INTRODUCTION
Confirming the diagnosis of asthma in children by demonstrating variable
expiratory flow limitation, expressed by a significant bronchodilator
response (SBDR) or the presence of bronchial hyperresponsiveness (BHR)
to methacholine or exercise, avoids over and underdiagnosis(1–4). The finding of an SBDR in spirometry, defined
as a bronchodilator change ≥ 12% in FEV1, confirms the
diagnosis of asthma in a child with clinical suspicion, without the need
for other studies (4).
The methacholine test (MT) measures BHR directly and has high
sensitivity and low specificity for the diagnosis of asthma, which
increases with lower PC20 (methacholine concentration that produces a
20% drop in FEV1) (2). A PC20 between
4 and 8 mg/dl is considered borderline, and below 4 mg/dl, the lower the
PC20, the greater the possibility of diagnosing asthma in patients with
clinical suspicion of this disease (5,6).
The exercise test (ET) is an indirect bronchial provocation test, which
has low sensitivity and high specificity for the diagnosis of asthma. A
positive ET is considered to be a drop in FEV1 ≥ 10%
post-exercise (7).
Although SBDR in spirometry, a PC20 ≤ 2 mg/dl and a positive ET measure
different pathophysiological aspects, all three indicate with high
probability the presence of asthma (1,2,7).
In recent years, the value of FEF25-75 and forced
expiratory flows have been questioned in the interpretation of
spirometry, because flow values depend on the volume at which they are
measured and have high variability (8).
Spirometry has low sensitivity for the diagnosis of asthma(9). Frequently, spirometry in children with suspected
asthma is often normal, without SBDR in FEV1, but with
SBDR in FEF25-75 (>30%), without a
>5% change in FVC (which indicates that the curves pre and
post bronchodilator were measured at similar volumes). This result, in a
child with clinical suspicion of asthma, raises doubts in its
interpretation, due to the publications that suggest that the
FEF25-75 does not contribute to clinical decisions(8,10). Despite this, this parameter is still
frequently used (11).
We postulate that if the only SBDR in FEF25-75 in the
spirometry, of children with suspected asthma, indicates a reversible
obstruction of the airway that allows to certify the diagnosis, it will
be more frequently associated with a BHR in the MT and/or ET than , in
children with normal spirometry, without SBDR in
FEF25-75.
Due to the fact that the evidence is not conclusive regarding the role
of FEF25-75 for the evaluation of spirometry, this study
aims to relate SBDR to FEF25-75, when
FEV1 and FEV1/FVC are normal, with MT
and/ET, in children aged 5 to 15 years with clinical suspicion of
asthma.