Inspiratory obstruction
Inspiratory
airway obstruction is best assessed during a forced inspiration.
It leads to a typically shaped flow-volume curve. Upon forced
inspiration from residual volume (RV) inspiratory flow first
increases rapidly but then reaches a plateau. Often inspiratory
flow then declines progressively. A ‘normal’,
unimpeded inspiratory maneuver is not associated with a plateau.
The forced inspiratory volume in 1 second (FIV1) is a useful index to assess inspiratory flow limitation. The ‘normal range’ of FIV1 has not been studied well; according to one source it should be more than 80% of the IVC (see references).
Inspiratory obstruction occurs predominantly in diseases associated with extrathoracic airway narrowing, as this causes the intraluminal pressure to drop upon inspiration. This may lead to dynamic airway compression during inspiration (an inspiratory check valve), in particular when a tumor or swelling (e.g. from the thyroid) compresses the airway. Even so intrathoracic airway obstruction may also lead to diminished inspiratory flow, e.g. some forms of bronchitis.
Reference
values for FIV1
Tammeling GJ. Standard values for lung volumes and ventilatory
capacity of sanatorium patients. Selected Papers. Royal Neth
Tuberc Ass 1961; 1: 65-89.