Flow-Volume Loops plot inspiratory and expiratory flow against volume during maximally forced inspiration and expiration.
The normal inspiratory portion is convex and symmetrical; the expiratory portion usually shows a quick rise to peak flow rate, followed by a linear fall in flow until residual volume is reached.
In obstructive lung diseases, the loop shifts to the left, because lung volumes are higher than normal (air is trapped);
FEV1 decreases more than FVC, so FEV1/FVC is reduced.
In restrictive lung diseases, the loop shifts to the right, because lung volumes are lower than normal (lung compliance is reduced).
FEV1 and FVC decrease proportionally, so FEV1/FVC is normal (or elevated).
Be aware that these loops also provide information about the type of obstruction (variable intrathoracic, variable extrathoracic, or fixed).
For References, please see full tutorial on Respiratory Pathology Overview
