About 6 percent of patients in both groups lived to be discharged from the hospital.
But in the 10 percent of patients who had also received bystander CPR and were candidates for defibrillation, longer CPR from paramedics actually decreased the odds of survival.
"Before, the theory was to do a bunch of CPR to make the heart stronger and more responsive to the shock, but we didn't show that," said Stiell. "We showed that too much is not a good thing."
At this point, Stiell recommends that paramedics and firefighters deliver just one minute of CPR.
In a second CPR study published in the same journal issue, researchers report disappointing results for the "impedance threshold device" (ITD), which was designed to increase the beneficial effects of CPR.
The device increases blood flow to the heart while someone is administering CPR.
Nearly 9,000 patients in the United States and Canada were split into two groups, one of which received ITD treatment and another that got sham ITD treatment. The researchers, noting that about the same number in each group survived to hospital discharge, said the ITD did not significantly improve results.
Visit the American Heart Association for help finding a CPR class.
SOURCE: Ian Stiell, M.D., senior scientist, Ottawa Hospital Research Institute and chair of emergency medicine, University of Ottawa; Joseph Feldman, M.D., chairman of emergency services, Hackensack University Medical Center, Hackensack, N.J.; Sept. 1, 2011, New England Journal of Medicine
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