CHANGES IN LUNG-VOLUME AND STATIC EXPIRATORY PRESSURE-VOLUME DIAGRAM AFTER SURFACTANT RESCUE TREATMENT OF NEONATES WITH ESTABLISHED RESPIRATORY-DISTRESS SYNDROME

The effect of natural surfactant on respiratory system mechanics in in fants with respiratory distress syndrome (RDS) is incompletely underst ood, possibly because the analysis has usually been confined to the ti dal breath. We studied 11 paralyzed neonates weighing 540 to 1,850 g b efore and approximately 30 min after surfactant, which was instilled a t 4 to 41 h of age. Diagrams relating airway pressure to expired volum e were obtained by having the infant exhale passively through a flowme ter, starting at 30 and ending at 0 cm H2O of pressure. An interrupter intermittently stopped the flow so that pressure could be recorded un der static conditions. FRC was measured by sulfur hexafluoride washout , and TLC was calculated from FRC and the pressure-volume (P-V) curve. Ventilation homogeneity was assessed from the washout curve as pulmon ary clearance delay (PCD). TLC increased by 10% or more in five infant s, but it remained unchanged in the others. Median TLC was 19 ml/kg be fore and 21.5 ml/kg after surfactant (p = 0.39). The P-V curve became markedly steeper at low pressures after surfactant in most infants, th e slope of the steepest segment, i.e., maximal compliance, increasing from 0.65 to 1.22 ml/cm H2O/kg (medians, p = 0.008). Dynamic complianc e (Cdyn) was unchanged at 0.28 ml/cm H2O/kg, whereas specific dynamic compliance (Cdyn/FRC) decreased (p = 0.04). There was no significant i mmediate change in PCD. The findings imply that during the first 30 mi n surfactant acted mainly by stabilizing already ventilated air spaces .