Inouye, 88, the longest-serving member of the current Senate, has been at Walter Reed since Thursday, NBC News reported. Doctors are working to regulate his oxygen intake, and he has been seen recently on the Capitol grounds with a portable oxygen supply, NBC said.
"For the most part, I'm OK. Much to my frustration, I have to remain in the hospital for my own safety and to allow the necessary observation. I will be back on the Hill as soon as my doctors allow it," Inouye said in a statement. He was first elected to the Senate in 1963.