When Red Sox slugger David Ortiz was removed from the lineup Monday night because of an "irregular heartbeat," fans were wondering, what could that mean?

"Only about a thousand things," said Dr. Alfred E. Buxton, director of the cardiology division at Rhode Island and Miriam Hospitals in Providence.

But in a healthy athlete, Buxton said, the most likely cause of the irregular heartbeat is atrial fibrillation, a condition that is usually benign in young people.

Dr. Michael A. Coady, a heart surgeon who is chief of surgery of Landmark Medical Center in Woonsocket, agreed that heartbeat irregularities -- or "palpitations," as a Red Sox statement yesterday called the problem -- are typically benign in young athletes.

Drugs, including anabolic steroids, can cause heartbeat irregularities, but people should not jump to that conclusion because there are many possible causes, Coady said. Stress and overtraining can also make the heart beat unevenly.

The two local doctors asked to comment about "palpitations" and "irregular heartbeat" emphasized that the range of possible explanations for those symptoms is vast, making it difficult to offer much information to worried baseball fans.

"Atrial fibrillation is the most common sustained rhythm abnormality that we see in our society," Buxton said, and it gets more common with age.

In atrial fibrillation, the electrical signals in the heart are poorly coordinated, so that upper chambers of the heart quiver rather than beating in concert with the lower chambers. People with atrial fibrillation may feel their hearts pounding or racing, and sometimes experience weakness or shortness of breath. But some don't even notice that anything is amiss.

"We treat it primarily because it bothers people," Buxton said. "In general it's not the kind of thing that makes people faint or have a cardiac arrest."

If an athlete had atrial fibrillation, Buxton said, "I wouldn't be so concerned about the person getting hurt, but making him feel lousy -- not being able to perform optimally." Once the symptoms are treated, however, he is not likely to be impaired. Drugs to slow the heartbeat or to correct the electrical problem are commonly prescribed.

Generally, Buxton said, atrial fibrillation poses little risk, because the heart's lower chambers -- the ventricles -- are responsible for pumping the blood out to the body. The biggest potential problem is that clots can form in the atria, and if they get into the bloodstream they can cause a stroke.

"For someone like David Ortiz, who presumably is healthy -- nothing wrong with his heart structurally, never had a stroke -- the risk to him of having a stroke is way less than one percent," Buxton said.

Still, no one is saying that David Ortiz has atrial fibrillation.

"In some cases, arrhythmias can be pathological," said Landmark's Coady. "They can lead to major cardiac events, including cardiac arrest and sudden death."

The fact that Ortiz was screened and then allowed to return to play last week "would imply that the arrhythmia is more likely to be benign," Coady said.

Both doctors said it was unusual for a person with palpitations to be admitted to the hospital, but Ortiz stayed overnight at Massachusetts General Hospital on Aug. 19.

"The reason to keep someone would if you're able to capture something on the EKG, or it's some celebrity and you really want to have him screened," Coady said.

ffreyer@projo.com 277-7397