Researchers are not sure of the reasons for the improvement. But they suspect it's changes in how hospitals treat cardiac arrest, and possibly the way bystanders respond when they see someone suddenly collapse.

The study, which appears in the journal Circulation, found that in 2009, the death rate among Americans hospitalized after cardiac arrest was just under 58 percent. That was down from almost 70 percent in 2001.

Researchers based their findings on a national hospital discharge database that included nearly 1.2 million Americans who were hospitalized for cardiac arrest. They stress that the numbers account only for cardiac arrest victims who survive long enough to be admitted to the hospital.

Many people die before ever reaching the hospital. And in fact, of the more than 300,000 Americans who go into cardiac arrest outside of a hospital each year, less than 8 percent survive.

"So this does not say anything about the rates of mortality of all (cardiac arrest) resuscitation attempts," senior researcher Dr. Alejandro A. Rabinstein, of the Mayo Clinic in Rochester, Minnesota, said in an email.

"But the study does have a clear message," Rabinstein said. "If you have a cardiac arrest and you get to be hospitalized, your chances of surviving the hospitalization are getting better."

CPR ADVANCES

Cardiac arrest occurs when a problem in the heart's electrical activity causes it to stop beating normally, making it unable to pump blood to the body. It's fatal within minutes unless the normal rhythm can be restored with a shock from a defibrillator.

Since 2001, a few advances have been made in treating cardiac arrest, Rabinstein's team says.

One is that experts now recommend that bystanders do "hands-only" CPR, which means chest compressions alone and no mouth-to-mouth breathing. It's believed to be easier for laypeople to perform, and may have made more people willing to perform CPR on a stranger. (The American Heart Association has video instructions at bit.ly/Nzb1SU.)

Some research has suggested that the switch to hands-only has helped cardiac arrest survival inch up.

The public also has more access to devices called automated external defibrillators (AEDs) -- portable, layperson-friendly versions of the equipment doctors use to shock an arrested heart.

It's possible, Rabinstein said, that those steps have helped.

But even if cardiac arrest victims get to the hospital alive, they are not out of the woods.

In recent years, doctors have started doing angioplasty in certain patients soon after they arrive at the hospital. And Rabinstein said he thinks that may be helping.

COOLING TO THE RESCUE?

He also suspects that a newer in-hospital treatment for people who remain comatose after cardiac arrest is making a difference.

It's called therapeutic hypothermia, and it involves cooling the patient's body using cold IV saline, cooling blankets or ice packs. A day later, doctors slowly re-warm the patient. (For more on therapeutic hypothermia, see reut.rs/edTRww.)

The cooling therapy helps protect the brain, which is often severely damaged by cardiac arrest. And research suggests it allows more people to survive with their brain function intact.

The rising survival rate in this study came at the same time hospitals starting using therapeutic hypothermia.

"The effect of therapeutic hypothermia is mainly improving survival with (brain) function, rather than survival itself," Rabinstein said.

But he added that there is some evidence the therapy is boosting overall long-term survival as well.

Still, the researchers point out, U.S. hospitals have been fairly slow to adopt therapeutic hypothermia. So it's hard to know how much of an impact the therapy might be having on the national level.

SOURCE: bit.ly/NIMoms Circulation, online June 27, 2012.

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