First of all, we would like to thank the authors of the letter above for their comments on our article entitled "Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome" (OSAS).(1) In the article, we showed that the patients in our sample, which consisted of obese individuals with untreated OSAS, had reduced lung function, reduced inspiratory muscle strength, and reduced physical capacity. In addition, we found that, in these patients, reduced lung function, but not reduced respiratory muscle strength, was associated with reduced shuttle walk distance. However, no correlation was found between lung function or respiratory muscle strength and six-minute walk distance (6MWD).

With regard to the comment made by those authors regarding the study design, which is cross-sectional and observational and therefore is not the most appropriate for establishing a cause for the reduction in strength and lung function found in our patients, we agree with it. In fact, we consider this to be one of the limitations of our study. We suggested that, in order to establish causality accurately, studies with greater methodological rigor, such as randomized clinical trials, should be performed, given that our study investigated only one group of obese individuals with OSAS and there were no groups for subsequent comparisons. With regard to the presence of comorbidities, this is a common finding in patients with OSAS. The intermittent episodes of hypoxia and reoxygenation present in OSAS can promote oxidative stress associated with the release of inflammatory markers, contributing to the emergence of comorbidities and of consequences for peripheral and cardiorespiratory muscles, and this can directly affect exercise tolerance. As for the authors' remarks regarding the use of body mass index to assess obesity, we agree that this is not the most reliable method to classify obesity because it does not take body composition into account. Nevertheless, according to an editorial in the BMJ in 2018,(2) body mass index remains the most commonly used and widely accepted measure of obesity in adults and children, as well as having a strong correlation with gold standard measures of body fat.

Finally, regarding the comment on the lack of correlation between lung function and 6MWD, we would like to emphasize that the correlation between the two variables is not associated with the fact that the six-minute walk test has been validated in obese subjects and found to be reproducible, as highlighted in the letter above. What a lack of correlation tells us, from a statistical point of view, is that a change in the value of an independent variable (i.e., lung function) did not cause changes in the value of a dependent variable (i.e., 6MWD). Similar results have also been reported by Ferreira et al.,(3) who found no correlations between lung function and 6MWD when analyzing obese children and adolescents.

In conclusion, we would like to thank once again the authors of the letter above for the continuing discussion about the methodological aspects and results of our paper,(1) making it possible to broaden the debate on OSAS, a topic that is so current and important in the field of respiratory and sleep medicine.

REFERENCES

1. Carvalho TMDCS, Soares AF, Climaco DCS, Secundo IV, Lima AMJ. Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome. J Bras Pneumol. 2018;44(4):279-284. https://doi.org/10.1590/s1806-37562017000000031
2. Is BMI the best measure of obesity? BMJ. 2018;361:k2293. https://doi.org/10.1136/bmj.k1274
3. Ferreira MS, Mendes RT, de Lima Marson FA, Zambon MP, Paschoal IA, Toro AA, et al. The relationship between physical functional capacity and lung function in obese children and adolescents. BMC Pulm Med. 2014;14:199. https://doi.org/10.1186/1471-2466-14-199