The Effect of Maternal Smoking During Pregnancy on Respiratory Responses To Changes in Inspired Levels of Oxygen in Infancy

Abstract □ 112

It is now well established that maternal smoking during pregnancy is a major risk factor for sudden infant death syndrome (SIDS). It has been suggested that deficits in respiratory control may be implicated in SIDS. Our aim was to test the hypothesis that infants born to mothers who smoke have a reduced ability to mount a ventilatory response to changes in inspired levels of oxygen compared to infants born to nonsmoking women. Women were recruited in early pregnancy and reports of smoking habits were verified by urinary cotinine levels. Forty women (17 smokers) were divided into 17 closely matched sets with each set containing one smoker and between 1-3 nonsmokers. All individuals within a set were matched for social class, maternal age and parity, feeding, infant's gender, birthweight and gestational age. At around 10 weeks of age respiratory control was measured overnight during quiet sleep using the alternating breath test, in which the infant breathed alternately 2 breaths of 40% O₂ and 2 breaths of 0%₂ for 2 minutes. Inspired and endtidal oxygen (E_TO₂) levels were measured by mass spectrometry and ventilation by Respiratory Inductance Plethysmography. The most striking finding was that even though the inspired levels of oxygen received were similar in the two groups the mean difference in E_TO₂ during the test was significantly higher (p = 0.022) in the smoking group (11.86% versus 10.61%). This may reflect differences in the relationship between the tidal volume and resting lung volume in the two groups. The table below presents the mean (SEM) % alternation in five respiratory parameters measured in the two groups. Although the smoking group had a tendency for lower responses these were not significantly different when matching and confounding factors were accounted for. (Table)

Table 1

In conclusion, although the infants born to smoking women had a greater change in E_TO₂ their respiratory responses were not greater than those of the nonsmoking group. These results suggest that infants born to smoking mothers do have slightly reduced respiratory responses to changes in inspired oxygen.