U.S. Sen. Daniel Inouye has been hospitalized since Thursday after suffering a fainting spell before a vote in the U.S. Capitol, his office confirmed.

Inouye was initially taken to the George Washington University Hospital but has been at the Walter Reed National Military Medical Center since Sunday.

“For the most part, I am OK,” Inouye said in a statement issued by his office today. “However, I am currently working with my doctors to regulate my oxygen intake. Much to my frustration, while undergoing this process, 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. Thank you all for the kind words and continued support.”

This is the second trip to a hospital in less than a month for the 88-year-old Hawaii Democrat. Inouye sustained a minor cut to the back of his head following a fall at his home on Nov. 15 and was taken to the Bethesda Naval Hospital as a precaution. He was released from the hospital and returned to his home in Maryland.

Inouye, who has served in the Senate since 1963, is the chairman of the Appropriations Committee and the Senate’s president pro tempore, making him third in the presidential line of succession.