Updated: 7/12/2019

Pulmonary Function Tests

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Snapshot
  • A 23-year-old man presents to the emergency department with weakness.  He states his legs were weak yesterday but now he is also having trouble breathing.  He recently recovered from a diarrheal illness.  The patient's forced vital capacity is significantly reduced and he is subsequently intubated (Guillain Barre syndrome).
Introduction
  • Clinical definition
    • spirometry
      • measures volume of expired air during forceful and complete exhalation
        • forced vital capacity (FVC)
          • total exhaled volume after a maximal inspiration
        • forced expiratory volume in 1 second (FEV1)
          • the volume exhaled in the first second of maximal expiration
        • FEV1:FVC ratio
          • low = obstructive disease
            • air can not get out quickly
            • concerning for obstructive disease (such as asthma)
          • normal/high = restrictive disease
            • no obstruction of airway
            • concerning for restrictive disease (such as pulmonary fibrosis)
      • flow-volume loops
        • plot out spirometry findings
          • normal 
          • obstructive disease 
          • restrictive disease 
    • bronchoprovocation challenge
      • administration of bronchospastic agent (such as methacholine) followed by spirometry
        • can aid in diagnosis of asthma
Pulmonary Function Tests
 
Interpretation of Pulmonary Function Tests
Metrics Definition
Obstructive
Restrictive
Elasticity
  • Ease of recoil after lungs have expanded
Compliance
  • Ease of alveoli expansion
Volumes
Minute volume
  • Volume of air expired in 1 minute
  • =/
  •  
Residual volume (RV)
  • Air left in lung after max expiration
Total lung capacity (TLC)
  • All the air in the lung = vital capacity + residual volume
Tidal volume
  • Volume of air breathed in and out for a normal breath
Vital capacity
  • Total volume exhaled after full inspiration.
Spirometry
Forced expiratory volume (FEV1) normal = 4L
  • Volume expelled in one second
  • =/
Forced vital capacity (FVC)
normal = 5L
  • Entire volume that can be expelled
  •  =/

(FEV1)/FVC
normal = 80%

  • Considered best screen, values of less than 50% correlate with postoperative pulmonary complications.
  • =/



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