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Though Rep. Gabrielle Giffords (D—Ariz.) remains in critical condition this morning at Tucson's University Medical Center, doctors are optimistic about her odds.

The bullet that struck Rep. Giffords on Saturday traveled the length of the left side of her brain — it entered from the back and exited through the front of her head. She was responsive to voice commands after the shooting and was in the operating room within 38 minutes, according to the medical center.

Though it may take weeks or months for doctors to be able determine the extent of any permanent damage, the bullet's trajectory is cause for the doctors' optimism, experts say.

A person's chances of surviving such a trauma to the brain depend on the areas of the brain that are struck, the velocity of the bullet and whether the bullet exits the brain, said Dr. Keith Black, chairman of neurosurgery at Cedars-Sinai Medical Center in Los Angeles.

If a bullet passes through both the right and left hemispheres of the brain, instead of being confined to one side, as it was in the case of Rep. Giffords, then the damage is likely to be much worse, Black said.

"The brain is somewhat redundant — it can sometimes tolerate losing one half," Black said. Like a twin-engine plane that has lost one engine, he said, a person whose brain was pierced by a bullet on only one side has a better chance than someone who has suffered injury to both sides.

It's also a positive sign if the bullet misses the brain's "high-value real estate," such as the brain stem and the thalamus, Black said. These deep brain structures are crucial to consciousness and basic functions such as controlling breathing and the heartbeat. And a person has a better chance of recovering if the bullet misses the major blood vessels that bring oxygen to areas where it's needed.

The left side of the brain, where Giffords was struck, controls language and speech, so the fact that Giffords was responding to those speaking to her after her injury shows that she may be able to understand and process language – a very good sign for her recovery, Black said.

A bullet that misses the brain's ventricles — the cavities within the center of the brain that are filled with cerebrospinal fluid — also leaves a person in better shape than one that strikes these regions. If struck, the cavities may fill with blood, which may lead to complications such as hydrocephalus (a swelling of the brain), which can further endanger the victim.

The speed of the bullet as it travels through the brain makes a difference as well.

"A high-velocity bullet does more damage than a low-velocity bullet," Black said. High-speed bullets, such as those fired by an AK-47 or other military weapon, do more peripheral damage to the regions of the brain around their path as they pass through than slower-moving bullets, such as those fired by handguns.

And, "if it stays in the brain, it does more damage," Black said, than a bullet that exits the brain, as it did in Rep. Giffords' case.

A shooting victim stands a better chance if they don't stop breathing and if their blood pressure remains high enough — both functions are needed to maintain an adequate oxygen supply to the brain. Arriving at a Level 1 trauma center — the highest level and most prepared to provide care for such injuries — shortly after such a trauma can help maintain or sometimes restore these functions, Black said.

The medical team treating Giffords removed part of her skull, and Black said this allows the brain to swell without becoming compressed.

"Inside the skull, the brain is like jello in a jar," Black told MyHealthNewsDaily. "If it doesn't have any place to expand, there can be even more damage. Confinement can prevent blood flow."

Later, when the swelling subsides, the part of the skull that was removed is replaced, he said. Swelling often peaks on the third day after such an injury, but in Rep. Giffords' case, doctors may wait as long as several months to replace the bone, he said.

"With a gunshot wound, they may be worried that the bullet brought in bacteria. They may want to make sure there's no evidence of infection before they replace the skull," Black said.

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