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.