MIFx%, maximum inspiratory flow at x% of the FVC
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.
Inspiratory flow limitation
- Miller RD, Hyatt RE. Obstructing lesions of the larynx and trachea. Clinical and physiologic characteristics. Mayo Clin Proc 1969; 44: 145-161.
- Miller RD, Hyatt RE. Evaluation of obstructing lesions of the trachea and larynx by flow-volume loops. Am Rev Respir Dis 1973; 108: 475-481.
- Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Official Statement of the European Respiratory Society. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.