Correlation of lung mechanics with saturated phosphatidylcholine ratios and surfactant protein A in bronchoalveolar lavage fluid from infants with RSV induced respiratory failure

Aim

Infants with respiratory syncytial virus (RSV) induced respiratory failure have been shown to be deficient in surfactant, both in quantity and ability to reduce surface tension. Theoretical evidence suggests that surfactant may have a role in maintaining patency of small airways, which has implications for RSV bronchiolitis. In addition, a minimum ratio of surfactant protein A (SPA) to fully saturated dipalmitoylphosphatidylcholine (DPPC) must be present for surfactant to be fully functional. We wished to investigate the relationship between (a) lung mechanics and SPA/DPPC ratios, and (b) lung mechanics and the ratio of DPPC to the monounsaturated palmitoyloleoylphosphatidylcholine (POPC) in bronchoalveolar lavage (BAL) fluid from ventilated RSV positive infants.

Methods

Nineteen infants were studied, median age 7 weeks (range 3–25 weeks), median weight 4 kg (range 2–6 kg). BALs were taken within 24 h of commencing mechanical ventilation. BAL surfactant phospholipid composition was detected by electrospray ionization mass spectrometry, and SPA levels by enzyme-linked immunosorbent assay. Static respiratory system compliance and resistance were measured using the single breath occlusion technique with a commercially available device (PEDS^® 4.1, Medical Associated Services, USA).

Results

The median (interquartile range) values for compliance 0.36 (0.31–0.47) ml/cmH₂O/kg, and resistance 234 (184–277) cmH₂O/l/s. The DPPC/POPC ratio correlated significantly with both compliance and resistance (see Figure). There was no correlation (neither linear nor exponential) between SPA/DPPC ratio and lung mechanics (compliance: r = 0.1, resistance r = 0.24).

Conclusion

Surfactant containing higher ratios of saturated phosphatidylcholine has a role in maintaining compliance and small airway patency in RSV infected infants. Although a certain minimal level of SPA is necessary for surfactant function, additional benefit is not seen with increasing SPA/DPPC ratios. These findings have implications for exogenous surfactant supplementation in this disease.