DISCUSSION
The results of our study confirm the findings of previous studies that
showed worse asthma control in children who are overweight or obese
compared to normal weight ones13-17. On the other
hand, airway resistance and reactance measured by IOS were not found to
be significantly different between these two groups suggesting a
mechanism apart from airway resistance or caliber underlying the
influence of obesity on asthma control.
Epidemiological studies have shown that obesity itself has a significant
effect on respiratory function, and some suggest that obesity is a risk
factor for development of asthma13. While body mass
index (BMI) has been shown to have little effect on spirometry results,
expiratory reserve volume and functional capacity are decreased in obese
people compared to normal weight people. Body mass index (BMI) has been
shown to be negatively related to various lung volume
measurements14. However, the relationship between
overweight and lung function in children is less clear despite
observation of increased FVC and FEV1 in overweight and obese
children15,16. More recently, the aggregation of
various cohorts from healthy and asthmatic children and adolescents has
confirmed that overweight or obesity is linked to higher than initial
FVC, TLC and FEV1 and decreasing maximum flows and FEV1 /
FVC17. In our study, we wanted to show the effect of
obesity on pulmonary resistance in asthmatic children. There are many
studies investigating the effect of obesity on spirometry in asthmatic
children, but patient compliance is required for spirometry and
interpretation may be difficult in younger children. Therefore, we
measured airway resistance by IOS that is a tidal breathing method not
requiring compliance, thus, allowing enrollment of younger children. We
found that the differences shown by spirometry in older age groups were
not present in resistance and reactance values measured by IOS. IOS
measures airway impedance, which is a function of resistance and
reactance, thus predicts about the peripheral airway obstruction and
detects inspiratory and expiratory changes in airway resistance in
asthmatic children7,18,19.
In adults, IOS was found to be correlated with spirometry results but
was not found to be correlated with asthma control20.
Measurements of R5Hz and AX have been found to be more sensitive to
bronchodilator response compared to FEV1 values in
children21. Similarly, in our study asthma control was
significantly worse in overweight/obese children but this was not
reflected on the IOS measurements of resistence.
Obesity is associated with more severe asthma symptoms, poorer asthma
control and poorer response to asthma treatment. This has been
attributed to many factors including but not limited to increased
oxidative stress, chronic inflammation and endothelial
dysfunction22-24. Similar to these previous
researches, our results confirmed that asthma control is worse in
overweight/obese children compared to normal weight ones.
Family history of allergic diseases is a well-known independent risk
factor for development of asthma in the child8,25. In
our study, family history of allergic diseases or asthma was not
significantly different between normal weight and overweight/obese
children. Lucas et al reported a higher rate of allergic sensitization
in obese children with asthma compared to normal weight
ones26. However, we did not detect a significant
difference in the rate of allergic sensitization in normal weight and
overweight/obese children suggesting that the association between body
weight and asthma is beyond increased inflammation related to obesity.
This difference might be related to the enrollement of overweight
subjects as well obese subjects in our study.
Major limitation of this study is the lack of IOS normal for our
population that precluded us from interpreting the deviation of the two
groups from normal. However, since we aimed to evaluate the effect of
being overweight or obese on airway resistance in asthma, we chose to
compare overweight and normal weight asthmatic children with each other.
The second limitation is the cross-sectional design which precluded us
from seeing the change in resistance with change in time and weight.
One of the strengths of our study was the enrollment of young children,
by the use of a tidal breathing method, IOS, to measure airway
resistance and reactance. IOS is a reliable method in young children who
can’t perform spirometry. Moreover, resistance and reactance
measurements at different frequencies allowed us to interpret about
different parts of the airways.
In conclusion, asthma control in children who are overweight or obese
are worse compared to the normal weight ones but, airway resistance and
reactance measured by IOS at different frequencies are not significantly
different between these two groups. This implies that airway resistance
change may not be the main pathogenetic mechanism underlying the
uncontrolled asthma and obesity coexistence. Further cohort studies,
looking at the change in resistance values with age and body mass index
in children will provide further insight about the relationship of
obesity and airway resistance in asthmatic children.