• Paediatric Lung Disaese

Dear editors

We thank Drs Rosewich, Eckrich and Zielen for their interest and comments1 on our recent Thorax publication.2

We entirely agree that lung growth is clearly related to somatic growth. The main focus of the paper was based on the data obtained from 200 spirometries and not in lung volumes because they were not available in all patients.

There is no contradiction between the rise in FVC and FEV₁ values and the conclusion that pulmonary function remained severely impaired. As patients had an extremely low initial lung function, despite their increment, lung function remains extremely low. Moreover, an annual decrease in percentage of theoretical values of total lung capacity and residual volume (12±4% and 12±11.6%/year, respectively) was observed.

We understand that there are different ways to inform lung function related to growth. Based on our large data obtained after a 12-year follow-up period and in order to be as clear as possible, we decided to analyse it using generalised linear mixed effects models, as shown in figure 1 in the paper. We presented the data of the lung function as a relationship between pulmonary function values and the development of height to show that FVC and FEV₁ values increased at the same time as they grew up from 5 to 20 years of age. This way of showing the data emphasises that lung growth is clearly related to somatic growth, thereby avoiding misunderstanding. This study allows us to determine the annual rate of FEV₁ and FVC growth as it was done for other chronic lung diseases like bronchopulmonary dysplasia.3