TV + IRV$; ~5800 mL. (TLC < 80% predicted: Restriction)
• FRC & RV Measurement Methods:
  • Helium dilution technique
  • Nitrogen washout method
  • Body Plethysmography

⭐ Body plethysmography measures total thoracic gas, including trapped gas (e.g., in bullae), making it the gold standard for FRC & TLC measurement.

DLCO & Specialized Tests - Gas Exchange & Challenges

• Diffusing Capacity (DLCO/TLCO): Measures gas transfer across alveolo-capillary membrane.

  • Principle: CO uptake reflects O₂ exchange. $V_{gas} = \frac{A \cdot D_M \cdot (P_1 - P_2)}{T}$.
  • Abnormal: < 80% predicted.
  • Factors affecting DLCO:
    • ↓ DLCO: Anemia, emphysema (↓area), ILD (↑thickness), pulmonary embolism (↓blood flow), smoking (↑COHb).
    • ↑ DLCO: Polycythemia, pulmonary hemorrhage, exercise, asthma (can be normal/↑).
  • ⭐ > DLCO ↓ in emphysema (alveolar destruction), often normal/↑ in asthma & chronic bronchitis (without emphysema).

• Bronchodilator Reversibility Test:

  • For asthma diagnosis/assessment.
  • Significant response: Post-bronchodilator FEV₁ or FVC ↑ > 12% AND > 200mL.

• Bronchoprovocation Test (e.g., Methacholine Challenge):

  • Diagnoses airway hyperresponsiveness (AHR), e.g., in asthma.
  • Positive: FEV₁ ↓ by 20% (PC₂₀) at low agonist concentration.

High‑Yield Points - ⚡ Biggest Takeaways

• FEV1/FVC ratio is key: ↓ in obstructive, normal/↑ in restrictive diseases.
• DLCO (diffusing capacity) is ↓ in emphysema and most ILDs.
• Spirometry measures dynamic lung volumes (FVC, FEV1) and flow rates.
• RV & TLC are ↑ in obstructive (air trapping), ↓ in restrictive patterns.
• Flow-volume loops: characteristic scooped (obstructive), miniature (restrictive), or flattened (fixed obstruction) patterns.
• Bronchodilator reversibility (FEV1 ↑ >12% & >200mL) suggests asthma.
• Methacholine challenge assesses bronchial hyperresponsiveness in suspected asthma_._