Improved Survival Accounts for Most, but Not All, of the Increase in Bronchopulmonary Dysplasia

¹ From the Department of Preventive Medicine, Division of Biostatistics, and the Department of Pediatrics, Division of Neonatology, Vanderbilt University School of Medicine, Nashville, TN.

The incidence of bronchopulmonary dysplasia (BPD) increased from 10.6% in 1976 through 1980, to 21.7% (1981 through 1985), and to 32.9% (1986 through 1990) in very low birth weight neonates (1500 g or less) admitted to the Vanderbilt Neonatal Intensive Care Unit, while there was a concurrent decline in incidence of neonatal death (NEOD) during the same periods (26.4%, 18.3%, and 15.9%, respectively). Population changes in risk factors (birth weight, sex, race, location of birth, gestational age, diagnosis of hyaline membrane disease, and 5-minute Apgar score) over time do not account for this increase. To estimate the proportion of the increase in BPD attributable to the concurrent decline in NEOD during these periods, separate logistic regression models for NEOD and BPD were calculated from patients born during 1976 through 1985. These results were used to predict the expected number of cases of NEOD and BPD during 1986 through 1990, assuming that the adjusted incidence of NEOD and BPD remained constant from 1976 through 1985, to 1986 through 1990. The increase in the combined outcome, NEOD/BPD, over the three time periods (34.2%, 36.1%, and 43.5%) remained statistically significant after adjustment for the risk factors listed above. During 1986 through 1990, the predicted number of NEOD was 83 more than the number observed, while the predicted number with BPD was 115 less than the number observed. If all 83 averted cases of NEOD during 1986 through 1990 had developed BPD, then 83 (72%) of the 115 excess cases of BPD could be attributed to averted NEOD. The approximate 95% confidence interval for this estimate was 58% to 87%. These results indicate that while most of the excess cases of BPD in 1986 through 1990 could be explained by neonates who previously would have died, there were still additional cases of BPD that were not explained by the risk factors included in this analysis.

Key Words: neonatal survival • bronchopulmonary dysplasia • statistical modeling

Submitted on February 24, 1992
Accepted on April 6, 1992

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