WASHINGTON — Dick Cheney kept a secret resignation letter in a safe while he was vice president in case his shaky health suddenly deteriorated, he said in an interview released Wednesday.

Cheney, who has a long history of heart problems, said that he signed the letter two months after entering office with president George W. Bush in 2001. It was always in a safe and only one staff member knew about it, he said.

"I did it because I was concerned... for a couple of reasons," Cheney told NBC television in an excerpt of an interview that will air on Monday.

"One was my own health situation. The possibility that I might have a heart attack or a stroke that would be incapacitating. And there is no mechanism for getting rid of a vice president who can't function," Cheney said.

The US Constitution's 25th Amendment allows for the removal of the president if the vice president and the majority of the cabinet agree that the head of state is incapacitated. It does not mention a scenario of an ill vice president.

Cheney, who suffered his first heart attack in 1978 at age 37, remains a prominent Republican and will soon publish a book entitled, "In My Time."

"There are gonna be heads exploding all over Washington," Cheney said of the book.

In the NBC interview, Cheney stood firm in his support for waterboarding of terrorism suspects. President Barack Obama on taking over in 2009 declared an end to techniques considered torture.

"I would strongly support using it again if we had a high value detainee and that was the only way we could get him to talk," Cheney said.

Cheney, often portrayed as a hawk and Washington insider who encouraged the younger Bush to pursue the war in Iraq, said that his book would not upset the former president.

"I didn't set out to embarrass the president or not embarrass the president," Cheney said. "If you look at the book, there are many places in it where I say some very fine things about George Bush. And believe every word of it."