Flow-volume curves: Fixed large airway obstruction
The inspiratory and expiratory flow-volume curves exhibit a flow plateau at low flows. Both the expiratory and inspiratory patterns are abnormal. The overall pattern is compatible with fixed obstruction in large extra- or intrathoracic airways, e.g. carcinoma of the larynx, or obstruction due to goiter. Also shown is the flow-volume curve during normal tidal breathing, and the MEFV-curve (interrupted line) for a healthy person of the same age, standing height and gender. It is generally assumed that obstruction is fixed if the ratio of forced expiratory and inspiratory flow halfway the FVC (MEF50/MIF50) is between 0.9 and 1.1.
Poor co-operation does not lead to reproducible curves. In general, therefore, if the patient appears to produce satisfactory efforts leading to a reproducible flow-volume curve with an abnormal pattern, take this seriously. Check carefully whether there is an inspiratory or expiratory stridor.