Approach to PFTs

    .

    Introduction

    • Common test used to obtain lung function 
    • Spirometry
      • Inhale fully, then blow into mouth piece for 6-12s.
      • Need 3 cruves that are technically adequate. (may take up to 8 times).
      • FVC and FEV1 in at least two of the curves should not vary more than 5% (reproducible).
        • Use the highest FEV1 and FVC.
        • Age gender height race comparison. 
    • FEV1 - Forced expiratory volume in 1 second.
      • Tells you severity of the defect.
    • FEV1 / FVC   (Fraction of FVC)
      • Tells you if there is an airflow obstruction or not.  Normal lungs should blow off at least 70% of breath in first second.
      • I.e. if low < 70%  (<0.70) = indicates airflow obstruction.
    • If Obstruction, see if it's reversible:
      • If obstruction is reversible, can benefit from therapy, and maybe point to something like asthma.
      • Improvement in FEV1 of 200 mL
        AND
      • Improvement of 12% total FEV1
    • Bronchial Challenge Testing
      • Indicated for those who have symptoms suspicious for asthma, but have normal spirometry. 
      • Very sensitive, but not very specific.
        • False positive: URTI, Allergies, smoking, bronchitis, CF can cause false positive).
        • False negative: Taking LABA before etc.
      • Used histamine or methacholine, gradiated doses of exposure looking for a drop of FEV1.
        • If drops by 20% from baseline = positive bronchial challenge test. 
        • Inhaled mannitol also approved. (triggers mediators of smooth muscle constriction). 
    • Lung Volumes:
      • If reduced to <80% = restriction.
      • Restriction = pulmonary fibrosis or other parenchymal lung disease.
    • DLCO (Difficuse capacity for carbon monoxide)
      • DLCO <70% considered abnormal (not as reproducible as spirometry), <50% is considered quite moderate.
      • Pt inhales small amount of CO, and then exhaled CO is meausred.  In normal lungs CO will difficuse out and bind RBC, so exhaled CO will be low. 
      • Causes of low DLCO :
        • Loss of alvelolar surface area (emphysema)
        • Fibrotic lung disease (emphysema, fibrosis) destroys capillary bed. 
        • Pneumonia (fluid/pus in pulmonary capillaries)
        • Anemia (often reported "corrected DLCO" to correct for low Hb).
        • Pulmonary HTN (decreases blood flow through pulmonary capillaries) --> often will have normal spirometry.
        • Pulmonary hemorrhage (falsely high DLCO)

    Approach to PFTs

    PFT Approach.png

    (Created by the University of Toronto Department of Resporology - Dr. Minz)

     

    Another Approach to PFTs:

    PFT Approach 2.png

    Credit: Toronto Notes

     

    Step-By-Step Approach

     

    1. Look at the demographic
      • Age
      • BMI (High -- restrictive lung disease)
      • Smoking Hx (COPD/Emphysema risk)
    2. Look at the Pressure-Volume Loop
      • Look for rapid upstroke (good effort)
      • Look for sharp point at top (good effort)
      • flow_volume.gif
    3. Look at FEV1/FVC and FEV1
      • if FEV1/FVC < LLN (5% normal)  --> Obstructive
      • if FEV1/FVC ≥ LLN --> Restrictive or normal.
      • Look at FEV1
        • FEV1 80%+ --> Mild obstruction
        • FEV1 50-79% --> Moderate obstruction
        • FEV1 30-49% --> Severe obstruction
        • FEV1 <30% --> Very severe obstruction
    4. Look at Lung Volumes
      • Need to see lung volumes to call "restricted", not just FEV1/FVC.  Volumes must be low!
      • If low volumes (<80%) --> consistent with restriction
      • If large volumes --> consistent with obstruction  (HYPERINFLATION)
      • Is there GAS TRAPPING? Look at RV or TLC/RV
        • (i.e. if restricted volumes, but normal/high RV, then likely neuromuscular weakness, but in pulmonary fibrosis will get restriction + reduction of all lung volumes)
    5. DLCO
      • Is diffusion affected?
      • High in emphysema, or parenchymal lung disease.
      • IF RESTRICTION + DLCO --> Parenchymal Lung Disease
      • IF RESTRICTION + NORMAL DLCO --> Extra-Parenchymal Restriction (consider scloliosis, neuromuscular disease etc..)
    6. Chemical Challenge
      • Methacholine --> GOLD standard for asthma.  If 20% drop in FEV1 --> asthma
        • Cholingeric bronchoconstriction
        • Not done anymore in many labs b/c dangerous.
      • Bronchodilator
        • if FEV1 ↑ by 12% AND ↑ by 200ccs --> Reversible obstruction --> Asthma
        • Also important in COPD --> If bronchodilators cause increase in FVC then can increase lung ventilation with bronchodilators --> Ventolin/atrovent can make a huge difference.

    Notes

    • Patterns:
      • Low lung volumes, Low DLCO, high FEV1/FVC, low FEV1 --> Parenchymal Restriction
      • Restrictionw/ air trapping, low lung volumes, normal DLCO, high FEV1/FVC, low FEV1 --> Extra parenchymal Restriction (Neurological, Neuromuscular, or Scoliosis)
    • Upper Airway Obstruction
      • If extrathoracic obstruction: narrowing of the inspiratory limb of flow/volume loop (lower part)
        • I.e. glottic strictures, tumors, vocal cord paralysis.
      • If intrathoracic obstruction: flattening of the expiratory limb (upper part)
      • If fixed upper airway obstruction: both inspiratory and expiratory limbs flattened.
        • I.e. tracheal stenosis post intubation, gotre (doesn't matter if intra/extrathoracic)
        • (will need tracheoplasty - stents, dilation via ENT or Thoracics)

     

    Restrictive Lung Disease

    • Extra-pulmonary
      • Kyphoscoliosis (can develop hypercapnea with pulmonary HTN)
        • Decompensation usually precipitated by infection, PE, volume overload.
        • NIPPV often has a role to improve entilation (esp during sleep).
      • Increased intra-abdominal pressure
        • Ascites, bowel edema, gas insufflation.
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