He’s “resting comfortably,” but it’s serious enough that Liz had to cancel an appearance in Chicago and rush home.

Two doctors at George Washington University Hospital said Cheney was stable but may have additional treatment Tuesday. “We don’t know what the plan is,” one physician told NBC News.

Cheney apparently had an angiogram to allow doctors to look in his coronary arteries with the results indicating the possibility of additional treatment. No further information was immediately available, including when he was admitted to the hospital.

Cheney has had a long history of heart problems, including four heart attacks, starting when he was 37 years old. He has had quadruple bypass surgery and two artery-clearing angioplasties.

Tapper says the word was out about his condition in Republican circles as early as 11 a.m. this morning. No clue yet as to whether this came on suddenly or whether, like Clinton, he started feeling unwell days earlier. If so, he didn’t show it at CPAC.

Stand by for updates.