Portland, Ore. - Fish oil supplements have proved effective in reducing the risk of sudden cardiac death, but a new study shows the oil may actually increase life-threatening abnormal heart rhythms in patients with implanted defibrillators.

The study, led by heart researchers at the Portland Veterans Affairs Medical Center and Oregon Health & Science University will be published in the June 15 issue of The Journal of the American Medical Association.

"We were very surprised by the study results. Previous studies have suggested that fish oil might actually prevent life-threatening abnormal heart rhythms," said Merritt Raitt, M.D., physician at the Portland Veterans Affairs Medical Center and associate professor of medicine (cardiology) in the Oregon Health & Science University School of Medicine. "But our results indicate that this supplement might actually be harmful and increase the risk of life-threatening heart rhythms in this group of patients."

Researchers enrolled patients who had suffered from recent episodes of either ventricular tachycardia (VT) or ventricular fibrillation (VF), dangerous rapid heart rhythms that originate in the ventricles of the heart, the main pumping chambers, and can lead to cardiac arrest and sudden death. The study specifically looked at 200 patients from six medical centers between February 1999 to January 2003. All the patients had implantable cardioverter defibrillators (ICD) that protected them against these life-threatening abnormal heart rhythms by automatically detecting and treating VT and VF with pacing and/or shocks.

In this randomized, double-blind trial, half the patients received a placebo, in this case olive oil, while the other half received 1.8 g of fish oil, omega-3 polyunsaturated fatty acids. The patients who received the fish oil supplements had blood levels of omega-3 polyunsaturated fatty acids that in previous studies were associated with a reduced risk of sudden death.

Every three months during a two-year period, the ICD memories were reviewed for episodes of VT and VF. Researchers then compared the number of episodes in the placebo group with that of the fish oil group.

The results showed fish oil did not prevent life-threatening abnormal heart rhythms, rather, it tended to increase the risk of VT and VF. After 6 months in the study, 36 percent of patients assigned to the placebo had an episode of VT or VF compared with 46 percent of patients who were in the fish oil group.

The effect was most prominent in the subset of 133 patients, who had suffered from VT at the time they entered the study. Researchers found that by 6 months, 37 percent of these patients assigned to placebo had experienced an episode of VT or VF compared with 61 percent of patients in the fish oil group. Over the entire course of the study recurrent episodes of VT or VF were significantly more likely to occur in the patients assigned to fish oil.

"We still believe that fish oil supplements or dietary fish intake can be beneficial by reducing sudden death in patients with a recent heart attack, as large studies have shown. However, it appears that this benefit is either not due to a reduction in abnormal heart rhythms or perhaps fish oil may have different effects on life-threatening heart rhythms in different situations," said Raitt. "Based on this research, we recommend that patients who have had life-threatening abnormal heart rhythms and implanted defibrillators should avoid fish oil supplements."

Other institutions who participated in the study were: Providence St. Vincent Medical Center, Portland, Ore.; Oregon State University, Corvallis, Ore.; Sacred Heart Medical Center, Eugene, Ore.; Southwest Washington Medical Center, Vancouver, Wash.; and Baystate Medical Center, Springfield, Mass.

The study was funded by the National Institutes of Health, a Public Health Service grant and Hoffman-LaRoche, Inc.