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.