17
Table 3. Lung Function Indices Capable of Differentiating
Extrathoracic from Intrathoracic Obstruction in Adults
Extrathoracic Obstruction Intrathoracic Obstruction
Fixed Variable
PEF Decreased Normal or
decreased
Decreased
Forced inspiratory flow
(FIF)
50
Decreased Decreased Normal or decreased
FIF
50
/Forced expiratory
flow (FEF)
50
~1 < 1 > 1
Table 2. Summary of Types of Spirometrically Defined and
Lung Volume Defined Ventilatory Impairments
Ventilatory
Impairments Patterns
Restriction • FVC < 5th percentile.
• Reduced FVC does not prove restrictive impairment but may be
suggestive of restriction when FEV
1
/FVC is normal or increased.
• Low TLC from single breath test not reliable, especially with low
FEV
1
/FVC.
• A normal FVC usually excludes restriction.
Obstruction
• FEV
1
/FVC < 5th percentile.
• Decrease in flow at low lung volume may reflect small airway disease
in individuals.
• Concomitant decrease in FEV
1
and FVC most commonly due to poor
effort but may reflect airflow obstruction. Recommend lung volumes.
• Measurement of absolute lung volumes may assist in diagnosis and
assessment of hyperinflation.
• Measurement of airflow resistance may assist in diagnosis.
Mixed
FEV
1
/FVC and TLC both < 5th percentile.
