TWU Research Links Smoking to SIDS

When parents put their newborn infants to bed each night with a lullaby or quiet words, they look forward to a morning filled with smiles, bottles and diapers. But the fear of SIDS - Sudden Infant Death Syndrome, or crib death - sometimes creeps into their minds, and research among members of the medical and scientific communities continues as they seek to find its cause.

Population studies of large numbers of people already have shown that pregnant women who smoke or who are exposed to others who smoke may more than double their babies' risk of suffering from SIDS; the reason for this increased risk of SIDS is unknown. However, a recent study conducted by Dr. Jan Wheeler-Sherman at the Texas Woman's University Institute of Health Sciences Houston Center has found a relationship between prenatal nicotine exposure and the heart rate of newborn infants. Dr. Wheeler-Sherman's doctoral dissertation for the TWU College of Nursing comprised the study. She currently is a neonatal nurse practitioner at the University of California-San Francisco.

"Exposure to cigarette smoke during pregnancy has been shown to increase the risk of Sudden Infant Death Syndrome, or SIDS," says Dr. Wheeler-Sherman. "The reason for this increased risk is not known, but nicotine has been shown to cause cardiac effects in animals and in adult humans. This study examined the relationship between nicotine exposure during pregnancy and the heart rate of newborn infants after birth."

She adds, "The time after birth is a period in which the heart function of the infant is changing. I believed that any changes in the infant's heart rate would be more obvious immediately after the birth. Also, measuring the heart rate during the time immediately after birth allowed me to assess the effects of fetal drug exposure (specifically nicotine) under the challenging conditions associated with birth. Measuring the heart rate under challenging conditions, rather than at a time when the baby was not under stress, more likely would reveal problems."

A total of 130 infants and their mothers were involved in the study, which was done in Kingwood, Texas, and completed in May 1999. All of the infants were full-term, well newborns whose mothers had no health problems, as well as an uncomplicated labor and delivery.

Following each baby's delivery, blood was drawn from the umbilical cord, which is attached to the placenta. (The umbilical cord provides nourishment, oxygen and any substances to which the mother is exposed directly to the fetus.) The sample of blood then was analyzed for cotinine. Cotinine is the primary product that is made when the body breaks down nicotine; it also is considered the best method currently available to measure exposure to nicotine. The heart rate of each infant then was measured at one-minute intervals during the baby's first four hours of life.

The cotinine samples were analyzed by Dr. John T. Bemert at the Tobacco Exposure Biomarkers Laboratory of the Centers for Disease Control.

The study found that newborn babies with higher levels of cotinine, had a significantly lower maximum heart rate and a lower variance in heart rate when compared with infants with lower levels of cotinine. "In other words, newborn infants with higher levels of cotinine had much lower heart rates. They were not able to increase and vary their heart rates as well as infants with lower levels of cotinine," explains Dr. Wheeler-Sherman. "The output of blood from the heart of an infant depends primarily on heart rate. If the baby is not able to increase and vary the heart rate during times of stress, then the baby could possibly be at an increased risk for SIDS."

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©2006 Texas Medical Center

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