10:40 AM PT -- Arnold's people just told us the following ... the procedure was "to replace a pulmonic valve that was originally replaced due to a congenital heart defect in 1997. 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. During that procedure, an open-heart surgery team was prepared ... in case the catheter procedure was unable to be performed."

The rep says Arnold's recovering from surgery and is in stable condition. His first words after waking up: "I'm back." Naturally.

Arnold Schwarzenegger has undergone emergency open-heart surgery ... TMZ has learned.

Sources connected to Arnold tell TMZ, the 70-year-old Schwarzenegger went to Cedars-Sinai Thursday for a catheter valve replacement. We're told the surgery is somewhat experimental and he developed complications.

Doctors were prepared in case the catheter valve replacement failed ... and quickly decided Arnold needed emergency open-heart surgery, which we're told lasted several hours.

This is not the first time Arnold has gone under the knife for heart surgery. He had an aortic valve replaced in 1997. At the time doctors didn't feel the procedure was urgently needed, but Arnold decided to do it while he was still young. He also said at the time the condition was congenital and had nothing to do with steroids.

As for his condition ... we're told he's stable.