Mechanical ventilation effect on surfactant content, function, and lung compliance in the newborn rat

Studies of ventilator-associated lung injury in adult experimental animal models have documented that high tidal volume (TV) results in lung injury characterized by impaired compliance and dysfunctional surfactant. Yet, there is evidence that, in neonates, ventilation with a higher than physiologic TV leads to improved lung compliance. The purpose of our study was to evaluate how lung compliance and surfactant was altered by high TV ventilation in the neonate. We utilized a new model (mechanically air-ventilated newborn rats, 4-8 d old), and used 40 or 10 mL/kg TV strategies. Age-matched nonventilated animals served as controls. In all animals, dynamic compliance progressively increased after initiation of mechanical ventilation and was significantly greater than basal values after 60 min (p < 0.01). Lung lavage total surfactant with both TV strategies (p < 0.05) and the large aggregate fraction (only in TV = 40 mL/kg; p < 0.01) were significantly increased by 60 min of mechanical ventilation, compared with control animals. Ventilation with 40 mL/kg TV for 60 min adversely affected the lung surfactant surface-tension lowering properties (p < 0.01). After 180 min of ventilation with 40 mL/kg TV, the lung total surfactant content and dynamic compliance values were no longer distinct from the nonventilated animals' values. We conclude that, in the newborn rat, mechanical ventilation with a higher than physiologic TV increases alveolar surfactant content and, over time, alters its biophysical properties, thus promoting an initial but transient improvement in lung compliance.