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Lung volumes and alveolar expansion pattern in immature rabbits treated with serum-diluted surfactant

Department of Anesthesiology and Intensive Care Medicine, Graduate School of Medical Science, Kanazawa University, Kanazawa 920-8641, Japan

Submitted 29 September 2003 ; accepted in final form 8 June 2004

In acute respiratory distress syndrome, mechanical ventilation often induces alveolar overdistension aggravating the primary insult. To examine the mechanism of overdistension, surfactant-deficient immature rabbits were anesthetized with pentobarbital sodium, and their lungs were treated with serum-diluted modified natural surfactant (porcine lung extract; 2 mg/ml, 10 ml/kg). By mechanical ventilation with a peak inspiration pressure of 22.5 cmH₂O, the animals had a tidal volume of 14.7 ml/kg (mean), when 2.5 cmH₂O positive end-expiratory pressure was added. This volume was similar to that in animals treated with nondiluted modified natural surfactant (24 mg/ml in Ringer solution, 10 ml/kg). However, the lungs fixed at 10 cmH₂O on the deflation limbs of the pressure-volume curve had the largest alveolar/alveolar duct profiles (48,000 µm²), accounting for 38% of the terminal air spaces, and the smallest (<6,000 µm²), accounting for 31%. These values were higher than those in animals treated with nondiluted modified natural surfactant (P < 0.05). We conclude that administration of serum-diluted surfactant to immature neonatal lungs leads to patchy overdistension of terminal air spaces, similar to the expansion pattern that may be seen after dilution of endogenous surfactant with proteinaceous edema fluid in acute respiratory distress syndrome.

acute respiratory distress syndrome; alveolar overdistension; dynamic surface tension; positive end-expiratory pressure