The fainting spell that put President Bush on the floor yesterday was probably caused by a common intestinal bug, although fatigue and jet lag may have heightened his vulnerability to the illness, doctors said.
Specialists in digestive illnesses agreed that Mr. Bush's unexpected collapse probably had nothing to do with the overactive thyroid that produced an irregular heartbeat last year, or with the sleep medication Halcion that the president took the night before his attack.
From media accounts, doctors surmised that the president was overcome by an intestinal virus that is "going around" this winter, usually producing symptoms of nausea and diarrhea. Casually called "stomach flu," it has nothing to do with true influenza, a respiratory infection that has also kept many people out of schools and offices this winter.
More properly called gastroenteritis, the illness can be caused either by a virus or bacteria. Most sufferers do not faint, but those who do are experiencing vasovagal syncope, the same response that occurs sometimes when people are overcome by fear or extreme pain.
"It's not common, but it sometimes occurs," said Dr. Marvin Schuster, a specialist in digestive diseases at the Johns Hopkins School of Medicine.
In some cases, infection can stimulate the vagus nerve, which runs between the brain and most of the major organs of the body. Among other things, the nerve is responsible for making sure that blood vessel walls are kept tense so that enough blood gets to the brain.
But when the nerve is overstimulated -- by infection, pain or fright -- it relaxes its control.
"Then, the blood vessels dilate, and the blood sort of pools up in your lower extremities," said Dr. Schuster, who is also director of digestive diseases at the Francis Scott Key Medical Center. "Too little blood gets to the brain."
The person faints.
Although frightening, fainting usually has a happy ending. "You fall, and your head is low, and this is where it should be," Dr. Schuster said. As gravity pulls the blood to the brain, the person regains consciousness.
Although members of the Secret Service apparently did him no harm, they probably shouldn't have raised Mr. Bush to his feet so quickly after he fainted, Dr. Schuster said, since the best medicine is to keep the head low and the legs elevated.
The drug Tigan, which Mr. Bush received after his fainting spell, relieves nausea by suppressing the brain's vomiting center. There are no drugs to cure the infection itself, but the illness usually passes on its own after a day or two.
"People are laid so low, they can hardly raise their heads from the pillow," Dr. Schuster said. "But it's self-limited. You're over it, and you're back without any long-term effects."
Less is known about the effects of jet lag and fatigue on intestinal illnesses, although some doctors speculated that the president's grueling schedule in the air and on the ground may have predisposed him to sickness.
"We expect that kind of thing will probably influence whether people get the disease, but there's little known about that," said Dr. Mark Donowitz, chairman of gastroenterology at the Johns Hopkins School of Medicine. "The studies are not convincing that the more stress you have the more likely you'll get [intestinal] illness."
Dr. Schuster said fatigue can predispose people to infection by hampering the body's defense mechanisms. But the bug also hits people who feel well rested, he said.
President Bush probably caught the bug somewhere on his trip rather than at home, said Dr. Schuster.
"It can be picked up from other people sneezing and from hand to mouth," Dr. Schuster said. "Like shaking hands with 250 people."