Mucus rheology and transport in neonatal respiratory distress syndrome and the effect of surfactant therapy.

BACKGROUND Neonatal respiratory distress syndrome (RDS) is caused by a deficiency of pulmonary surfactant. Alveolar collapse and obstruction of conducting airways leads to mismatch of ventilation and perfusion and profound hypoxemia. We postulated that surfactant deficiency could alter the properties of respiratory mucus in such a way that it would be poorly cleared from airways and promote airway luminal obstruction and that these changes might be reversed by the exogenous administration of a synthetic surfactant preparation. METHODS Respiratory mucus coating an endotracheal tube (ETT) or suction catheter was collected from 14 neonates (gestational age, 24 to 36 weeks; birth weight, 600 to 2,400 g) with RDS who required tracheal intubation and ventilation. Eight of these neonates received 5 ml/kg of an intratracheal artificial surfactant preparation (Exosurf), given between 2 and 10 h of age and six neonates received 5 ml/kg of air. Mucus viscoelasticity, hydration (percentage of solid composition of mucus), and mucociliary clearability (NFPTR) were measured for each specimen. RESULTS The total volume of mucus collected from the surfactant-treated and control infants was similar, but mucus hydration was significantly less in babies with RDS who did not receive Exosurf (percentage of solid composition of mucus 18.7 vs 11.4; p = 0.013). Ciliary transportability was also less in the untreated babies (NFPTR 0.39 vs 0.86; p = 0.018) and this mucus was more rigid (increased viscoelasticity: log G* 1 rad/s 2.28 vs 1.50; p = 0.0001). CONCLUSIONS These data suggest that airway obstruction in RDS may be due, in part, to abnormal mucus properties and impaired ciliary transport. Surfactant therapy appears to improve mucus clearability. Exogenously administered surfactant may also be beneficial for the treatment of other selected respiratory conditions associated with impaired mucus clearance.