Results
The clinical and functional characteristics of the 295 asthmatic children are presented in Table 1. Three patterns were evident: tracking of lung function (null slope of FEV1% predicted change over time) in 68.8% of children (95% confidence interval (CI) [63.5%, 74.1%]), loss of lung function (negative slope) in 27.8% (95% CI [22.7%, 32.9%]), and gain in lung function (positive slope) in 3.4% (95% CI [1.3%, 5.5%]).
The children characterized by loss of lung function depicted a better initial lung function (highest z-scores of FEV1, FVC and FEV1/FVC) and a lower FEV1 variability (SD of the slope) during their follow-up than children with tracking or gain of lung function. At the last visit, these children were characterized by a lower lung function (lowest z-scores of FEV1, FVC and FEV1/FVC) and lower BMI than children with tracking or gain of lung function. At this last visit, these children had a better level of asthma control than the children with tracking of lung function, while receiving similar asthma treatment.
Overall, the children characterized by a gain in lung function evolved in an opposite way to those losing function. The Figure 1 shows changes in FEV1% predicted over time in the three different groups based on individual slopes and the Figure 2 shows the relationship between z-score of FEV1 at inclusion and individual FEV1 slopes in the three different groups.
Finally, we determined the number of asthmatic children with severe asthma (defined by airflow limitation): 45/295, 15%, 95% CI [11 to 19%], which was more frequent in children with a decline in lung function than in children with tracking of function (see Table 1). As compared to the 250 children without severe asthma, those with severe asthma had a lower initial FEV1 (87% ±14 versus 95 ±14, p<0.001), a lower initial z-score of FEV1/FVC (-1.67 ±0.88 versus -1.10 ±0.97, p<0.001) and a higher ICS dose at the last visit (423 µg ±223, n=34 versus 340 ±208, n=141; p=0.0418); the exacerbation frequency of these two groups was not different (p=0.367).