Vice President Dick Cheney was treated for an abnormal rhythm in the upper chambers of his heart, called an atrial fibrillation, at the GW Hospital on Wednesday afternoon.

White House doctors discovered the atrial fibrillation when Cheney met with them on Wednesday morning. As a result, he cancelled a fundraising appearance for Republican congressional candidate Marty Ozinga in Homer Glen, Ill.

Megan Mitchell, a spokeswoman for Cheney, said in a statement that the outpatient procedure was successful. Cheney was released at 2 p.m.

“An electrical impulse was delivered to restore the heart to normal rhythm,” Mitchell wrote. “The procedure went smoothly and without complication.”

Mitchell wrote that Cheney returned home and resumed his normal schedule after the procedure.

More than two million Americans have an atrial fibrillation, according to the American Heart Association. Although it can be treated easily with an electronic shock, the lack of blood being pumped out of the atrium can cause a stroke if left untreated.

Today marked the second time in less than a year that Cheney has been treated for atrial fibrillation at the GW Hospital. Last November, the abnormality was discovered when he visited doctors for treatment of a cough from a cold.

Cheney has had four heart attacks in the past three decades. A pacemaker was implanted in his chest in 2001.

The vice president is major benefactor of the GW Hospital, having donated $2.7 million to establish the Richard B. and Lynne V. Cheney Cardiovascular Institute. He has received cardiac treatment at the hospital since his first heart attack in 1978.