Small for gestational age birth may increase airflow limitation in bronchopulmonary dysplasia.

Objectives: To determine significant indices for assessing the pulmonary function of infants according to bronchopulmonary dysplasia (BPD) severity and to evaluate whether small for gestational age (SGA) could affect pulmonary function in BPD.

Methods: We evaluated 117 preterm infants who had undergone tidal breathing flow-volume loop and multiple-breath washout analyses within 7 months after birth. We categorized preterm infants according to BPD severity into mild/moderate BPD (n = 86), severe BPD (n = 21), and without BPD (n = 10) and the presence of SGA or appropriate gestational age (AGA) using the Fenton growth chart. We evaluated nine healthy term infants as controls.

Results: The tidal breathing ratio (time to peak expiratory flow/expiratory time [t /t ]) was significantly lower in infants with severe BPD than in those with mild/moderate BPD. Lung clearance index (LCI) was not different based on BPD severity. In the correlation analysis after adjusting for gestational age and sex, t /t was correlated with the duration of mechanical ventilation (r = -0.347, P < .001) and the duration of oxygen supply (r = -0.248, P = .013) in infants with BPD. The proportion of "lower t /t ," defined as below the cut-off value, was greater in SGA infants (P = .017), while no significant difference was seen in the percentage of "higher LCI," defined as above the cut-off value between SGA and AGA infants.

Conclusions: In infants with BPD, t /t could be a useful pulmonary index which shows lower values in severe BPD. The finding of SGA in infants with BPD could be associated with poor pulmonary function related to the t /t values.