The MMEF (maximum midexpiratory flow) is the average expiratory flow over the middle half of the FVC. It should be taken from the blow with the largest sum of FEV1 and FVC (procedures, ref. 2). This index is very highly correlated with FEF50%FVC, so that one index offers no advantage over the other. The MMEF is in use as an index of airway obstruction, but this is fraught with hidden dangers:
- As MMEF = ½FVC/Δt (where Δt = time required to expire the middle half of the FVC), every disorder that affects the FVC will also influence the MMEF, just as in the case of MEFx%. During a forced expiration exhaled volume does not increase linearly with time; in the case of airway obstruction and reduced FVC the change in Δt, and therefore the severity of airway obstruction, is underestimated. It follows that the index is highly dependent on the validity of the FVC measurement and the level of expiratory effort.
- The reverse situation arises after bronchodilatation: the improvement in airway patency is underestimated by an increase in the FVC. It is even possible that the MMEF remains the same or even decreases when the FVC and the FEV1 increase appreciably after a bronchodilator drug. Assessing the MMEF isovolumetrically (at the same lung volume), i.e. by timing the interval taken to exhale the middle half of the largest FVC recorded before and after the intervention can circumvent this problem. Commonly available instruments do not provide facilities to this end.
These pitfalls also apply to longitudinal observations, unless the FVC would remain the same. An improvement in MMEF after administration of a bronchodilator, if not accompanied by an improvement in FEV1 and/or VC, should be disregarded (ref. 1). If MMEF is measured serially this should be done isovolumetrically (ref. 2), otherwise e.g. an improvement in FEV1 can be associated with paradoxical falls in MMEF. Another problem is that, depending on age, sex and ethnic group, the between subject coefficient of variation varies between 20-62% for FEF25-75% (ref.3), severely limiting its usefulness for clinical purposes.
|Ref. 1 - Isolated improvement in FEF25-75%|
|Berger R, Smith D. Acute postbronchodilator changes in pulmonary function parameters in patients with chronic airways obstruction. Chest 1988; 93: 541-546.|
|Ref. 2 - Assess serial measurements of FEF25-75% isovolumetrically|
|1||Olsen CR, Hale FC. A method for interpreting acute response to bronchodilators from the spirogram. Am Rev Respir Dis 1968; 98: 301-302.|
|2||Newball HH. The unreliability of the maximal midexpiratory flow as an index of acute airway changes. Chest 1975; 67: 311-314.|
|3||Sherter CB, Connolly JJ, Schilder DP. The significance of volume-adjusting the maximal midexpiratory flow in assessing the response to a bronchodilator drug. Chest 1978; 73: 568-571.|
|4||Stinson JM, McPherson GL, Darveaux R. Use of the isovolume FEF25-75% to assess small airway obstruction. Respir Care 1980; 25: 59-62.|
|5||Cockcroft DW, Berscheid BA. Volume adjustment of maximal midexpiratory flow: importance of changes in total lung capacity. Chest 1980; 78: 595-600.|
|6||Berger R, Smith D. Acute postbronchodilator changes in pulmonary function parameters in patients with chronic airways obstruction. Chest 1988; 93: 541-546.|
|Ref. 3 - Large coefficient of variation|
|Quanjer PH, Stanojevic S, Cole TJ, Baur X, Hall GL, Culver B, Enright PL, Hankinson JL, Ip MSM, Zheng JP, Stocks J and the ERS Global Lung Function Initiative. Multi-ethnic reference values for spirometry for the 3-95 year age range: the Global Lung Function 2012 Equations. Eur Respir J 2012; 40: 1324–1343.|
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.
Miller MR et al. Standardisation of spirometry. ATS/ERS task force: standardisation of lung function testing. Eur Respir J 2005; 26: 319-338.