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Hollywood action star Arnold Schwarzenegger is in a stable condition after undergoing emergency open-heart surgery, his representatives said Friday, adding that his first words on waking were "I'm back."

The 70-year-old actor turned activist -- famous for the catchphrase "I'll be back" -- was in a Los Angeles hospital Thursday to have a valve replaced and developed complications, according to his spokesman Daniel Ketchell.

Doctors rushed the "Terminator" and "Predator" star into theater for open-heart surgery, operating for several hours, according to reports in the US entertainment press.

Schwarzenegger, a former Mr Universe, underwent non-urgent heart surgery 21 years ago to have the valve replaced, due to a condition he said was congenital and nothing to do with steroids.

"That 1997 replacement valve was never meant to be permanent, and has outlived its life expectancy, so he chose to replace it yesterday through a less-invasive catheter valve replacement," Ketchell said.

"During that procedure, an open-heart surgery team was prepared, as they frequently are in these circumstances, in case the catheter procedure was unable to be performed.

"Governor Schwarzenegger's pulmonic valve was successfully replaced and he is currently recovering from the surgery and is in stable condition."

Schwarzenegger revealed in his book "Total Recall: My Unbelievably True Life Story" that he initially kept that first operation a secret from his wife, Maria Shriver, by telling her he was on vacation in Mexico.

In an interview with CBS's "60 Minutes" he said his doctor told him he was "crazy" when he said he planned to keep the operation quiet from Shriver.