Flow-volume curves: Variable extrathoracic airway obstruction
The pattern of the expiratory flow-volume curve is normal, but the inspiratory flow reaches a low plateau value. This pattern is characteristic of variable extrathoracic airway obstruction (an upper airway abnormality which may cause airway obstruction), for example due to paralysis of the vocal cords. Typically the FVC and FEV1 are in the normal range, as there is no intrathoracic airway obstruction, and the high pressure in extrathoracic airways distends the airway (see literature), unless there is fixed obstruction. However, the FIV1 and FIV1/VC are typically abnormally low. Also shown is the flow-volume loop during normal tidal breathing.
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.
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Hyatt RE. Evaluation of major airway lesions using the flow-volume loop. Ann Otol Rhinol Laryngol 1975; 84: 635-642.