system
Given the following post, respond as if you're commenting on Reddit:
human
How do doctors induce a medical coma?

Following on from the recent coverage of Schumacher's accident, I have been looking for details on what exactly a 'medically induced coma' is, how it helps and how long it can persist for.
gpt
A "medically induced coma" is probably more of media term. A barbiturate coma is a medical term for a treatment option for severe head injury, but is quite a specific entity, whereas "medically induced coma" when used in the press will often cover the general run of the mill sedation that is used in Intensive Care Units.

So, a barbiturate coma is an option when treating someone with a severe head injury. The problem that arises in these patients is that when the brain is injured, just like other parts of the body, it has a tendency to swell. Unlike other parts of the body though, the brain is encased in the bony box of the skull, so as it swells it compresses itself and can cause more damage. This brain swelling is often the mechanism of death for people who die from brain injury.

The swelling is monitored by placing a transducer either just into the surface of the brain, or more deeply into the fluid filled structures at the centre. This then monitors the brain pressure, the ICP (Intracranial Pressure). The aim is to try to keep the ICP low, ideally below 20mmHg.

If the ICP of a brain injured patient starts to rise, there are a number of options to try to treat it. The simpler things include adjusting the patients breathing to control the carbon dioxide level in the blood, (this directly effects ICP), giving the patient sedation to limit things like coughing that can cause ICP to rise,loosening dressings or tapes that might be compressing the neck. It is also important to rule out an expanding blood clot that might need urgent surgery.

If these simpler measures fail, then there are a number of alternatives. Several of these are based around trying to lower the metabolic activity of the brain cells, so that they need less oxygen, and so are less sensitive to the effects of the swelling. 

A barbiturate coma is one of these options. It is induced by giving very high doses, in a continuous infusion,  of the drug sodium thiopentone, which used to be used for general anaesthesia. Often the drug is given until the patients EEG shows a specific pattern which indicates deep coma. The drug can be given for several days, and sometimes even longer but has serious side effects. These include the lowering of blood pressure, an increased susceptibility to infection, gastric standstill, and fluctuations in body chemistry which can be fatal. 

It is a rarely used technique, for although it is effective at lowering ICP, it has never been shown to improve outcome in properly controlled trials, and it does have a high incidence  of serious side effects.