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Former U.S. Vice President Dick Cheney.
Former Vice President Dick Cheney secretly put a signed resignation letter in a safe shortly after taking office, in part because of concerns about his health, according to excerpts from an NBC News interview.

Cheney, 70, who suffered four heart attacks before becoming President George W. Bush's vice president, was worried about the possibility "that I might have a heart attack or a stroke that would be incapacitating," he said in the interview. "There is no mechanism for getting rid of a vice president who can't function."

Cheney signed the letter in March 2001, two months after the inauguration. Bush knew about it as did a member of the vice president's staff, according to the NBC excerpts.

The former vice president has been beset by heart trouble. In February 2010, he was hospitalized for what doctors described as a "mild" heart attack, his fifth, and he underwent surgery later that year to implant a pump to assist his heart.

The interview, set for broadcast on the network's Dateline program Aug. 29 and the Today show Aug. 30, was conducted in conjunction with the release next week of Cheney's memoir, In My Time. Cheney said the reaction to some of the revelations in the book will have "heads exploding all over Washington." The excerpts of the interview released by NBC didn't elaborate.

Cheney also defended the use of harsh interrogation techniques of suspected terrorists, including "waterboarding," which simulates drowning.

"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," he said.

In the book, Cheney talks about differences he had with Bush over striking a target where Iraqi leader Saddam Hussein was believed to be hiding after the U.S. invasion of Iraq in 2003. Cheney said he didn't think revealing the differences would embarrass Bush or betray a trust.

"I say some very fine things about George Bush and believe every word of it," Cheney told NBC.