EDITOR'S NOTE: This weekly column by reporter Bill Kettler answers readers' questions about topics of general medical interest with information provided by doctors from PrimeCare, Jackson County's independent practice association.

Some med icines for high blood pressure reduce the pulse rate as well as blood pressure. Can such a reduction in pulse rate bring about atrial fibrillation a kind of irregular heartbeat?

-- Jim V., Medford

An irregular heartbeat is sure to get nearly anyone's attention, so it's no surprise that cardiologists see many patients with symptoms of atrial fibrillation, a rapid fluttering in the heart's upper chambers.

"It's one of the most common things we see," said Dr. Brian Gross, a Medford cardiologist. "Sometimes we see three or four cases a day."

One of Gross' patients likened the feeling to "a flounder flopping in my chest." Another described it as "a squirrel running around in my chest." Still another said it felt like there was "a rabbit trying to squirm out of my chest."

Gross describes atrial fibrillation as "chaos" in the heart's auricles, the upper chambers that collect blood and push it to the ventricles, the pumping chambers. Usually the heart's complex electrical system allows it to ignore the irregular, rapid pulsations in the upper chambers, but sometimes those impulses are transmitted to the ventricles resulting in a fluttering sensation in the chest.

To understand why atrial fibrillation occurs, it helps to know how the heart functions. Think back to high school biology. The heart has four chambers: two auricles, two ventricles. Oxygen-poor blood comes into the right auricle from the veins and goes to the right ventricle, which pumps it to the lungs, where it picks up oxygen. Oxygen-rich blood returns to the heart via the left auricle, and then goes to the left ventricle, which pumps it around the body, in a cycle that happens about 70 times a minute for people in good health.

A structure called the SA node in the upper right-hand corner of the heart provides the electrical impulse that controls the heart muscle's rhythmic contractions.

"The current spreads over the heart like the ripples in a puddle when you toss in a pebble," Gross says. The electrical current causes the auricles to squeeze, forcing blood into the ventricles, and stimulates the ventricles to pump.

The SA node can be influenced by stress and hormones, part of the evolutionary mechanism that helped our ancestors survive in a prehistoric world when "fight or flight" could easily be a life or death decision. When the stressful moment ends, the heart typically returns to its normal rhythm, but sometimes, especially among older people, the heart doesn't "re-set" itself. Instead of beating regularly, the auricles may get several hundred impulses per minute.

Those unsettling feelings often prompt a call for an ambulance, or a call to a cardiologist at a minimum.

Gross says there's no research data that indicates any connection between high-blood-pressure medicines and atrial fibrillation. He says there could be any number of causes, because when somebody has hypertension "there are often lots of other things going on with the heart."

That said, high blood pressure remains one of the most common causes of atrial fibrillation. Other contributing factors can be excessive consumption of alcohol, a weak heart muscle, blood clots, disease in the heart valves or hardening of the arteries. Sometimes it just happens to an otherwise normal heart.

Physicians treat high blood pressure aggressively because it can lead to other serious health problems such as heart attack and stroke. Fortunately, medicines that bring down blood pressure, such as beta blockers and calcium blockers, are some of the safest drugs on the market. There is no evidence that these medications trigger atrial fibrillation.

High blood pressure is one of the most common ailments in the United States. By some estimates there are as many as 65 million Americans who would be healthier if they reduced their blood pressure.

The same medicines that lower blood pressure can be used to manage atrial fibrillation. In older patients, generally people over 70, doctors prescribe blood thinners such as Coumadin to reduce their chances of having a stroke in conjunction with an episode of atrial fibrillation.

E-mail questions to: bkettler@mailtribune.com, or send them to: Mail Tribune, Ask Your Doctors, P.O. Box 1108, Medford, OR 97501, or call 776-4492.