Inspiratory airway 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.
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.