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 United States.

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 history.

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.

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 structure.

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.

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 colleagues wrote.

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 conclude.

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 98?"

"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 Unit, agrees.

"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 assessment."

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.