To understand the mechanisms behind improved oxygenation after intratracheal surfactant instillation, the immediate and late effects on lung volume and compliance of the respiratory system (CRS) were analysed. Infants received modified porcine surfactant (Curosurf) or modified bovine surfactant (Alveofact). Measurements of functional residual capacity (FRC) and CRS were successfully performed in 90 ventilated preterm infants (birth weight 1264 +/- 435 g; gestational age 28.2 +/- 2.5 weeks) with severe respiratory distress syndrome. FRC and CRS were measured during mechanical ventilation prior to and 1, 3, 6, 24, 48, 72, 96, 120 and 168 h after surfactant replacement. Oxygenation rapidly improved. FRC increased significantly from 7.64 +/- 1.58 ml/kg to 15.35 +/- 3.27 ml/kg (P < 0.01) at 1 h after surfactant instillation. CRS remained virtually unchanged during the first hours after surfactant replacement and a concomitant decrease in specific compliance was seen.
Conclusion: the changes in lung function following surfactant treatment can only be explained by initial stabilisation of already aerated alveoli followed by recruitment of new gas exchange units as mechanisms involved in mediating the effect of surfactant on gas exchange. However, since no significant correlation between changes in functional residual capacity and improvement in arterial-to-alveolar oxygen tension ratio was seen, other effects of surfactant must be considered. These include local and/or systemic changes in haemodynamics.