A controversial therapy to remove heavy metals from the bloodstream was shown in a large trial to cut the risk of another major heart problem in patients who have already suffered a heart attack, but researchers cautioned that the benefit was small and more study is needed.

Chelation therapy, an alternative treatment dismissed by many medical professionals as quackery, has its origins in unproven 50-year-old theories about the cause of arterial plaques, the fatty deposits that can cause heart attacks.

Despite no clear evidence of a benefit, and the real risk of side effects such as low blood calcium levels, a 2007 survey found that over 100,000 U.S. heart patients had undergone the $5,000 treatment.

The results, released here at a meeting of the American Heart Association, showed that 26 percent of patients given a series of chelation infusions had a heart attack, stroke, coronary revascularization, were hospitalized for angina, or died, compared with 30 percent of patients treated with a placebo.

Much of the difference between the two groups was in the need for repeat artery-clearing procedures and patients with diabetes showed the biggest benefit.

“Our findings are unexpected and additional research will be needed,” said Dr. Gervasio Lamas, chief of Columbia University’s Division of Cardiology at Mount Sinai Medical Center in Miami Beach, Fla., and the trial’s lead investigator. “This does not constitute sufficient evidence to recommend chelation therapy.”

The 1,708-patient trial to assess chelation therapy (TACT) was launched in 2002 under the auspices of the National Heart Lung and Blood Institute and the National Center for Complementary and Alternative Medicine.

Patients in the study were treated with either a solution containing ethylene diamine tetra-acetic (EDTA), vitamin C, B-vitamins, electrolytes, a local anesthetic and the anti-clotting drug heparin, or a placebo infusion of salt water and a small amount of sugar.

Dr. Lamas and others said the level of statistical difference between the groups was small and more work needs to be done to rule out whether the results were due to chance.

“This is a diverse multi-component intervention,” said Dr. Paul Armstrong, senior cardiologist at the University Hospital in Edmonton and a professor at the University of Alberta. He cited the absence of a clear biologic rationale for chelation therapy, and described the trial findings as “hypothesis-generating, not practice-changing.”

The size of the trial was scaled down from an initial target of more than 2,300 patients due to enrollment difficulties. In addition, regulators in 2008 temporarily suspended enrollment to clear up issues surrounding patient consent.

“It is striking that there was difficulty in enrollment despite an increase in the use of this therapy,” Armstrong said.

There was also an unusually high number, 30 percent, of patients who discontinued the therapy during the trial.

The findings “need to have some confirmation, but nevertheless the trial results are very provocative,” said Dr. Mark Hlatky, director of the cardiovascular outcomes research center at Stanford University.

Chelation treatment has become popular among some parents trying alternative treatments for children with autism, even though the therapy can be dangerous and is based on the unproven idea that such children have heavy metal poisoning.

“Intriguing as the results are, they are unexpected and should not be interpreted as an indication to adopt chelation therapy into clinical practice,” said Dr. Elliott Antman, chairman of the AHA Scientific Sessions Committee and professor of medicine at Harvard Medical School.

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By Deena Beasley and Bill Berkrot