The influence of surfactant on functional residual capacity measured by nitrogen washout method in preterm newborns

Functional Residual Capacity (FRC) is an important lung function parameter which can be measured by the nitrogen washout method. The accuracy and reproducibility was tested in a lung model starting with two known volumes (22.0 and 70.0 ml) at six different oxygen levels (0.21, 0.3, 0.5, 0.7, 0.8, 0.9). The measurements of the FRC were highly accurate and reproducible in the range to 0.21-0.8. The study was followed by in vivo application in 20 premature infants (mean gestational age 30 weeks, mean birth weight 1.526 kg) with Respiratory Distress Syndrome (RDS) who received surfactant replacement therapy. FRC was measured five times: 1/2 hour before, 1/2 hour after, 1 hour after, 6 hours after receiving the first dose of surfactant, and before extubation. A significant increase in the FRC (p < 0.02) was found half an hour after the surfactant was administered, accompanied by a highly significant decrease in the FiO2 (p < 0.00001). The increase in the FRC is followed by a decrease in the initial value during the following six hours and before extubation, despite a persistent significant decrease of the Fraction of Inspired Oxygen.