Tests of healthy preterm infants younger than 12 weeks of age show prematurity to be independently associated with reduced lung function. Predictors of this reduced expiratory flow during the first months of life include male sex, low gestational age and increased weight gain.
The results appear in the second issue for February 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
Marcus H. Jones, M.D., Ph.D., of the Department of Pediatrics at the Hospital Sco Lucas in Porto Alegre, Brazil, and four associates examined 62 preterm infants who had no significant neonatal respiratory disease. They also tested 27 full-term infants as controls. Both the preterm and full-term groups included more females than males.
"There was a noticeable reduction in expiratory flows in the preterm groups compared with control infants and reference values," said Dr. Jones.
Using a model that adjusts for differences in body size, the investigators found male sex and lower gestational age significantly and independently associated with reduced flows in the preterm group.
"The increased risk of wheezing, chronic cough and hospital readmissions early in life suggests that some degree of airway obstruction is present even in preterm infants without neonatal respiratory distress," said Dr. Jones.
The researchers defined "healthy preterm infants" as those who had not required significant ventilatory support after birth and had not suffered prior lower respiratory infections. A minority of the premature infants in the study cohort required supplemental oxygen in the first two days of life.
"This study demonstrates an independent effect of gestational age on expiratory flows," said Dr. Jones. "There is up to a 7 percent increase per week of gestation. It also reinforces the hypothesis that prematurity alone has an important role in the development of persistent airway obstruction."
He added that lower expiratory flows in boys have also been observed in other lung function studies on full-term infants.
"However, in our sample, after adjusting for length, gestational age and weight, flows were up to 30 percent lower in males, a greater effect than previously reported in full-term infants," said Dr. Jones. "One could speculate, from the magnitude of lung function loss, that the described disadvantage of male infants in relation to respiratory disorders is heightened by prematurity. This might contribute to the increased neonatal morbidity and mortality related to respiratory illness for preterm male infants."
They also noted that in full-term and preterm infants, exposure to cigarette smoke while in the womb was not associated with lower air flows in their sample. "This absence of association was unexpected and may reflect the limitation of self-reported smoking among mothers and possible misclassification," Dr. Jones explained.
The investigators concluded that understanding the mechanisms associated with lower expiratory flow in preterm infants, particularly in the first year of life, could have potential implications for the prevention of respiratory diseases in this age group.