Notes
Lung Flow-Volume Loops
Flow Rate Changes in Disease
- Flow-Volume Loops plot inspiratory and expiratory flow against volume during maximally forced inspiration and expiration.
- First, we show that 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.
- Then, we show that, 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. - Show that, 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).