Surfactant Phospholipid Kinetics in Ventilated Children after Therapeutic Surfactant Supplementation

Abstract

Acute lung Injury leads to alterations in surfactant lipid composition and metabolism. Although several mechanisms contribute to dysregulated surfactant metabolism, studies investigating in vivo surfactant metabolism are limited. The aim of this study is to characterise surfactant phospholipid composition and flux utilising a stable isotope labelling technique in mechanically ventilated paediatric patients. Paediatric patients (<16 years of age) received 3.6 mg/kg intravenous methyl-D₉-choline chloride followed by the endotracheal instillation of 100 mg/kg of exogenous surfactant after 24 h. Bronchioalveolar fluid samples were taken at baseline and 12, 24, 36, 48, 72 and 96 h after methyl-D₉-choline infusion. Nine participants (median age of 48 days) were recruited. The primary phosphatidylcholine (PC) composition consisted of PC16:0/16:0 or DPPC (32.0 ± 4.5%). Surfactant supplementation resulted in a 30% increase in DPPC. Methyl-D₉ PC enrichment was detected after 12 h and differed significantly between patients, suggesting variability in surfactant synthesis/secretion by the CDP-choline pathway. Peak enrichment was achieved (0.94 ± 0.15% of total PC) at 24 h after methyl-D₉-choline infusion. There was a trend towards reduced enrichment with the duration of mechanical ventilation prior to study recruitment; however, this was not statistically significant (p = 0.19). In this study, we demonstrated the fractional molecular composition and turnover of surfactant phospholipids, which was highly variable between patients.

Keywords: intensive care; paediatric; phospholipids; surfactant; ventilation.

Figure 1

The bronchoalveolar lavage fluid phosphatidylcholine molecular composition at recruitment (T = 0) (A) and the fractional composition of major PC species over time (B). (N = 9 for time points 0 and 12, N = 8 for time points 24, 36, 48 and 72 h).

Figure 2

The total PC (A) and PC16:0/16:0 (B) methyl-D₉ enrichment variability between patients, summary total PC and fractional PC enrichment (C) and comparison with other PC species (D). (N = 9 for time points 0 and 12, N = 8 for time points 24, 36, 48 and 72 h).

Figure 3

The fractional composition of unlabelled and methyl-D₉-labelled PC at the first enrichment time point (T = 12 h) (A) and the compositional variation in unlabelled and methyl-D₉-labelled PC16:0/16:0 over time (B). Data are presented as the mean and standard error of the mean. Student t-test analysis, * p < 0.05. (N = 9 for time point 12, N = 8 for time points 24, 36, 48 and 72 h).

Figure 4

The gradient slope of the methyl-D₉ PC16:0/16:0 enrichment for patients (N = 8) (A) and the correlation (95% confidence intervals) between the gradient and the log-transformed duration of mechanical ventilation prior to methyl-D₉ choline infusion (B).

Figure 5

Study schedule. BALF, bronchoalveolar lavage fluid; PICU, paediatric intensive care unit.

Figure 6

Example BAL Mass Spectrometry data. (A,B) Typical BAL direct infusion mass spectrum. (A) The newly synthesised methyl-D₉-labelled PC species which have a diagnostic collision gas-induced product of m/z 193. (B) The endogenous PC species which have a diagnostic product of m/z 184.