CHICAGO, April 28 (UPI) -- U.S. researchers said an analysis of studies involving the use of hemoglobin-based blood substitutes suggests an increased risk of heart attack.

The study by the Journal of the American Medical Association, released Monday, said randomized controlled trials completed as early as 1996 raised questions about the safety of the products.

"Sponsors are required by law to report their results to the Food and Drug Administration in a timely fashion after studies are completed, even if they do not publish their findings," the report said. "Had the agency placed a moratorium on trials at that point, product-related deaths and (heart attacks) in subsequent trials most likely would have been prevented."

Dr. Charles Natanson of the National Institutes of Health and colleagues looked at 16 trials involving five different products and more than 3,700 patients to examine the association between hemoglobin-based blood substitutes and the risk of heart attack and death. Overall, the blood substitutes were associated with a 30 percent increased risk of death, the report said.