INTRODUCTION
Preterm birth, defined as birth at less than thirty-seven weeks gestation, is the leading cause of mortality in children under five years of age and an important cause of morbidity, associated with prolonged hospitalizations and multisystem complications that persist throughout childhood and into adulthood1. Rates of preterm birth have increased globally over the past twenty years and currently preterm births account for more than 11% of livebirths2-3. Bronchopulmonary dysplasia (BPD) is a major complication of preterm birth and the most common cause of chronic lung disease in infancy. Based on the 2001 National Heart, Lung, and Blood Institute (NHLBI) definition4, it is estimated that up to 50,000 infants develop BPD each year in the United States5-6. While advances in neonatal care and technology, such as gentler ventilation strategies, exogenous surfactant, and the widespread use of postnatal steroids, have led to improved survival and outcomes for these infants, the incidence of BPD, which is defined as the need for supplemental oxygen for at least twenty-eight days after birth, has remained unchanged6.
Several previous studies, including a meta-analysis by Kotecha et al. 7, have shown that preterm patients, and especially those with BPD, display significant lung function impairment later in life compared to their term peers. These individuals are at an increased risk of respiratory morbidity later in childhood, with increased respiratory symptoms, structural abnormalities, and decreased lung function, primarily characterized by airway obstruction, altered peripheral lung mechanics, and hyperinflation8. A study of two hundred preterm children in Australia, including 126 with BPD and 74 without BPD, demonstrated that lung function trajectories in survivors of preterm birth are impaired and those with BPD, ongoing respiratory symptoms, or findings on chest computed tomography (CT) have the poorest trajectories, and are at the greatest risk of developing significant lung disease in adulthood8. Similar results were appreciated in a longitudinal observational study of the EPICure cohort9, a cohort of extremely low birth weight infants which showed that individuals displayed impaired lung function and increased respiratory morbidity that persisted into middle childhood, defined as eleven years of age, with findings especially notable in infants with BPD. Interestingly, this impairment was appreciated in infants born in both the pre-surfactant era as well as the modern era during which new management techniques have been employed10-11. These findings all suggest that long-term follow-up is especially important in this population given the concern for chronic obstructive pulmonary disease during adult life. This population has remained steady and even grown in size in the past several years12, and as a result, the global burden of lung disease following preterm birth is set to rise which has significant implications for adult providers and the health care system as a whole.
Overall, however, little is known about the potential modifiable predictors of lung function in this population specifically during the first two years of life, a period of postnatal life in which alveolar growth occurs most rapidly13. One retrospective study of 322 preterm infants with BPD showed that the initial severity of an individual’s BPD was an important predictor of pulmonary function abnormalities (at eight years old and then subsequently at fifteen years old) as well as health care usage during childhood14. The aim of this study was to investigate demographic and clinical characteristics as well as growth measurements at birth and at two years of age to determine what factors are most strongly predictive of lung function at six years of age. We hypothesized that in the outpatient setting, socioeconomic factors, growth parameters, and need for respiratory support would influence lung function at 6 years of age in children with a history of bronchopulmonary dysplasia.