INTRODUCTION: Little is known about surfactant metabolism in infants as radioactive isotopes cannot be used in humans. We used the stable isotopes[U-13C]-palmitic (PA) and [U13C]-linoleic acid (LA) as precursors for endogenous surfactant phosphatidylcholine (PC) synthesis to study surfactant turnover in infants.

METHODS: We infused albumin bound U13C-PA and U13C-LA in 7 critically ill infants who were mechanically ventilated and received 20 kcal/kg/day of IV glucose and no oral feeding (weight 3.6±1.3 kg;, range 1.9-5.8; age 57±64 days, range 1-149). We measured by gas chromatography-mass spectrometry the 13C-enrichment of plasma free PA and LA and of PA and LA of surfactant PC obtained from tracheal aspirates (TA). 13C-enrichments of plasma free fatty acids were measured 5 times (t=0, 3, 6, 12, 18 and 24 hours)during the intravenous isotope infusion (24 h) and enrichments in surfactant PC every 12 hours till extubation. Kinetic data [fractional synthesis rate,FSR; secretion time,ST; time at peak enrichment,PEAK; and half-life,HL] were calculated from the enrichment curve of surfactant PC over time and are reported below in the table (mean±SD, no significant differences found).

Table 1 No caption available.
Full size table

All patients had steady state plasma PA and LA concentrations and enrichments.

CONCLUSIONS: This novel and safe method is applicable to the study of surfactant kinetics in human infants. The synthesis of surfactant from plasma PA and LA is a slow process and half lifes of surfactant PC were 67.9 and 53.3 hours for PA and LA respectively. Kinetic data with both fatty acids gave similar results. However marked differences were found between patients, possibly reflecting differences in type and severity of disease.