In the case of traumatic brain injury—such as the bullet wound sustained by U.S. Rep. Gabrielle Giffords in Saturday's assault outside a Tucson supermarket that killed six people and wounded 13 others—doctors sometimes induce a coma. This effective shutdown of brain function naturally occurs only in cases of extreme trauma, so why would doctors seek to mimic it in patients, as they have with the congresswoman, already suffering from head wounds and other issues?

The answer lies in the science behind general anesthesia, which some 60,000 patients undergo every day. A review paper in the December 30, 2010, issue of The New England Journal of Medicine reveals that such anesthesia is, essentially, a reversible coma.

That is exactly what doctors are aiming for in the case of a true medically induced coma, often using the same drugs or extreme hypothermia induced by exposure to a cold environment to halt blood flow entirely and permit surgery on the aorta. Shutting down function can give the brain time to heal without the body performing radical triage by shutting off blood flow to damaged sections. To find out more about such medically induced comas and the reasons why doctors employ them, Scientific American spoke with anesthesiologist Emery Brown of Harvard Medical School, co-author of the NEJM review.

[An edited transcript of the interview follows.]

What is a medically induced coma?
So basically what happens with a medically induced coma is that you take a drug and administer it until you see a certain pattern in the monitor that follows the patient's brain waves, the EEG [electroencephalogram]. Patients with brain injuries who are in a coma have a similar pattern. If that pattern is there, then you feel comfortable that the patient is in a drug-induced coma. You are doing it so that you can hopefully protect the brain.

What are you protecting the brain from?
If you've had a brain injury, what happens is the metabolism of the brain has been significantly altered. You may have areas without adequate blood flow. The idea is: "Let me reduce the amount of energy those different brain areas need." If I can do that then, as the brain heals and the swelling goes down, maybe those areas that were at risk can be protected.

But the main thing about a drug-induced coma, as opposed to a coma, is that it's reversible. If you do this to someone with a normal brain, they would come right out of it once you removed the drugs.

But it's used in the case of people with brain injuries. So what are the risks?
Speaking generally, the main effects that these drugs have outside the brain is they reduce blood pressure. So people trying to do this are giving a lot of other medicines to keep blood pressure up and keep the heart pumping in a nice way. You're protecting the brain on one hand and, on the other hand, all areas of the brain are not getting the blood they need necessarily.

If you do this for an extended period of time, the drugs can accumulate and it may take them a while to wash out of the system as well. As long as you're mindful of these things you can see someone through a period like this.

How long is that period?
It really depends on the injury, whether it's a brain injury or seizing. One patient was kept in [a drug-induced coma] for six months. Obviously, that's the tail end of the distribution.

It depends on how the person is progressing and the nature of the injury. What the neurologists or [intensive care unit] doctors do is try to have them come out as soon as possible. In a case like Gifford's they have swelling. If they see the swelling recede, then they may try to lighten up the coma to see if she can come back and see what her level of function is.

How safe are drug-induced comas?
A drug like propofol, we use this every day in the operating room. It is probably the most used drug in all of anesthesia. Every day essentially, when patients go under general anesthesia that whole state is a reversible coma. It's a difference in dosage.

How does a medically-induced coma differ from a natural coma?
The body doesn't usually decide to enter a coma. A coma is a profound shutdown of brain function. It typically results from profound trauma, brain injury, a drug overdose, stroke—some very gross insult. There isn't a natural analogue for [a medically induced coma].

Are there after effects?
It's hard to sort out, because if you're going to these extremes you're already dealing with a very dire situation. If there are effects later on, it's an extremely difficult distinction to make whether it is an effect of the drug-induced coma. People who do this are very mindful of watching and monitoring. They make every effort to only use this option for as long as they need to.