Surfactant and physiologic responses of preterm lambs to continuous positive airway pressure.

Although continuous positive airway pressure (CPAP) is used frequently for preterm infants, the relationships between the amount of surfactant and lung physiologic and injury responses to CPAP are unknown. Therefore, saturated phosphatidylcholine (Sat PC) was measured to quantify the surfactant necessary for preterm lambs to breathe successfully on a CPAP of 5 cm H(2)O (CPAP 5). Five of 21 lambs delivered at 130-136 days gestation failed to keep PCO(2) below 100 mm Hg by 2 hours. The lambs that failed had less than 1.9 micromol/kg Sat PC in bronchoalveolar fluid (approximately 3% the pool size at term), less surfactant secretion, and less large aggregate surfactant. Physiologic responses of other 132-day preterm lambs after 2 or 6 hours of CPAP 5, 8 cm H(2)O CPAP (CPAP 8), or mechanical ventilation were then characterized. At 6 hours, oxygenation and lung gas volumes were higher with CPAP 8 relative to the other groups and VE was decreased with CPAP 8 relative to CPAP 5. Lung dry/wet ratios were greater for the CPAP groups than for the mechanical ventilation group. A small amount of endogenous Sat PC is required for preterm lambs to breathe successfully with CPAP. CPAP 8 improves early newborn respiratory transition relative to CPAP 5.