New research links sudden infant death syndrome to declining heart rate

January 07, 1994|By Jonathan Bor | Jonathan Bor,Staff Writer

A study of premature babies has found that a declining heart rate is responsible for at least some cases of sudden infant death syndrome, challenging the widespread belief that SIDS occurs when an infant unaccountably stops breathing.

Dr. Robert G. Meny, a pediatrician at the University of Maryland Medical Center, said the finding should push research in a new direction -- prompting doctors to explore why the heart rate slows and what might be done to prevent it.

"With these recorded deaths we find there was a heart rate problem before the babies stopped breathing," said Dr. Meany, director of clinical services at the university's SIDS Institute. "We need to see what is causing those low heart rates."

Physicians at the University of Maryland, the Johns Hopkins Children's Center, the Harvard University School of Medicine and the Robert Wood Johnson School of Medicine in New Jersey participated in the study. It appears this week in the journal Pediatrics.

The findings were based on recordings by "memory monitors" that were attached to six babies who were considered at high risk for SIDS because they were born early. Besides providing real-time records, such monitors are programmed to set off a high-pitched squeal when infants stop breathing for more than 20 seconds or when the heart rate surges or falls abnormally.

Monitors revealed that the heart rate dropped to dangerously low levels before or at the same time the babies stopped breathing. In four of the cases, parents responded to the alarm within a minute and immediately tried to resuscitate the babies -- but their efforts failed.

In two of the cases, parents didn't respond until much later. Describing one tragic case, Dr. Meny said the parents had apparently slept through the warning because they had become accustomed to false alarms triggered by the baby's healthy cry. A rapid heart rate is a natural and harmless response to crying.

Once desensitized to the alarms, the parents were not awakened when their baby's heart slowed to a dangerous pace and set off the ear-piercing squeal.

Every year, about 5,000 infants in the United States -- 100 or so of them in Maryland -- die unexpectedly and for no apparent reason.

When doctors exclude any other verifiable cause, they categorize thedeaths as SIDS.

The largest risk factor is prematurity -- about 20 percent of SIDS babies were born early. Babies whose mothers abused narcotics during pregnancy or whose siblings died of SIDS are also at high risk. So are babies who were exposed to their parents' smoking during the first few months of life.

Although it remains controversial, an Australian study also found a higher incidence of SIDS among babies who slept face-down on soft bedding.

In the new study, doctors examined the deaths of three babies whose only risk factors were prematurity; three others had a lung impairment known as bronchopulmonary dysplasia, a condition that is often arises when premature babies spend a long time on mechanical ventilators.

For this reason, three of the deaths could be defined as SIDS while the other three could be attributed to the lung impairment. All six, however, suffered from the same declining heart rate before they died. Also, the same observation of a declining heart rate was made in a 1991 UM study involving four SIDS deaths.

Doctors said a plausible explanation for the slow heart rates could be a low level of oxygen in the bloodstream, a condition known as hypoxia.

Dr. John L. Carroll, a Johns Hopkins pediatrician who took part in the recent study, said many of the risk factors for SIDS also predispose children to breathing problems that could lower the oxygen content of blood, triggering a low heart rate.

Premature infants, for instance, have poorly developed lungs; babies exposed to smoke inside and outside the womb may not get enough oxygen; infants sleeping face-down may have trouble getting air.

Such breathing problems, he said, are completely different from apnea -- the sudden cessation of breathing. Unlike apnea, these problems could cause a more gradual decline that could be reversed if doctors knew what to look for.

"It could mean that something is going on for a few hours, or even a few days before they die," Dr. Carroll said. "There's growing evidence that these weren't absolutely, perfectly normal kids who up and died. I think we are getting closer to figuring out what it is."

Dr. Carroll said infants at high risk for SIDS may someday be sent home with a different type of monitor -- one that measures blood-oxygen levels.

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