Volume-Adjusting the FEF25–75%: When is Change Significant?

Abstract

Measurement of the FEF25–75% has proven to be an effort-independent, simply obtained and possibly more sensitive1McFadden ER Kiser R DeGroot WJ Acute bronchial asthma: Relations between clinical and physiologic manifestations.N Engl J Med. 1973; 288: 221-226Crossref PubMed Scopus (378) Google Scholar indicator of airflow obstruction. Simple equipment is all that is required and published data are available on normal subjects.2Morris JF Koski A Johnson LC Spirometric standards for healthy nonsmoking adults.Am Rev Respir Dis. 1971; 103: 57-67PubMed Google Scholar It can be seen in plots of maximum flow-static recoil curves that maximum expiratory flow rates may be reduced either by loss of elastic recoil with normal airways, or by airways obstruction with normal elastic recoil.3Mead J Turner JM Macklem PT Little JB Significance of the relationship between lung recoil and maximum expiratory flow.J Appl Physiol. 1967; 22: 95-108Crossref PubMed Scopus (643) Google Scholar Frequently, the combination of loss of elastic recoil and increased airways resistance is responsible for the reduced flow rates. Since there is little or no immediate change of elastic recoil with either bronchodilators or bronchoprovocation, spirometric testing before and after administration of a bronchodilator or bronchoprovocation attempts to measure changes in airways resistance only. By measuring flow over the same segment of volume, one is minimizing the variability of differing elastic recoil pressures which may obviate true changes in airways resistance.4Sherter 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: 568-571Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar In this issue of Chest, Cockroft and Berscheid (see page 595) have provided evidence that even small changes in total lung capacity (TLC) may cause large changes in volume-adjusted measurements of the FEF25–75%. The question that now needs to be answered is … Do the greater numerical changes observed in measurements of volume-adjusted FEF25–75% increase the sensitivity of this pulmonary function test? We4Sherter 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: 568-571Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar demonstrated that in 25 asthmatic patients given ephedrine, there was a statistical improvement in FEV1. FEF25–75% not corrected to the same volume segment did not change. When volume adjusted, the statistical changes in the FEF25–75% were greater than those for FEV1. These data suggest that volume adjusting will increase the sensitivity of measurements of the FEF25–75%. The variability and error of the method were not studied. In their article, Cockroft and Berschied attempt to identify more accurately the volume segment over which the volume-adjusted FEF25–75% should be measured. They note that small changes in total lung capacity (TLC) lead to large changes in volume-adjusted FEF25–75% There are some difficulties in their method. The accuracy of identifying TLC by body plethysmography requires measurement of two volumes. Functional residual capacity (FRC) is measured in the body box and has variability. Inspiratory capacity, a variable and effort-dependent measurement, is added to FRC to give TLC. If not carefully controlled, small and possibly insignificant errors in estimating TLC could yield large errors in the volume-adjusted FEF25–75% We agree that measurements of FEF25–75% should be adjusted to the same volume segment before and after both bronchodilation and bronchoprovocation. Further proof is needed that the variables of the technique have been identified and solved. More importantly, it needs to be demonstrated that even sensible manipulation of the FEF25–75% will give us more information than other simple measurements such as the FEV1. The authors caution us not to over-interpret small changes, but we do not know when a small change becomes a big change, or more importantly, a clinically significant change.