The study suggests that exposure to cigarette smoke in the womb causes impaired respiratory responses in early infancy to thermal stress (primarily overheating from too high temperatures or too much clothing) and hypoxia (low oxygen), which can occur when infants are put to sleep belly down, thus raising the risk of SIDS.
Although maternal smoking is considered a major risk factor for SIDS, a causal role has not been established, Dr. Shabih U. Hasan at the University of Calgary in Alberta, Canada, explains in a press release.
To investigate the compounding effects of cigarette smoking on other known risk factors for SIDS - namely thermal and oxygen stress - Hasan and colleagues exposed pregnant rat pups to either room air (control) or mainstream cigarette smoke equivalent to that a pack-a-day smoker would experience.
A total of 30 control and 39 cigarette smoke-exposed one-week-old rat pups were randomised to undergo either thermo-neutral or hyperthermic exposure to an oxygen-depleted environment. Hasan's team then analysed the respiratory responses to these challenges.
Overall, just 13 percent of the control animals exhibited gasping, whereas 36 percent of the cigarette smoke exposed animals did. None of the control animals exhibited gasping under low oxygen conditions during thermo-neutral experiments, whereas 25 percent of the cigarette smoke exposed animals did.
Under hyperthermic conditions, just 29 percent of the control group displayed gasping behaviour, compared to 49 percent of the cigarette smoke exposed group.
The investigators say these observations provide "important evidence" of how prenatal cigarette smoke exposure, low oxygen episodes, and high temperatures might place infants at higher risk for SIDS.