MONDAY, April 16, 2018 (HealthDay News) -- Caffeine can send the heart racing, but for some people coffee may help prevent abnormal heartbeats, Australian researchers report.

Many doctors advise patients with abnormal heart rhythms ("arrhythmias") to avoid caffeine. But, for most heart patients, coffee and tea are safe and may sometimes reduce the frequency of arrhythmias, the researchers said.

"Although coffee increases your heart rate, it does not make it abnormal," explained senior researcher Dr. Peter Kistler. He is director of electrophysiology at Alfred Hospital and Baker Heart and Diabetes Institute in Melbourne.

"We found that there is no detrimental effects of coffee on heart rhythm and, in fact, coffee at up to three cups per day may be protective," he said.

Some people, however, may notice palpitations after drinking coffee, Kistler added, and those folks should avoid caffeine.

To see how caffeine affects patients with the common heart rhythm disorder known as atrial fibrillation, Kistler and colleagues looked at eight previously published studies.

"We have completed the largest review of the medical literature to date to determine the relationship between coffee and arrhythmias, or abnormal heartbeats," he said.

Kistler's group found that, among more than 228,000 patients, drinking coffee cut the frequency of episodes of atrial fibrillation by 6 percent. A further analysis of nearly 116,000 patients found a 13 percent risk reduction.

One cup of coffee contains about 95 milligrams of caffeine and acts as a stimulant to the central nervous system. Caffeine also blocks adenosine, a chemical that can trigger atrial fibrillation, Kistler explained.

In addition, the researchers found that caffeine had no effect on abnormal heartbeats in the lower chambers of the heart, called ventricular arrhythmias.

As many as six cups of coffee a day, about 500 milligrams of caffeine, did not increase the severity or rate of ventricular arrhythmias, the study authors said.

Moreover, one study of 103 heart attack patients who got about 353 milligrams of caffeine a day found improved heart rate and no significant arrhythmias, according to the new report.

Two studies, however, found an increase in the risk for ventricular arrhythmias among patients who drank as much as nine or 10 cups of coffee a day.

But Kistler's team did find that patients with heart conditions should avoid caffeinated energy drinks. These drinks can each contain 160 to 500 milligrams of concentrated caffeine.

About 75 percent of patients with heart conditions who drank two or more energy drinks in a day had palpitations within 24 hours, the researchers found.

Studies suggest that up to 300 milligrams of caffeine a day may be safe for patients with abnormal heartbeats.

But some patients who are particularly sensitive to caffeine should avoid it, Kistler said. In fact, about 25 percent of patients said that coffee triggered episodes of atrial fibrillation, the researchers found.

According to Dr. Byron Lee, director of the University of California, San Francisco's electrophysiology laboratories and clinics, "This study is certainly reassuring to coffee drinkers worried about arrhythmias."

Having said that, Lee cautioned that a lot is still not known about the effect of caffeine on the heart.

"Three cups of coffee per day appears safe, but we are uncertain about more daily intake," he said. "Some patients find that coffee is a clear trigger of their arrhythmias, and they should still be cautious."

The report was published April 16 in the journal JACC: Clinical Electrophysiology.

More information

For more on abnormal heart rhythms, visit the American Heart Association.