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.