Millions of Americans have the potentially dangerous irregular heartbeat known as atrial fibrillation.

Now, research suggests that being obese might undercut the effectiveness of certain drugs meant to treat AFib.

The new study followed more than 300 patients listed in the University of Illinois at Chicago's AFib Registry. Researchers found that a class of medicines called sodium channel blockers, which are often used to treat AFib, were less effective in obese patients.

In fact, the recurrence rate for the heart arrhythmia was 30% for obese patients taking sodium channel blockers, compared with 6% for non-obese patients.

That could be bad news for many patients, since obesity is a big risk factor for AFib, said a team led by Dr. Dawood Darbar, head of cardiology at the university's College of Medicine.

Still, obese patients may have a viable option: Darbar's team found that another class of drugs, called potassium channel blockers, worked better in obese patients.

"This is the first time anyone has shown that there is a differential response to anti-arrhythmic drugs for AFib," Darbar said in a university news release. "As 50% of the patients in our AFib Registry are obese, this provided us with a unique opportunity to determine whether obesity affected response to drug treatment."

Dr. Satjit Bhusri, a cardiologist at Lenox Hill Hospital in New York City, said the new findings make sense, given what's known about how sodium channel blockers work.

"Most antiarrhythmic drugs distribute throughout the body and are absorbed by many organs," explained Bhusri, who wasn't involved in the Chicago study. "Since these drugs require a large dose to reach a steady level in the blood, those patients that are obese may well be under-treated — they may require increased dosing compared to thinner patients."

But upping the dose could lead to unwanted side effects, he added, so doctors may want to consider other treatment options in obese patients.

According to Bhusri, sodium channel blockers commonly prescribed for AFib include quinidine, procainamide, felcianide and propafenone.

Darbar agreed that the new study offers obese patients a route to better treatment.

The obesity epidemic is increasing, he said, so "having treatment options to better manage AFib would greatly improve quality of life and could prevent the risk of serious complications, like stroke, which can cause early death."

Dr. Laurence Epstein is system director of electrophysiology at Northwell Health in Manhasset, New York. Reading over the new findings, he agreed that "treating atrial fibrillation remains a challenge."

"What we are learning is that all AFib is not the same," he said. "If you asked an oncologist how do you treat cancer, they would ask you, what type of cancer? The same is true for AFib. The present study adds additional support to the notion that all AFib is not the same, and we need to be tailoring treatment to individual patients."

The study was published online Nov. 27 in the journal JAMA Cardiology.