How Anesthesia Works

The Basics of General Anesthesia

When you get general anesthesia, you're "put under," which means that you're totally unconscious and immobilized. You "go to sleep" and don't feel, sense or remember anything that happens after the drugs begin to work on your system.

It's not completely clear exactly how general anesthetics work, but the current accepted theory is that they affect the spinal cord (which is why you end up immobile), the brain stem reticular activating system (which explains the unconsciousness) and the cerebral cortex (which results in changes in electrical activity on an electroencephalogram).


Major, complex surgeries that require a long period of time to perform typically require general anesthesia. Patients may be under for just a few hours for a knee replacement, or as many as six hours for something more complicated, such as heart bypass surgery.

If you're preparing for a surgery requiring general anesthesia, you'll typically meet with the anesthesiologist to give him or her your medical history. This is important because people certain with conditions might require special care under anesthesia -- a patient with low blood pressure might need to be medicated with ephedrine, for example. Patients who are heavy drinkers or drug users also tend to react differently to anesthesia. During this meeting, you'll be instructed not to eat for several hours before surgery. It's possible for someone under general anesthesia to aspirate, or breathe in, the contents of the stomach.

When you're under general anesthesia, you'll be wearing a breathing mask or breathing tube, because the muscles become too relaxed to keep your airways open. Several different things are continuously monitored while you're under -- pulse oximetry (oxygen level in the blood), heart rate, blood pressure, respiratory rate, carbon dioxide exhalation levels, temperature, the concentration of the anesthetic and brain activity. There's also an alarm that goes off if your oxygen level drops below a certain point.

There are four stages of general anesthesia:

  • During the first stage, induction, the patient is given medication and may start to feel its effects but hasn't yet fallen unconscious.
  • Next, patients go through a stage of excitement. They may twitch and have irregular breathing patterns or heart rates. Patients in this stage don't remember any of this happening because they're unconscious. This stage is very short and progresses rapidly to stage three.
  • During stage three, the muscles relax, breathing becomes regular and the patient is considered fully anesthetized.
  • Stage four anesthesia isn't a part of the regular process. This is when a patient has received an overdose of drugs, which can result in heart or breathing stoppage, brain damage or death if swift action isn't taken.

We'll look at the drugs administered during general anesthesia, as well as recovery, next.