June 2, 2008 A new study provides the most direct evidence that there exists a causal link between smoking during pregnancy and Sudden Infant Death Syndrome (SIDS). Clinicians have long considered prenatal cigarette smoke exposure a major contributing risk factor for SIDS, but researchers had not proved a casual relationship. Other contributing factors include disturbances of breathing and heart rate regulation and impaired arousal responses, thermal stress (primarily overheating from too high temperatures or too much clothing) and sleeping in the prone (belly-down) position.
"Since the advocacy of 'back to sleep position,' smoking during pregnancy has become the principal risk factor for SIDS," said Dr. Shabih Hasan, staff neonatologist and associate professor in the department of pediatrics at the University of Calgary, and the principal investigator of the new study, which appears in the first issue for June of the American Journal of Respiratory and Critical Care Medicine, a publication of the American Thoracic Society.
"Our results provide some of the most direct evidence to date suggesting that prenatal cigarette smoke exposure can contribute to the destabilizing effects of hypoxia and thermal stress on neonatal breathing," said Dr. Hasan.
To investigate the compounding effects of cigarette smoking on other known risk factors for SIDS, namely thermal and oxygen stress, researchers exposed pregnant rat pups to either room air (control) or mainstream cigarette smoke equivalent to that a pack-a-day smoker would experience.
"Our approach sought to quantify the effects of cigarette smoke holistically, rather than using nicotine exposure as a proxy for cigarette smoke. Nicotine is just one of the 4,700 known toxins in cigarette smoke that could have protracted effects on embryonic development and postnatal growth," said Dr Hasan.
In this study, both plasma nicotine levels in the mothers and reduced birth weight in the pups were comparable to those of moderate to heavy smoking human mothers and the infants born to them.
A total of 30 control and 39 cigarette smoke-exposed one-week-old rat pups were randomized to undergo either thermoneutral or hyperthermic exposure to an oxygen-depleted environment. Researchers then analyzed the respiratory responses to the challenges.
Overall, just 13 percent of the control animals exhibited gasping, whereas nearly three times that--36 percent--of the cigarette smoke exposed animals did. Furthermore, none of the control animals exhibited gasping under hypoxic conditions during thermoneutral experiments, whereas 25 percent of the cigarette smoke exposed animals did. Under hyperthermic conditions, just 29 percent of the control group displayed gasping behavior, compared to nearly half--49 percent--of the cigarette smoke exposed group. "These results also indicate the adverse effects of low oxygen and thermal stress even in pups, which were not exposed to cigarette smoke during pregnancy," said Dr. Hasan.
But the effects were much more pronounced in pups that head been exposed to cigarette smoked prenatally. Under hyperthermic conditions, hypoxia induced gasping in both groups, but only the cigarette smoke-exposed animals exhibited a pronounced and longer lasting respiratory depression following the termination of hypopxia.
"Our results show that prenatal cigarette smoke exposure compounds the risk by increasing the likelihood of gasp-like respiration and prolonging the time that it takes for neonates to return to normal breathing following hypoxia," said Dr. Hasan. "These observations provide important evidence of how prenatal cigarette smoke exposure, hypoxic episodes and hyperthermia might place infants at higher risk for SIDS and further support efforts to foster prenatal smoking cessation programs."
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