METHODS
Population and Study Design :
A retrospective chart review was performed using patients (n=598) who were recruited from an outpatient BPD clinic between January 2008 and February 2020 and were at least five years of age as of February 2020. All subjects were born at less than or equal to 36 weeks gestation and presented with respiratory symptoms. BPD severity was defined by the 2001 NHLBI workshop definition4. While at the time of enrollment in the study, all subjects were outpatients at a single center, they received their neonatal care at a variety of neonatal intensive care units (NICUs) across the state prior to referral to the outpatient BPD clinic for follow-up. All caregivers were consented per study protocol as approved by the Johns Hopkins University Institutional Review Board (Protocol NA_00051884).
Clinical Data :
Clinical data were obtained through chart review for birth parameters, clinical characteristics, growth parameters at both two and six years of age, and pulmonary function testing data obtained at six years of age. Bronchopulmonary dysplasia was defined as the need for oxygen supplementation for twenty-eight days of life and a “physiologic” assessment of the oxygen requirement at thirty-six weeks corrected age for determining severity4. Percentile for birth weight was corrected for gestational age using national norms15, and later height, weight, and body mass index measurements were expressed as quantitative data and percentile-for-age measurements based on CDC norms.
Pulmonary Function Testing :
Patients performed spirometry at approximately six years of age at a routine clinic visit when well and without symptoms of an acute respiratory tract infection. Testing was performed using MedGraphics Platinum Elite Series Plethysmograph at two different clinic locations and according to the American Thoracic Society (ATS) and European Respiratory Society (ERS) standards16. Patients were adequately instructed and trained before proceeding with testing. Forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) were recorded and adjusted for age, height, and race per protocol. Quantitative measurements as well as percent predicted values were recorded17. Spirometry measurements were accepted if they met the acceptability and repeatability criteria of the ATS and ERS16. All spirometry performed and used in analysis was interpreted by an attending board-certified pediatric pulmonologist.
Statistical Methods :
Chi-square and t -tests were used to compare demographic and clinical features between the portion of the population with pulmonary function testing data available, deemed the study population, and the remainder of the registry population to determine whether the study population was representative of the entire population of children presenting to BPD clinic. Linear regression analysis was performed employing both FEV1% predicted and FVC% predicted as dependent variables and individual predictors as independent variables in univariate models. Predictors were selected based on prior literature. Multivariate regression analysis was performed by dropping non-significant predictors identified as significant in univariate analysis in a stepwise fashion. STATA IC 14 (StatCorp LP, College Station, TX) was used for analyses. P -values <0.05 were considered statistically significant.