8
Figures
FEV
1
/FVC
>5th percentile?
Possible restriction or
non-specific pattern
Normal spirometry
FVC >5th percentile?
Beginning with the ratio of FEV
1
/FVC, determine whether obstruction is present based on whether
the ratio is low (right side of figure). If obstruction is present, then assess the FVC to determine
whether there is simply obstruction or whether there may be concomitant restriction ("mixed
disorder"). Measurement of total lung capacity (TLC) will define restriction, so if TLC is normal,
then there is only obstruction, but if TLC is low, then there is concomitant restriction. If the FEV
1
/
FVC is normal, signifying no obstruction (le side of figure), then once again assess the FVC.
Figure 5. Approach to Interpretation of Spirometry
YES
YES
NO
Classification of Ventilatory Impairments Defined by Spirometry
a
FEV
1
FVC
FEV
1
/FVC
Obstructive impairments
Normal/
Normal
Restrictive impairments
Normal/
Non-specific pattern
Normal
Muscle weakness
Normal
Suboptimal effort
Normal
Mixed disorder
Dysanapsis Normal
Normal/
a
Reduced or elevated results are defined by the lower and upper limits of normal (ULN) respectively.
