If a woman goes into labor before her baby is full term, her obstetrician must make a crucial recommendation: delay labor or allow it to continue. Delivering the baby prematurely may increase the baby's risk of suffering from neonatal respiratory distress syndrome (RDS), a potentially fatal condition.
Now medical researchers at Washington University in St. Louis have generated new risk estimates for RDS that allow physicians to make delivery decisions with far greater confidence. They reported the results of their study in the January 2005 issue of the American Journal of Obstetrics and Gynecology.
RDS ranks as the sixth most common cause of death in newborns in the U.S. The syndrome occurs when newborn babies' lungs have not fully matured and lack the normal coating in the tiny sacs where air is exchanged. RDS causes difficulty breathing and not only threatens survival, but also poses a risk of long-term health effects caused by oxygen deprivation.
Because of the health risks associated with RDS, physicians who must consider delivering a preterm baby estimate lung maturity by testing the amount of the lung coating, or surfactant, present in amniotic fluid. Previous recommendations for prediction of RDS listed one surfactant level indicating mature lungs and another lower number indicating immature lungs, but that left a gray zone in the middle that required educated guesswork by physicians.
"Not only was there a gray zone, but when the surfactant tests were designed, they didn't consider the gestational age of the fetus," says senior author Ann Gronowski, Ph.D., associate professor of pathology and immunology and of obstetrics and gynecology. "Everyone suspected that the cutoff numbers changed over the weeks of gestation, which meant the old guidelines weren't adequate."
So, the Washington University researchers set out to create recommendations that took gestational age into account. "We felt decision-making could be improved by building a statistical model that showed the amount of risk associated with different surfactant levels at each week of gestation," says Curtis Parvin, Ph.D., associate professor of pathology and immunology and of biostatistics, who designed the statistical model used in the study.
The research group combined data from three studies of women and babies whose doctors had administered a test for amniotic surfactant shortly before delivery, a total of 509 cases. The data showed whether the newborns had RDS and what their gestational age was, as well as surfactant level.
The results of the statistical analysis were arranged into convenient tables and graphs to which physicians can refer to determine the potential risk of RDS based on both surfactant level and gestational age.
"For example, the absolute risk chart we built shows that at 34 weeks of gestation, a baby with a surfactant level of 20 has a 73 percent risk of RDS, while a baby showing a level of 60 has only a 3 percent risk," Parvin says. "You can also see that a surfactant level of 40 gives a 55 percent risk at 30 weeks and an 8.5 percent risk at 37 weeks. The old cutoff numbers didn't provide anywhere near as much information as these figures do."
"Physicians are faced with balancing potential risks to the mom and the baby," Gronowski says. "Say the mother has severe hypertension, for example, but it can be controlled. If the surfactant levels and gestational age indicate a high risk of RDS, the doctor can delay delivery. But if our estimates indicate that risk of RDS is relatively low, the doctor can decide to deliver right away to better protect the mother's health."
Having detailed information about the amount of risk of RDS for each situation gives physicians more choice in patient treatment, according to Gronowski. "I presented this data to our physicians last year. They started using the tables immediately and have been very pleased with them."
Parvin CA, Kaplan LA, Chapman JF, McManamon TG, Gronowski AM. Predicting respiratory distress syndrome using gestational age and fetal lung maturity by fluorescent polarization. American Journal of Obstetrics and Gynecology 2005;192:199-207.
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