Arnold Schwarzenegger is "back" after having heart surgery.

The actor underwent a procedure last week to replace his pulmonic valve for the second time, and awoke afterward to announce, "I'm back," his representative said at the time.

He reiterated the sentiment Monday in a tweet, adding that he's grateful to be alive.

"It's true: I'm back! I went to sleep expecting to wake up with a small incision and woke up with a big one -- but guess what? I woke up, and that's something to be thankful for," he wrote. "Thank you to the doctors & nurses. And I'm truly filled with gratitude for all of the kind messages."

Schwarzenegger, 70, had his pulmonic valve replaced in 1997 due to a congenital heart defect and knew at the time the fix wasn't permanent. Though he'd elected to have a less-invasive procedure this time, an open-heart-surgery team was prepared to step in, Schwarzenegger's rep noted last week.

According to the Mayo Clinic, the pulmonary valve helps regulate blood flow to the heart. If it malfunctions, it can force the heart to work harder than usual to supply the body with blood.

The former California governor said in 1997 that he had not experienced any discomfort prior to his original procedure.

"Choosing to undergo open-heart surgery when I never felt sick was the hardest decision I've ever made," Schwarzenegger said in a statement at that time. "I can now look forward to a long, healthy life with my family."