DISCUSSION
We found that children with suspected asthma and normal spirometry, with SBDR in FEF25-75, have greater bronchial hyperreactivity demonstrated in MT and/or ET than those who, having normal spirometry, do not have SBDR in FEF25-75. Despite this difference, we observed that the SBDR alone in FEF25-75 is not always accompanied by a HRB that can confirm the diagnosis of asthma with other exams, such as a PC20 ≤ 2 mg/dl in the MT and/or a positive ET. It was even possible to rule out the diagnosis of asthma in 14 children with SBDR in FEF25-75, due to a PC20 greater than 8mg/dl in the MT, which has a high negative predictive value for this pathology (4). This finding suggests that the SBDR alone in FEF25-75 would not be sufficient in all cases to establish the diagnosis of asthma, unlike what happens with the finding of SBDR in FEV1, which always confirms the diagnosis (4,17). We cannot rule out that these 14 results are false negative MT, which can occur when the patient is in an asymptomatic period or is on inhaled corticosteroids, which may have influenced the results of some children, although both conditions did not change between spirometry and the MT.
One of the reasons that may explain the lack of correlation between HBR and SBDR in FEF25-75 in some children studied, is that the spirometric curves before and after bronchodilator are not performed at isovolume, and that the SBDR in FEF25-75 responds to this change in volume and not to a real response, since the measurement of the flows are dependent on the lung volume at which they are measured(8). Serial measurements of FEF25-75 in the same individual can change due to the progression or improvement of lung disease due to changes in lung volume(8). Although only spirometries with a change of less than 5% between pre- and post-bronchodilator FVC values were included in the study to avoid this effect, it is not enough to demonstrate that the measurements were made at isovolume, since FVC is not only determined by the expired volume, but also the effect of gas compression during the forced expiration maneuver, which is more important in patients with airway obstruction (18).
Another factor that can determine a false SBDR in only FEF25-75 is the poor quality of the spirometric curves, especially when the child is not capable of performing a maximum effort, which would be more frequent at a younger age (8).
The diagnostic value of SBDR in FEF25-75, when the rest of the parameters are normal and there is no SBDR in FEV1, has not been described in the literature, although it has been suggested that a response of at least 30% in FEF25-75 is more sensitive for identifying asthmatics than a 12% response in FEV1 (9,19).
For a long time, FEF25-75 was considered a parameter with greater sensitivity than FEV1 to assess early small airway obstruction (SAO) in children (20). In recent years it has been shown that expiratory flows, including FEF25-75, are not very useful in clinical practice, both in adults and children (8,10). They are also not more sensitive in detecting SAO in children diagnosed with asthma and cystic fibrosis as previously described (10).
As reported in the literature, we found a very low sensitivity of FEV1 to determine variable airway obstruction in asthmatic children (9).
The MT presented a sensitivity of almost 70% to detect BHR. Although it is not a very specific test for diagnosing asthma, half of the children in the study with positive TM showed PC20 ≤ 2 mg/dl, which highly likely supports this diagnosis (2,5).
ET showed a lower sensitivity than that reported in the literature, which can be explained by the use of inhaled corticosteroids and the conditions of humidity (average 34.8%) and temperature (20.5 °C) in which it was performed (7,16). This is a finding to take into account for our pulmonary function laboratory, since it is performed very frequently and, in our study, it proves to have very little utility, since a negative exercise test does not rule out the diagnosis of asthma (7).
One of the limitations of this study is given by the lack of a subsequent analysis of the evolution of the patients to confirm or rule out the diagnosis of asthma and thus be able to relate it to the results of the studies carried out. Another limitation is that it is a retrospective study, in which the indication to request the MT and ET was not controlled and varied with the criteria of each pediatric bronchopulmonary who requested the studies in each patient.
We conclude that children with suspected asthma and normal spirometry, with SBDR in FEF25-75 studied here, have greater bronchial hyperreactivity than those who, having normal spirometry, do not have SBDR in FEF25-75. The SBDR in FEF25-75 is not always accompanied by an HRB that can confirm the diagnosis of asthma.
The sensitivity of the SBDR of the spirometry and exercise test for the diagnosis of asthma were very low, being higher for the MT. We believe that in our laboratory it would be more efficient to perform a MT than a TE to assess HRB in children with suspected asthma.