The cause for former Vice President Dick Cheney's hospitalization is related to his recurring heart trouble, a family friend of Cheney's told CNN Sunday.

Cheney was suffering from atrial fibrillation, or irregular heart rhythm, the friend said.

Cheney was admitted to George Washington University Hospital on Friday as a result of "progressive retention of fluid related to his coronary artery disease," Cheney's office said.

At the hospital, Cheney was placed on intravenous medication and his condition has "markedly improved," his office said in a statement Saturday....