In a first ever experimental study to compare breathing reflexes of preemies born to smokers with those born to non-smokers, researchers in Canada found that premature babies whose mothers smoked during pregnancy are likely to be at a higher risk of Sudden Infant Death Syndrome (SIDS) than premature infants whose mothers did not.

The study was the work of principal researcher Dr Shabih Hasan and colleagues from the Department of Pediatrics and Institute of Maternal and Child Health, Faculty of Medicine, at the University of Calgary in Alberta, Canada, and is published in the first of the September issues of the American Journal of Respiratory and Critical Care Medicine. Hasan is a a staff neonatologist and professor in the Department of Pediatrics at the University.

Hasan and colleagues found that babies whose mothers had smoked while pregnant showed a number of signs of poor breathing. Smoking during pregnancy appears to deliver a double whammy, as Hasan explained:

"Not only does it raise a mother's likelihood of having a preterm baby, who is already among the most vulnerable to SIDS, but it increases the infant's susceptibility to SIDS even further."

Scientists already knew that low oxygen or hypoxia, coupled with an excess of carbon dioxide (hypercarbia), were predictors of SIDS, since babies at greatest risk of the syndrome tend to respond less effectively to low oxygen and/or excess carbon dioxide.

Babies born prematurely have greater difficulty breathing, which increases in proportion to how early they arrive. And cigarette smoke is known to cause apneas (when breathing stops) in full term babies. But the two effects had not been looked at together.

"Cigarette smoke exposure and preterm birth have not been investigated together with respect to their potential effects on respiratory dysfunction" said Hasan.

For the study, Hasan and colleagues recruited 22 preterm babies who had been born spontaneously, with no other complicating respiratory factors, between weeks 28 and 32 of pregnancy. 12 of the babies' mothers smoked 5 or more cigarettes a day during pregnancy, while the other babies' mothers did not (these babies were the control group).

Before starting the experiment (baseline), the researchers measured the breathing rate, pauses in breath, recovery period and heart rate of all the babies in both the cigarette smoke exposed group and the control group. They also measured how much oxygen was in their blood (oxygen saturation levels).

After this, they "challenged" the babies' breathing ability by putting a tube in their nose and giving them air with less than the normal amount of oxygen in it for five minutes, and took the readings again, under the poorer oxygen conditions. The babies were monitored closely, with resuscitation equipment on standby.

The results showed that:
  • Some of the breathing pattern readings were very similar in the group of babies born to mothers who smoked during pregnancy and in the group of babies whose mothers did not.

  • But the heart rate and recovery period differences between the two groups differed significantly.

  • The babies who had been exposed to cigarette smoke as fetuses showed increased heart rate while breathing oxygen poor air compared to when they breathed the normal air, while the babies who had not been exposed to cigarette smoke had the same heart rate under both conditions. (This suggested that lack of oxygen put the smoke-exposed babies under more stress than the control group babies).

  • Also, while blood oxygen went down the same in both groups under poor oxygen conditions, the babies who had been exposed to cigarette smoke did not recover as well, as quickly, or as often as the control group babies.
The researchers concluded that:

"We provide evidence of how prenatal CS [cigarette smoke] exposure and hypoxemic episodes affect the duration and recovery of breathing pauses in preterm infants. These observations could help explain why these infants are at a particularly high risk for sudden infant death syndrome."

Hasan commented that the study showed preterm babies sometimes experience incomplete or delayed recovery when their breathing is interrupted, and this obviously affects the risk of SIDS. But there is another reason to be concerned, even after this risk of SIDS has passed. As Hasan explained:

"Inability or delayed recovery from repeated low oxygen episodes can also be detrimental to brain development. There is increasing evidence that infants exposed to prenatal cigarette smoke are at high risk for developmental and behavioral disorders."

Hasan and colleagues speculated that nicotine interacts with particular and highly sensitive brain cell receptors that are involved in key processes that affect respiratory control. Another reason they suggested might be that changes to the way the lungs develop and work mechanically could lead to poor exchange of gas (when oxygen comes into, and carbon dioxide goes out of the lungs), which results in the poorer recovery observed in the babies who were exposed to cigarette smoke as fetuses.

Whichever explanation it might be, the findings are significant on their own, in helping to identify the babies that could be at higher risk of poor recovery when exposed to lower than normal oxygen levels at home.

Hasan said their study might also help to:

"Distinguish the infants, who will arouse in response to hypoxemia. The infants identified to be at risk can subsequently be further investigated and/or monitored at home," he said.

"Prenatal Cigarette Smoke Exposure Attenuates Recovery from Hypoxemic Challenge in Preterm Infants."
Jennifer Schneider, Ian Mitchell, Nalini Singhal, Valerie Kirk, and Shabih U. Hasan.
Am. J. Respir. Crit. Care Med. 178: 520-526.
First published online as doi:10.1164/rccm.200803-432OC

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Sources: American Thoracic Society.

Written by: Catharine Paddock, PhD