Extrathoracic airway obstruction
Clinical signs develop when the extrathoracic
trachea is occluded by 50% or more. Flow limitation may also
occur by pathological processes in the subglottis and vocal
cords. In these instances a forced inspiration leads to a
large pressure drop across the obstructing lesion, so that
intratracheal pressure falls far below that of surrounding
tissue. This then leads to inspiratory narrowing of the trachea
downstream from the obstruction, and to flow limitation.
The inspiratory flow-volume curve in the second example,
rather than being semicircular, now displays a flat 
plateau.
In the case of variable extrathoracic obstruction the shape
of the maximum expiratory flow-volume curve is still normal.
A flow plateau during both inspiratory and expiratory forced
ventilatory maneuvers is indicative of fixed inspiratory and
expiratory airway obstruction.