Effect of surfactant on regional lung function in an experimental model of respiratory distress syndrome in rabbit

We assessed the changes in regional lung function following instillation of surfactant in a model of respiratory distress syndrome (RDS) induced by whole lung lavage and mechanical ventilation in eight anaesthetized, paralyzed, and mechanically ventilated New Zealand White rabbits. Regional specific ventilation (s(V) over dot) was measured by K-edge subtraction synchrotron computed tomography during xenon washin. Lung regions were classified as poorly aerated (PA), normally aerated (NA), or hyperinflated (HI) based on regional density. A functional category was defined within each class based on s(V) over dot distribution (High, Normal, and Low). Airway resistance (Raw), respiratory tissue damping (G), and elastance (H) were measured by forced oscillation technique at low frequencies before and after whole lung saline lavage-induced (100 ml/kg) RDS, and 5 and 45 min after intratracheal instillation of beractant (75 mg/kg). Surfactant instillation improved Raw, G, and H (P < 0.05 each), and gas exchange and decreased atelectasis (P < 0.001). It also significantly improved lung aeration and ventilation in atelectatic lung regions. However, in regions that had remained normally aerated after lavage, it decreased regional aeration and increased s(V) over dot (P < 0.001) and s(V) over dot heterogeneity. Although surfactant treatment improved both central airway and tissue mechanics and improved regional lung function of initially poorly aerated and atelectatic lung, it deteriorated regional lung function when local aeration was normal prior to administration. Local mechanical and functional heterogeneity can potentially contribute to the worsening of RDS and gas exchange. These data underscore the need for reassessing the benefits of routine prophylactic vs. continuous positive airway pressure and early "rescue" surfactant therapy in very immature infants.