Lung function at 6 and 18 months after preterm birth in relation to severity of bronchopulmonary dysplasia

Many preterm infants with bronchopulmonary dysplasia (BPD) demonstrate impaired lung function and respiratory symptoms during infancy. The relationships between initial BPD severity, lung function and respiratory morbidity are not fully understood. We aimed to investigate the association between BPD severity and subsequent lung function and whether lung function impairment is related to respiratory morbidity.

In this longitudinal cohort study, 55 infants born preterm (23-30 weeks of gestation) with mild or moderate/severe BPD, based on oxygen requirement at 36 gestational weeks, were followed up at 6 and 18 months postnatal age. Respiratory symptoms, such as recurrent or chronic chough and wheeze, were noted and patient records were scrutinized. Lung function was assessed by passive lung mechanics, whole body plethysmography, and tidal and raised volume rapid thoraco-abdominal compression techniques. Results were related to published normative values.

Besides residual functional capacity (FRC) and respiratory system compliance (Cso ) assessed at 18 months, all measures of lung function were significantly below normative values. Moderate/severe BPD differed significantly from mild BPD only with respect to reduced Cso . At follow-up at 6 and 18 months, participants with respiratory symptoms showed lower; maximal forced expiratory flow at FRC (V'maxFRC) (P = 0.006, P = 0.001), forced mid-expiratory flows (MEF50 ) (P = 0.006, P = 0.048), and Cso (P = 0.004, P = 0.015) as compared to participants without symptoms.

In the present study BPD severity did not predict lung function, but may be associated with impaired alveolarization, indicated by reduced Cso . Respiratory morbidity was associated with reduced airway function and respiratory compliance in infancy after preterm birth.