Maximum inspiratory flow is determined by the forced developed by respiratory muscles and the resistance of the upper (extrathoracic) airways in particular. Therefore inspiratory flow is always effort dependent in healthy subjects. The inspiratory flow-volume curve normally looks more or less like a half circle. However, stenosis of upper airways may give rise to a flow limiting segment in extrathoracic airways (see reference below and illustration); inspiratory flow is then effort independent, giving rise to an inspiratory flow plateau (see the figure). Muscle weakness (e.g. myasthenia gravis) or pain may also limit inspiratory flow.
MIFx%FVC or FIFx%FVC in itself has not been shown to have clinical utility, unlike the shape of the MIFV-curve as a clue to inspiratory airway obstruction.
Recommended procedures: