Fish oil appears to do more harm than good for heart patients who have surgically implanted defibrillators to shock their weakened hearts back into rhythm, researchers said on Tuesday.

In a study of 200 patients with the electrical devices, half took fish oil supplements and the other half olive oil. Those who consumed fish oil had more episodes of dangerous heart arrhythmia that often precede Heart attacks, according to a report in this week’s Journal of the American Medical Association.

Fish oil - whether from eating fish such as cod or in supplement form - has previously been found to reduce by about 25 percent the risk of fatal Heart attacks in survivors of a previous attack.

Scientists believe the omega-3 polyunsaturated fatty acids contained in fish oil eases blood flow by reducing lipid levels in the blood. Fish oil’s ingredients may also induce changes in heart cell walls that smooths passage of electrical charges that trigger heartbeats.

“We went into the study with the pretty strong belief that fish oil was going to prevent arrhythmia and lessen the shocks that are so uncomfortable,” but records from the devices showed fish oil had the opposite effect, study author Merritt Raitt of the Portland, Oregon, Veterans Affairs Medical Center said in an interview.

Among patients who suffered from tachycardia, or rapid heartbeat, nearly two-thirds of those who took fish oil had episodes over a six-month period - twice the percentage of those taking a placebo. Among the patients who suffered from fibrillation, where the heart flutters, and 46 percent of those took fish oil had episodes compared to 36 percent of placebo-taking patients.

“What may be happening is that fish oil in these patients is proarrhythmic - making these abnormal rhythms occur when they wouldn’t otherwise,” Raitt told.

“Proarrhythmia is a danger with all drugs that affect heart rhythm and is more common the sicker the heart is. So we may just be seeing this particular drug, fish oil, is proarrhythmic in people who have defibrillators.”

Roughly 150,000 Americans each year receive the increasingly popular defibrillators, a $5 billion market shared by Medtronic Inc., Guidant Corp., and St. Jude Medical Inc.

Most defibrillator patients also take some type of heart medication, though there are few studies examining the combination, Raitt said.

SOURCE: Journal of the American Medical Association, June 15, 2005.