Timing and Driving Components of the Breathing Strategy in Children with Cystic Fibrosis During Exercise

The aim of this study was twofold: first, to determine the breathing strategies of children with cystic fibrosis (CF) during exercise, and secondly, to see if there was a correlation with lung function parameters. We determined the tension‐time index of the inspiratory muscles (T T0.1 ) during exercise in nine children with CF, who were compared with nine healthy children with a similar age distribution. T T0.1 was determined as followed T T0.1  = P 0.1 /P Imax  · T I /T TOT , where P 0.1 is mouth occlusion pressure, P Imax is maximal inspiratory pressure, and T I /T TOT is the duty cycle. CF children showed a significant decrease of their forced expiratory volume in 1 sec (FEV 1 ), forced vital capacity (FCV), and FEV 1 /FVC, whereas the residual volume to total lung capacity ratio (RV/TLC) ratio and functional residual capacity (FRC) were significantly increased ( P  < 0.001). Children with CF showed mild malnutrition assessed by actual weight expressed by percentage of ideal weight for height, age, and gender (weight/height ratio; 82.3 ± 3.6%). Children with CF showed a significant reduction in their P Imax (69.3 ± 4.2 vs. 93.8 ± 7 cmH 2 O). We found a negative linear correlation between P Imax and weight/height only in children with CF (r = 0.9, P  < 0.001). During exercise, P 0.1 , P 0.1 /P Imax , and T T0.1 were significantly higher, for a same percent maximal oxygen uptake in children with CF. On the contrary, T I /T TOT ratio was significantly lower in children with CF compared with healthy children. At maximal exercise, children with CF showed a T T0.1  = 0.16 vs. 0.14 in healthy children ( P  < 0.001). We observed at maximal exercise that P 0.1 /P Imax increased as FEV 1 /FVC decreased (r = −0.90, P  < 0.001), and increased as RV/TLC increased (r = 0.92, P  < 0.001) only in children with CF. Inversely, T I /T TOT decreased as FEV 1 /FVC decreased (r = 0.89, P  < 0.001), and T I /T TOT decreased as RV/TLC increased (r = −0.94, P  < 0.001). These results suggest that children with CF adopted a breathing strategy during exercise in limiting the increase of the duty cycle. Two determinants of this strategy were degrees of airway obstruction and hyperinflation. © 2005 Wiley‐Liss, Inc.