What killed Michael Jackson ?

Jackson's Death Focuses Attention on Cardiac Arrest.A misunderstood disease that is one of the world's biggest killers could get new attention.

Even before an autopsy has revealed exactly why Michael Jackson's heart apparently stopped, his death is focusing attention on one of the most mysterious and common killer diseases in America: sudden cardiac arrest.

Sudden cardiac arrest kills 200,000 to 300,000 Americans a year. It is often wrongly equated with a heart attack. In fact, what kills people who die suddenly is not the artery clogging (which destroys heart muscle but is not immediately lethal); it's the ventricular fib, a rapid quivering of the main pumping chambers in the bottom of the heart, called the ventricles. When this occurs, little blood gets out. Brain cells die within minutes. For every minute that goes by without the heart being restored to a normal rhythm, the patient's odds of survival drop by 10%. After 10 minutes they're gone.

Your Heart: A User's Guide

Heart attacks are one trigger but may account for only a minority of sudden cardiac deaths. Past heart attacks, which leave scar tissue that changes the electrical patterns of the heart, are another risk factor. Genetic disorders that cause subtle changes to the heart rhythm are a third. "The biggest bugaboo in the field is we don't know how to predict arrest," says Benjamin Abella, director of the Center for Resuscitation Science at the University of Pennsylvania. "The stars just align in the wrong way." Eighty percent of those who die suddenly from heart disease have some sign of coronary trouble.

Performing cardiopulmonary resuscitation (CPR) can keep a patient alive by pushing blood through the body. But there is only one treatment that halts the deadly arrhythmia: an electric shock, called defibrillation, that sets the heartbeat back into a regular rhythm. Defibrillators come in two forms: external versions used by paramedics and surgically implanted versions for patients at high risk.

Scientists developed defibrillation in the 1950s after linemen stringing electrical wires across the country were dying suddenly of cardiac arrest from electrocution. The idea emerged that maybe a counter-shock could set the heart right. In the 1960s, Harvard researcher Bernard Lown invented a device that could be used in hospitals called the cardioverter defibrillator.

Easy-to-use versions are now common in airports, casinos and other crowded public places. Studies have found they boost the cardiac arrest survival rate significantly. But a 2008 study failed to find any benefit from putting defibrillators in typical heart patients' homes. It isn't often that a patient suffers arrhythmia when there is someone present to help him.

Each year, 300,000 heart patients worldwide at very high risk of sudden death get automatic defibrillators surgically implanted. The gadgets reduce the death risk 25% in patients whose hearts, because of slow deterioration or scars from heart attacks, are inefficient at pumping blood. It's a $6.4 billion market for Medtronic ( MDT - news - people )Johnson & Johnson ( JNJ - news - people ) and Boston Scientific ( BSX - news - people ), according to analysts at Wachovia. But implantable defibrillators, which cost $30,000 installed, are an expensive insurance policy for something that might never happen, and there are potential complications from surgery. Some of the devices or their associated wires have had to be recalled because of potentially catastrophic flaws.

But sudden deaths often occur in people without known heart problems, so scientists are racing to find other risk factors. One clue to pinpointing who is at risk is a genetic disorder called long QT syndrome, named after an unusual reading on an electrocardiogram. In this condition, the heart is just a little slow to recover between beats. This and related disorders hit one in several thousand people and may account for 5% of sudden-death cases.

Common gene variations may alter the heart rhythm in slight ways that don't cause problems under normal conditions but may predispose people to sudden death during a heart attack or if they take certain drugs that affect heart rhythm. Two recent studies in Nature Genetics found 10 gene variations that slightly alter heart rhythm and could be involved in sudden cardiac death. "What we are beginning to understand is why one person who has a heart attack dies suddenly whereas another person who has a heart attack doesn't," says Duke University cardiac electrophysiologist Patrick Hranitzky. "A lot of it has to do with genetics."

Cardiologist Sumeet Chugh hopes he can uncover more conditions like weakened hearts or long QT that identify which patients will have sudden cardiac arrest. As a medical resident in Minneapolis in 1992, a beautiful 19-year-old woman was rushed into the emergency room after collapsing while dancing. Her heart had suddenly stopped. Chugh and his colleagues worked for almost an hour but couldn't save her. An autopsy found nothing. The only clue was her mother, who had also died abruptly at a young age.

"It was devastating to me," says Chugh, now associate director of the Cedars-Sinai Heart Institute in Los Angeles. He dedicated his career to solving the biggest mystery of heart disease, why many healthy people suddenly keel over and die. He runs a study that has tracked every sudden cardiac death in Portland, Ore. since 2002 to find some answers. "It's like a Rubik's cube," he says. "You have to put it together piece by piece."