When combined with a trigger, such as intense emotion or physical exertion, a long Q-T heart can go out of control and cause cardiac arrest and sudden death, he said.

"This often explains cases in which children die suddenly while playing in the sandbox, teenagers die of unexplained drowning, or previously healthy young adults die suddenly while jogging or shoveling snow," Ackerman said.

Cardiac arrest can be fatal within minutes unless the heart's electrical pattern is restored spontaneously or with the aid of a defibrillator, he said.

Ackerman said long Q-T syndrome may affect as many as one in 5,000 people.

According to U.S. Vital Statistics, about 3,000 infants die each year of sudden infant death syndrom, or SIDS. Several possible causes or triggers have been suggested for SIDS, including babies sleeping on their stomachs, nervous system problems related to breathing, abnormal metabolism in the liver, and flaws in the heart's electrical channels, Ackerman said.

The research team performed an autopsy on every unexplained infant death investigated by the Arkansas State Crime Laboratory between September 1997 and August 1999 -- a total of 93. Researchers extracted DNA from frozen heart tissue and studied the five genes linked to long Q-T syndrome.

They found that 5 percent of SIDS cases had genetic anomalies that were absent in the genes of 200 healthy, racially matched control subjects.

"Long Q-T syndrome is sometimes called the perfect killer, because it leaves no clues," Ackerman said. "Neither does SIDS. Our goal is to discover the truth.

"SIDS will probably turn out to have 20 different underlying causes. If we can figure out what they are, we can screen for them and hopefully one day, prevent future cases of SIDS."

Ackerman said there is no routine clinical test to screen for such cardiac causes. He said parents should continue to follow simple preventive measures that have decreased the frequency of SIDS:

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