Chest wall compliance in infants and children with neuromuscular disease

Respiratory muscle weakness is the primary cause of respiratory dysfunction in neuromuscular disease (NMD), but structural abnormalities of the chest wall also play a role. In adults with NMD, restrictive lung disease is in part caused by reduced chest wall compliance (C(W)), believed to reflect stiffening of connective tissue resulting from chronically reduced chest wall motion in the presence of respiratory muscle weakness. We hypothesized that chronic limitation of chest wall motion in young children with NMD leads to structural underdevelopment of the chest wall, and results in increased, rather than decreased, C(W). In 18 subjects with NMD, ranging from 3 mo to 3.8 yr of age, we compared C(W) with values obtained in children without NMD. A modification of the Mead-Whittenberger technique was used, with respiratory muscle relaxation provided by brief manual ventilation. Respiratory system compliance (Crs) and lung compliance (C(L)) were calculated from airway opening pressure, transpulmonary pressure, and tidal volume. C(W) was calculated as 1/C(W) = 1/Crs - 1/C(L) during manual ventilation. C(W)/kg was higher in subjects with NMD than in controls, at 5.2 +/- 2.8 (mean +/- SD) versus 2.4 +/- 0.8 ml/cm H2O (p < 0.001). In subjects who had normal lung compliance values during spontaneous breathing (C(Lspont)), C(W)/C(Lspont) was significantly greater in subjects with NMD (5.5 +/- 3.2) than in controls (1.9 +/- 1.0) (p < 0.001). By predisposing to rib cage deformation and reduced end-expiratory lung volume, abnormally high C(W) in infants and young children with NMD may contribute to respiratory dysfunction.