How to express the reversibility of bronchial obstruction?

Since the original description of the "epreuve de bronchodilatation par aerosols adrenaliniques" [1], testing the reversibility of bronchial obstruction has become one of the most common measurements in the pulmonary function laboratory. It is performed for clinical assessments as well as for fundamental investigations. However there are still many questions and controversies on the technical aspects of the tests, the type of analysis and the clinical relevance of the results. One of the most common ways of expressing the bronchodilating response is to quantify the change in forced expiratory volume in one second (FEV1) as a percentage of the basal obstruction (prebronchodilator or initial FEV1). In recent years, this mode of analysis have been challenged as reviewed by MESLIER and RActNBUX [2) for the following reasons: 1) this expression implies that the variation is related to the initial value, which has been shown to be false for FEV,; 2) it gives an unjustified advantage to low initial values; and 3) it is affected by the error related to the tendency of any result to regress towards the mean. Most of these criticisms also apply to the index recommended by the Societas Europaea Physiologiae Clinicae Respiratoriae (SEPCR) in 1983 [3] where the change in FEY 1 is expressed as a percentage of the mean between the initial and the final values of FEY,. A number of studies [4-7] have supported these criticisms through the analysis of the bronchodilator response of patients with asthma and chronic obstructive pulmonary disease (COPD). For several years. there has been a tendency to substitute this i11dex of bronchodilation (change in FEV, as a percentage or the initial FEV,) with another expressiou of bronchodilation: the change in FEY , as a percentage of the predicted value of FEY 1• EtlASSON and D EGRi\11> r4J. in a large review of published data. and MEsuER and eo-workers [2, 5] have shown that this index could be used to distinguish between patients with asthma and those with COPD and that it was not significantly correlated to initial FEY,. However, one must keep in mind the possibility that, at least in some patient groups, there may be a real negative correlation between response and baseline spirometry.