(CNN) -- 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.
On Sunday, Cheney's daughter, Liz Cheney, said that the former vice president was "feeling better" and would hopefully be released Monday.
Cheney has a long history of heart problems. He suffered his first heart attack in 1978, when he was 37.
He had his second heart attack in 1984 and a third in 1988 and underwent a quadruple bypass surgery to unblock his arteries. Shortly after Cheney was elected vice president in November 2000, he had a fourth heart attack and received a stent to open an artery.
In February, Cheney suffered his fifth heart attack.
CNN's John King contributed to this report