Table. 5—

Types of ventilatory defects and their diagnoses

ObstructionFEV1/VC <5th percentile of predicted
A decrease in flow at low lung volume is not specific for small airway disease in individual patients
A concomitant decrease in FEV1 and VC is most commonly caused by poor effort, but may rarely reflect airflow obstruction. Confirmation of airway obstruction requires measurement of lung volumes
Measurement of absolute lung volumes may assist in the diagnosis of emphysema, bronchial asthma and chronic bronchitis. It may also be useful in assessing lung hyperinflation
Measurements of airflow resistance may be helpful in patients who are unable to perform spirometric manoeuvres
RestrictionTLC <5th percentile of predicted
A reduced VC does not prove a restrictive pulmonary defect. It may be suggestive of lung restriction when FEV1/VC is normal or increased
A low TLC from a single-breath test should not be seen as evidence of restriction
Mixed defectFEV1/VC and TLC <5th percentile of predicted
  • FEV1: forced expiratory volume in one second; VC: vital capacity; TLC: total lung capacity.