It's a parent's worst nightmare. One moment your newborn is
sleeping peacefully. The next time you check the infant is dead.
Sudden infant death syndrome (SIDS) is the leading cause of death among infants between
1 month and 1 year of age and strikes one or two of every 1,000 babies. SIDS is blamed for
about 7,000 apparently healthy infants dying unexpectedly in their sleep each year in the
Other parts of the world have similar statistics for SIDS, often called "cot
death" outside the United States. SIDS is highest among premature babies and male
infants, but medical researchers don't know why. Physicians typically arrive at a
diagnosis of SIDS by excluding other causes of death through autopsy and extensive medical
Researchers have advanced a number of theories on the causes of SIDS. None have been
proved. Now, a study published in the June 11, 1998, issue of the New England Journal of
Medicine supports a theory that subtle, electrical disturbances in the heart could be
responsible for some SIDS cases.
The Multicenter Italian Study of Neonatal Electrocardiography and SIDS, coordinated by
Peter John Schwartz, M.D., of the University of Milan, Italy, recorded the
electrocardiograms (ECGs) of 34,442 newborns on the third or fourth day of life between
1976 and 1994.
Specifically, Dr. Schwartz and his colleagues studied the "QT" interval of
the infants' heartbeats on ECGs, then followed the newborns to learn whether those babies
who subsequently died of SIDS had QT intervals that were different from other babies.
What is the QT
Contractions of the heart are coordinated by an electrical signal. An ECG is a
recording of this electrical signal that resembles a series of sharp peaks and valleys and
plateaus. ECGs can be displayed on a strip of paper or on a monitor.
The ECG provides physicians with information such as the heart rate, rhythm, possible
inadequate supply of blood and oxygen to the heart muscle, and abnormalities of heart
A spike in the ECG known as the QRS complex occurs as the electrical impulse
"fires" within the cells of the ventricles, or lower heart chambers, causing
them to contract. The T wave reflects the heart's effort to recover electrically in
preparation for the next beat. The QT interval represents the time devoted to this
"firing and recovery" cycle that allows the heart muscle to pump blood
throughout the body.
QT interval and SIDS
When the QT interval is too long, the heart's electrical system can become chaotic and
"short-circuit," resulting in a potentially fatal heart rhythm.
"Our finding of a strong association between SIDS and prolongation of the QT
intervals suggests that some infants may have an increased susceptibility to
life-threatening arrhythmias (abnormal heart rhythms)," Dr. Schwartz and his
The study's findings support a hypothesis first proposed by Dr. Schwartz in 1976 that
abnormal development of the nerves controlling the heart's rhythm may predispose some
infants to potentially lethal, abnormal heart rhythms during the first year of life.
The authors found "that prolongation of the QT interval on the electrocardiogram
is an important risk factor, and . . . may be useful in the early identification of
infants at risk for SIDS." The authors also said that traditional SIDS risk factors
such as maternal smoking, bed sharing and sleeping on the abdomen, had markedly lower
predictive value than prolonged QT intervals among the infants studied.
Until now, there had been little evidence that SIDS is caused by abnormal heart rhythm.
Some researchers believe that SIDS may be related to breathing problems, especially long,
breathing pauses called apnea.
Given their findings, Dr. Schwartz and his colleagues suspect that the risk of SIDS may
be decreased by finding prolonged QT intervals on ECGs and using medicines like beta
blockers to reduce the likelihood of abnormal heart rhythms. But they caution, "our
study contains no data to justify new therapeutic recommendations."
More research is needed. The costs, risks and benefits must be weighed before such
steps are begun. "Even if our hypothesis were proved correct, however, such
identification and treatment would not be a simple task," the authors of the study
Screening and treatment
While infants born into families with a history of prolonged QT intervals are routinely
checked for ECG abnormalities during their first month of life, Co-burn J. Porter, M.D., a
pediatric cardiologist at Mayo Clinic, says the usefulness of routine ECGs on all newborns
is still unclear.
Dr. Porter points out that the T-wave measurement is imprecise in the first days and
weeks of life. "There may be a group with prolonged QT intervals who need to be
treated, but there isn't 100 percent agreement on who you treat and who you don't,"
Dr. Porter says.
"In fact, the study suggests that for every 100 babies with an abnormal QT
interval, only two will have SIDS," says Dr. Porter. "How do we identify the two
without introducing unnecessary drug treatment and undue parental concern for the other
"In addition, half of the babies studied who died of SIDS had measurements in the
normal range," says Dr. Porter. "So while the QT interval may be a factor in
some SIDS cases, I don't think we have a tool here that's sensitive enough to be used as a
predictor of SIDS."
Robert V. Johnson, M.D., a neonatologist and director of Mayo's Newborn Intensive Care
"As a screening tool, the ECG isn't good enough," Dr. Johnson says.
"Using the ECG to predict the risk of SIDS would cause groundless fears for many
parents and false reassurance for a few. We also don't yet know whether treating infants
with abnormal QT would be protective. I anticipate that molecular biology research will
soon give us better insight into cardiac causes of SIDS and perhaps more accurate risk
For now, Dr. Johnson says, babies with a family history of prolonged QT interval should
have an ECG. "For the rest, the best protection that parents can provide to reduce
the risk of SIDS is to place infants on their back to sleep and also to maintain
smoke-free households," Dr. Johnson says.
Researchers at the Mayo Clinic also are working to solve the SIDS puzzle. Michael J.
Ackerman, M.D., Ph.D., is pursuing the potential molecular causes of SIDS.
"Our work at Mayo is focused on searching for genetic defects that could explain
this tragic mystery and answer the question: Why do some SIDS infants have a prolonged QT
interval?" says Dr. Ackerman.