Objective: In normal infants, functional residual capacity using plethysmography (FRCpleth) exceeds FRC(N2) (using nitrogen washout) and the within-subject difference FRC(pleth-N2) indicates the part of pulmonary gas volume not ventilated during tidal breathing. We postulated that infants with bronchopulmonary dysplasia (BPD) have an elevated FRC(pleth-N2).
Design: In a prospective study, FRC(pleth-N2) in healthy newborns (controls n = 17) was compared to that in neonates recovering from the respiratory distress syndrome without BPD (noBPD, studied at discharge, n = 13) and with BPD (BPD1 n = 14, studied at 36 post-conceptional weeks; BPD2 n = 16, at discharge).
Measurements and results: Paired measurements, FRCpleth and FRC(N2), were performed using a plethysmograph (Jaeger, Germany) and an open-circuit nitrogen washout technique (SensorMedics 2600, USA). In comparison to the controls (5.2 ml/kg), FRC(pleth-N2)/kg in noBPD (12.7 ml/kg, p < 0.001), in BPD1 (24.9 ml/kg, p < 0.001) and in BPD2 (13.5 ml/kg, p < 0.001) was significantly higher. Compared with that of the controls FRCpleth was significantly increased in BPD1 (p < 0.001) and FRC(N2) was decreased in noBPD and both BPD groups.
Conclusion: The FRC(pleth-N2) is a reliable indicator for pulmonary inhomogeneities in infants with respiratory diseases.