Hey CPSolvers!


Scenario 

You’re admitting an 80-year-old man for chronic, progressive shortness of breath. 

He has a 65 pack-year smoking history.

The chest x-ray is clear, BNP is low, and hemoglobin is normal.

Your senior resident thinks you’re an all-star for looking for the pulmonary function tests and asks you to study and then interpret them on rounds. 

Were there always these many numbers and percentages and graphs?!

 

How do I… INTERPRET PULMONARY FUNCTION TESTS ? 

 

First, what are some common PULMONARY FUNCTION TESTS ?

  • Spirometry →  useful for the diagnosis of obstructive and/or restrictive lung disease
    • Includes post-bronchodilator → determines if reversible airway disease is present
  • Plethysmography → evaluates total lung volumes (total lung capacity or TLC)
  • Diffusion capacity (DLCO) → evaluates for impaired diffusion due to alveolar, interstitial, or pulmovascular dz
  • 6 minute walk test → evaluate for hypoxia with exertion

 

Let’s break down the interpretation of PFTs, starting with SPIROMETRY…

  1. First, look at the FLOW VOLUME LOOPS & ‘COMMENTS’ to determine the test’s VALIDITY
    • Did they breathe out all the way, did it last 6 seconds? 
    • Was there poor or inconsistent effort? 
      • Normal → 
      • Poor effort or patient unable to perform the test →
  2. Look at the RATIO of FEV1/FVC (not % of predicted) 
    • >0.7 → NON OBSTRUCTIVE PATTERN (skip to step 3)
    • <0.7 → OBSTRUCTIVE
      • Assess the flow volume loop for Scooping → LOWER AIRWAY OBSTRUCTION
      • Look at % PREDICTED of FEV1 to grade severity
        • >80% is MILD
        • 50-80% is MODERATE
        • 30-49% is SEVERE
        • <30% is VERY SEVERE
      • Look at POST-BRONCHODILATOR REVERSIBILITY
        • Improvement of FEV1 by 10-12% → likely ASTHMA
        • Partial improvement or no change → likely COPD
      • Look at CORRECTED DLCO/VA
        • Normal → Asthma, bronchiectasis, Chronic Bronchitis
        • Low → emphysema, Severe bronchiectasis or cystic fibrosis
    • BONUS: 
      • FEF 25-75%  (forced expiratory flow) is a sensitive way to assess for  SMALL AIRWAY OBSTRUCTION
        • <60% is generally abnormal

3. So your FEV1/FVC is >70%… What are the next steps?

    • Look at FVC and TLC to gauge lung volume 
        • FVC & TLC  normal → look at DLCO
          • DLCO normal → look at flow volume loop
            • flow volume loop normal → normal lung function
            • flow volume loop scooped out → concomitant RESTRICTIVE + OBSTRUCTIVE dz
          • DLCO low → EARLY ILD, PULMONARY VASCULAR DISEASE
        • FVC & TLC  low →  RESTRICTIVE LUNG DISEASES
          •  Look at DLCO to determine what is restricting the lung
            • DLCO normal → outside the lungs
              • NEUROMUSCULAR DISEASE, KYPHOSIS, OBESITY
            • DLCO low → inside the lungs
              •  ILD

 

Your patient’s PFTs showed:

  • Good consistency and effort
  • FEV1/FVC ratio 85% with a non-scooped flow volume loop
  • Low FVC and TLC
  • Low DLCO

Based on the PFT interpretation, you suspect interstitial lung disease and prioritize a high resolution chest CT, which shows findings concerning for UIP/idiopathic pulmonary fibrosis.

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