Millions of Americans experience atrial fibrillation every year, but doctors have never been able to determine precisely why an apparently healthy heart like President Bush's would suddenly stumble into arrhythmia after beating normally for 66 years.
"Unfortunately, it's really very common and relatively easily treated, and when it's not associated with extreme long-term risk, it's a little hard to get a lot of research money," said Dr. Thomas A Mattioni, assistant professor of medicine at the University of Maryland and director of cardiac electrophysiology at University Hospital.
There has been some research. Atrial fibrillation poses an increased risk of stroke for patients who experience it all the time. And researchers like Nathan H. Carliner, a cardiologist at University Hospital and the Baltimore Veterans Administration Medical Center, are working on that problem.
Carliner said yesterday that his clinical studies have shown that, for these chronic patients, anti-clotting drugs like Coumadin do "very dramatically reduce the risk of stroke."
Doctors have also learned that atrial fibrillation often occurs in patients with some underlying abnormality, Mattioni said, such as a hormonal imbalance, long-standing high blood pressure, cardio-myopathy (a weakening of the heart muscle), coronary artery disease, inflammation of the lining of the heart or heart valve defects.
During his weekend stay at Bethesda Naval Hospital, Bush has no doubt been examined for all of those things. But so far, no sign of any underlying disease has been reported.
If that holds up, Bush will be classed among many otherwise healthy patients who have "lone atrial fibrillation," the cause of which, Mattioni said, "is still unknown."
Clearly, in order for atrial fibrillation to occur, he said, "there must be something there that's different" in the electrical conductivity of the upper chambers of the heart.
Instead of an orderly firing of their electrical impulses, he said, "every small piece of the heart is . . . firing off, not really paying attention to what is going on in other areas." The result is an ineffective, chaotic quivering of the heart's upper chambers.
The vital lower chambers, or ventricles, beat on as best they can, supplying blood to the body and brain, with a faster pace to make up for reduced efficiency.
The key to the arrhythmia in these healthy patients seems to be a "trigger" event that kicks the heart out of its normal, steady rhythm.
"One of the things we do know is that the arrhythmia can commonly develop for the first time in a period of extreme physical exertion and stress," Mattioni said. Long-term emotional stress, however, such as running a country and a war, does not appear to be a triggering factor.
President Bush was jogging at Camp David Saturday afternoon when he experienced a shortness of breath, the first sign of the irregular heartbeat.
That may have been his "trigger," but fatigue, alcohol, nicotine and caffeine are often cited in other patients.
There is some belief, Mattioni said, that "in some individuals this is really a kind of random event that's not predictive of a serious, long-term abnormality or risk."
"Others say it may be a very early indication of perhaps some other cardiac pathology," he said. "But certainly the overwhelming majority of the time, it's a random event that happens in some people and may never come back or become chronic."
"This very likely could be the only episode he [Bush] will have," Mattioni said.
At this stage, Mattioni said, Bush's doctors will want to watch the President and "try to get a feeling for whether he is someone who has had an isolated incident that won't recur. And if that's the case, after an observation period, they will want to consider stopping his medication to see how he does."
If his heart rhythm remains normal, they may advise a more moderate exercise regimen. If the arrhythmias come back, long-term drug therapy may be needed to keep his heart beating normally.