The biggest risk to U.S. Rep. Gabrielle Giffords D-AZ is brain swelling, as she remains in critical condition on Monday, after a Saturday shooting.

Giffords, 40, sustained a gunshot to the head during a Congress on Your Corner event in Tucson that killed six people, including federal judge John Roll, and injured 14 others.

The bullet wound was on the left side of her head and traveled from the back to the front of her skull, Dr. Peter Rhee, medical director at University Medical Center in Tucson, told reporters Sunday.

Dr. Michael Lemole Jr, MD, who performed surgery on the Congresswoman, said UMC physicians are cautiously optimistic about Giffords' outlook.

Lemole said that as of Sunday morning, Giffords was responsive to voice commands. She is able to respond to simple commands such as can you open your eyes or can you raise two fingers, he said.

Lemole said that brain swelling was the biggest threat facing Gifford. To alleviate that, surgeons removed a part of Gifford's skull and are preserving it until swelling subsides and it can be replaced, according to a statement by UMC.

Rhee said Giffords was quickly transported by paramedics and emergency response teams to the center. A fast response was the reason she reached the operating room 38 minutes after the shooting, according to UMC.