Sensing a problem early Wednesday, Cheney saw the White House physician, who discovered the vice president was experiencing a recurrence of the irregular heartbeat. Cheney participated in regular morning briefings with President Bush, among other duties, and remained working at the White House until he went to George Washington University Hospital in the afternoon for treatment.
The process took nearly two hours, after which Cheney went home, said Megan Mitchell, a Cheney spokeswoman.
"An electrical impulse was delivered to restore the heart to normal rhythm," she said. "The procedure went smoothly and without complication."
Cheney told Bush of his condition. The president responded "like he would with any friend," said spokesman Tony Fratto, by wishing the vice president well and telling him to "go and make sure the doctors do what they need to do."
Later, in Ada, Mich., Bush told reporters that Cheney is "going to be fine."
"He said he was confident, the doctors are confident, and therefore I'm confident," Bush said.
Cheney also experienced atrial fibrillation in November 2007, and doctors also administered an electrical shock then in a treatment that took about 2 1/2 hours. That irregular heartbeat was discovered while White House doctors were treating the vice president for a lingering cough from a cold.
Dr. Zayd Eldadah, director of cardiac arrhythmia research at Washington Hospital Center, said it's not unusual for Cheney to have another such episode. An estimated 2.8 million Americans have atrial fibrillation, the most common type of irregular heartbeat and one that is not life-threatening in itself.
"This kind of rhythm problem generally does keep coming back over time," said Eldadah, who is not involved in Cheney's care. "The natural history of atrial fibrillation in people who have heart disease and are older is that it keeps coming back, and generally comes back more frequently."
The main risk from atrial fibrillation is not that Cheney will have another heart attack, but that he eventually could have a stroke if the rhythm problem is not treated.
Atrial fibrillation causes the upper chambers of the heart to quiver, instead of pump. As a result, some blood can pool in the heart. When blood settles, it tends to clot. And if those clots are then pumped out to the body, they can lodge in tiny blood vessels in the brain and cause a stroke.
The procedure Cheney underwent Wednesday is like resetting a computer, Eldadah explained. It involves sedation, and then an electrical charge delivered to the heart. "The heart will be turned off and on to reset it," said Eldadah. "It's a quick fix to restore normal rhythm."
If the procedure doesn't work, patients typically are put on blood thinners to prevent clotting.
"Atrial fibrillation in patients like Vice President Cheney is not a source of great worry or alarm," Eldadah said. "It's very treatable."
Cheney has had four heart attacks, starting when he was 37, and many heart-related doctor's and hospital visits over the years since. He has had quadruple bypass surgery and two artery-clearing angioplasties. In 2001, he had a special pacemaker implanted in his chest. The pacemaker's battery was replaced last year, and then the entire device was replaced.
The type of defibrillator Cheney has is used to prevent sudden death from a very different type of irregular heartbeat than atrial fibrillation, a much more serious kind that starts in the bottom of the heart.
In 2005, he also had surgery to repair an arterial aneurysm on the back of each knee.
In his checkup in July, doctors said Cheney's heart was beating normally for a man of his age and health history.
The campaign visit that Cheney canceled was for Marty Ozinga, a concrete company owner who is running for the House against Democrat Debbie Halvorson, a high-ranking Illinois state senator. Mitchell said Cheney called the event instead to express his support for the candidate.
Associated Press writers Ricardo Alonso-Zaldivar and Jennifer Loven contributed to this report.