airflow obstruction happens first in distal airways
- distal, smaller airways are impacted first when there's airflow obstruction
- this manifests as reduced airflow at lower volumes (end of expiration)
- this is why obstructive PFTs have characteristic appearance
- end of expiration tapers much slower while the first part (FEV1) is less affected
- FEF25−75% are sometimes used to describe this decrease in small airway flow between 25% and 75% of exhaled FVC (flow volume loops normal definition). However the measurements are widely variable and thus not used.1
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