Michael Jackson, the moon-walking pop star who once patented "anti-gravity" shoes to enhance his stage performances, died of cardiac arrest yesterday at age 50.

When Jackson was rushed to the University of California at Los Angeles Medical Center yesterday afternoon, he was reportedly unconscious and not breathing. His doctors were unable to resuscitate him.  Jackson was declared dead from cardiac arrest, but details of its cause are still unknown.

Brian Oxman, a spokesman for the family, told CNN that they had been concerned for his health for many months.  "His use of medications had gotten in the way," Oxman said, "his injuries which he had sustained performing, where he had broken a vertebrae, and he had broken his leg from a fall on the stage, were getting in the way."

In 2005, when he was tried and cleared on child molestation charges in California, Jackson's attorneys had said he became addicted to morphine and the painkiller Demerol, which he is said to have received an injection of yesterday. Jackson has a long history of health troubles, including rumors last year that he had alpha-1 antitrypsin deficiency disorder and needed a lung transplant for severe emphysema.

Could these conditions have played into Jackson's death?  And what is the difference between cardiac arrest and heart attack?

Christie Ballantyne, interim chief of cardiology at Baylor College of Medicine in Houston, Tex., tells Scientific American that cardiac arrest just means that someone has lost their blood pressure and their pulse.  "Usually, this means there has been a collapse of the circulatory systems and many times, it is due to an [accelerated] heartbeat," he says.  When the heart is contracting so quickly, blood can no longer be squeezed from the pumping chamber and circulate to the brain, leading to death within minutes.

This accelerated heartbeat, or arrhythmia, can be a sign of a heart attack, which stems from buildup of plaque in the arteries.  But not all heart attacks lead to cardiac arrest, and they are not the only cause of cardiac arrests, Ballantyne says.  Cardiac arrest can occur from a major blood clot in the lung or a heart enlarged from infection or other damage.  Previous heart attacks can also leave scars that injure the heart's pacemaking electrical system.

A recent article in Forbes highlights the challenges for doctors trying to reduce the 200,000 to 300,000 Americans who die each year from sudden cardiac arrest.  "The biggest bugaboo in the field is we don't know how to predict arrest," Benjamin Abella, director of the Center for Resuscitation Science at the University of Pennsylvania told the magazine. "The stars just align in the wrong way."  Implanted defibrillators have gone a long way towards reducing deaths among high risk heart patients, but many people who succumb to cardiac arrest every year have no history of heart problems, leading some researchers to begin the hunt for genetic clues.

Ballantyne points out that severe lung problems or the use of depressant drugs like Demerol leads to a condition called cardiopulmonary arrest, where the lungs stop working before the heart.  Stimulants like cocaine, on the other hand, can accelerate the heartbeat and lead to cardiac arrest.

Jackson's autopsy results are expected to be made public later this afternoon, but toxicology analysis, which could indicate what drugs, if any, he was taking will not be available for several weeks.

Image of Michael Jackson sticker courtesy Tobyotter via Flickr