Effect of single dose surfactant on pulmonary function

Sequential changes in pulmonary mechanics in response to single dose exogenous surfactant instillation were studied in 15 preterm neonates who had hyaline membrane disease (HMD). The infants were part of a larger double-blind national study. Birth weight ranged from 0.88 to 1.55 kg, and gestational age was between 27 to 32 wk. There were six infants in the surfactant group and nine in the placebo group. Pulmonary mechanics were studied before and at 2, 24, 60, and 96 h after surfactant or sham instillation using a pneumotachometer and an esophageal balloon catheter. The variables studied were dynamic compliance (Cdyn), pulmonary resistance, work of breathing, tidal volume, and minute ventilation. Infants in the surfactant group showed an immediate and significant (p less than .05) improvement in gas exchange ratio, decreased mean airway pressure (9.7 +/- 0.9 to 7.9 +/- 0.4 cm H2O) and airway resistance (133 +/- 6.3 to 92 +/- 14.9 cm H2O/L.sec) (p less than .05). Changes in Cdyn were noted only at 24 h after surfactant instillation. In the control group, gradual improvement occurred after the initial deterioration. The findings suggest that the immediate improvement in oxygenation after surfactant instillation is the result of factors other than changes in lung compliance, such as improved ventilation/perfusion and better capillary stability with decreased leakage of fluid into alveoli.