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.