Am. J. Respir. Crit. Care Med., Vol 154, No. 4, 10 1996, 918-923.
Changes in lung volume and static expiratory pressure-volume diagram after surfactant rescue treatment of neonates with established respiratory distress syndrome
LJ Bjorklund, CT Vilstrup, A Larsson, NW Svenningsen and O Werner
The Neonatal Intensive Care Unit, Department of Pediatrics, University Hospital, Lund, Sweden.
The effect of natural surfactant on respiratory system mechanics in infants
with respiratory distress syndrome (RDS) is incompletely understood,
possibly because the analysis has usually been confined to the tidal
breath. We studied 11 paralyzed neonates weighing 540 to 1,850 g before and
approximately 30 min after surfactant, which was instilled at 4 to 41 h of
age. Diagrams relating airway pressure to expired volume were obtained by
having the infant exhale passively through a flowmeter, starting at 30 and
ending at 0 cm H20 of pressure. An interrupter intermittently stopped the
flow so that pressure could be recorded under 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 pulmonary clearance delay (PCD). TLC increased by
10% or more in five infants, but it remained unchanged in the others.
Median TLC was 19 ml/kg before 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, the slope of the steepest segment, i.e., maximal compliance,
increasing from 0.65 to 1.22 ml/cm H20/kg (medians, p = 0.008). Dynamic
compliance (Cdyn) was unchanged at 0.28 ml/cm H20/kg, whereas specific
dynamic compliance (Cdyn/FRC) decreased (p = 0.04). There was no
significant immediate change in PCD. The findings imply that during the
first 30 min surfactant acted mainly by stabilizing already ventilated air
spaces.
