How Anesthesia Works

By: Shanna Freeman

Nurse preparing a syringe for local anesthesia. Hinterhaus Productions / Getty Images

Anesthesia is a mysterious concept to most of us, even if we've been anesthetized before. The term comes from the Greek for "loss of sensation," but that's not the only effect it causes in your body. Anesthesia, essentially a reversible condition induced by drugs, is intended to result in one or more different states of being. It can relieve pain, give you amnesia to knock out your memory of the procedure or how it felt, reduce anxiety (because who doesn't have anxiety when undergoing a medical procedure?) and paralyze your muscles.

It sounds a little scary, but anesthesia is made as safe as possible by careful calculation of the required dosages and diligent monitoring by medical professionals. And not all types of anesthesia are created equal.


When you think of anesthesia, it's likely you think of what's called general anesthesia, which is when you're completely unconscious during a medical procedure such as a major surgery. But there are several different types, and not all of them leave you oblivious to the world. Local anesthesia, for example, can affect just a small patch of skin. Which type you receive depends on a number of factors, including what kind of medical procedure you need and what your medical history looks like. There can also be some overlap between different types of anesthesia, and often, more than one drug is necessary to produce all the desired effects.

In this article, we will look at the different types of anesthesia so that you can understand what it is, how it works and what risks are involved. We'll also learn about anesthesia awareness and talk about the history of anesthesia (and what it has to do with cocaine). Let's start by looking at procedural sedation, also known as "twilight sleep."

Procedural Sedation, the Twilight Sleep

You may have undergone procedural sedation and not realized that it even qualified as anesthesia. If you've had your wisdom teeth taken out, for example, you've probably had procedural sedation. This type of anesthesia is used for short, relatively minor medical procedures and is also known as conscious sedation or twilight anesthesia. In addition to dental work, procedural sedation is used for things like setting broken bones, LASIK and minor cosmetic surgeries.

Under procedural sedation, you remain fully awake and can respond to questions and instructions. That doesn't necessarily mean that you know what's going on, though -- you'll be sleepy and relaxed. You typically won't remember the procedure or the short period of time following it. Some of the drugs used in procedural sedation can make you feel giddy or euphoric.


Procedural sedation has a lot in common with general anesthesia. That's because the same types of drugs used in general anesthesia are also administered in procedural sedation; they're just given in much smaller amounts. Usually, this means a sedative such as ketamine or nitrous oxide, which depresses the central nervous system. Sometimes a dissociative, which keeps nerve sensations from reaching the brain, is used instead, such as diazepam (more commonly known as Valium) or midazolam.

In high doses, these drugs induce sleep and paralysis and affect the cardiovascular system, but in lower doses, they calm the patient and reduce anxiety. For procedural sedation, one of these types of drugs is used in combination with an analgesic such as fentanyl for pain relief. These anesthetics may be inhaled, given orally, injected or used in a combination of the three methods. For example, nitrous oxide and other sedative gases are inhaled, but ketamine and Valium are injected into an IV line.

How long the procedural sedation lasts depends on the drugs administered -- it may be as few as five or 10 minutes or as long as an hour. Recovery is speedy, and you won't usually have the side effects associated with general anesthesia, such as vomiting, nausea or dizziness (although they can still occur). Patients under conscious sedation still have to be carefully monitored to ensure that they don't slip into deeper sedation.

We'll look at another type of anesthesia next -- the local kind.

Local Anesthesia: No Longer Cocaine!

If you've had procedural sedation and think you know everything there is to know about anesthesia, think again -- local anesthesia is very different. Patients still remain awake and conscious but don't get the dreamy or sleepy state that accompanies procedural sedation.

This is another form of anesthesia that you've likely experienced before. For example, in addition to procedural sedation during dental work, dentists also use a local anesthetic when they inject your gum to numb the area. Local anesthesia is used to make a very small area of the body, such as a patch of skin, insensitive to pain. It typically provides both analgesia and paralysis by blocking the nerves' impulses so they can't travel to the brain, but patients may still feel pressure and sensation.


Local anesthetics can be topical, or isolated just to the surface. These are usually in the form of gels, creams or sprays. They may be applied to the skin before the injection of a local anesthetic that works to numb the area more deeply, in order to avoid the pain of the needle or the drug itself (penicillin, for example, causes pain upon injection). Topical anesthetics aren't just used in medical procedures -- if you've ever used a pain relief product for a rash or hemorrhoids, you've used a topical anesthetic. The type of local anesthetic you inject might be given before procedures like stitches or debridement (the removal of dead or damaged tissue).

Drugs used in local anesthesia usually end in the suffix "-aine" (such as lidocaine, novocaine or tetracaine) because they're chemically very similar to cocaine -- the first local anesthetic. The problem with using cocaine as an anesthetic is that it's addictive and highly stimulating to the cardiovascular system, so synthetic alternatives were developed. Some of these drugs have negative side effects of their own, such as allergic reactions, and have fallen out of favor. This is why dentists today typically use lidocaine instead of novocaine. The other issue with these drugs is that they're vasodilators, which means that they widen blood vessels and could lead to excessive bleeding, but epinephrine can counteract this effect.

Local anesthesia usually wears off within four to five hours. The pain relief lasts longer than the actual procedure most of the time. There are usually very few side effects, but patients do have to be careful with the numbed area -- if you leave the dentist's office unable to feel half your face, you might accidentally bite your cheeks. In rare cases, injected local anesthetics can cause nerve damage, but they're typically low-risk if administered correctly to a healthy person.

In the next section, we'll look at a related type of anesthesia: regional.

Regional Anesthesia, from Peripheral to Central

Sometimes, the terms "local anesthesia" and "regional anesthesia" are used interchangeably. For the purposes of this article, we'll use regional anesthesia to describe anesthesia that's used in a wider region of the body. For example, while local anesthesia may be used to numb an area on the leg, regional anesthesia can numb the entire leg. This is known as peripheral regional anesthesia because it blocks a single nerve or specific bundle of nerves. The other type of regional anesthesia is central anesthesia, which usually involves an injection into the cerebrospinal fluid or the epidural space just outside the spinal canal.

Regional anesthesia is also known as a nerve block. Some of the same drugs that we learned about in the local anesthesia section are used in regional anesthesia -- they're just used in larger dosages and have a stronger effect on the central nervous system. Patients can remain conscious for procedures under regional anesthesia, but they may also be sedated during the administration of the block, during the medical procedure or during both. This depends on the procedure as well as the patient's preference -- some people would rather not be conscious.


Sometimes regional anesthetics are given with a single injection, but they can also be given intravenously or continuously through a catheter. One technique, called a Bier block, uses a tourniquet to keep blood from flowing through a limb before the drug is injected into a vein. (It can only be used with a relatively short procedure, though.)

Women who have given birth are probably very familiar with the central anesthetic technique known as an epidural. In this procedure, an anesthesiologist inserts a catheter into the epidural space, typically in the lower back area. This continuously feeds drugs such as lidocaine as well as fentanyl or clonidine to provide pain relief, resulting in a loss of sensation from the waist down.

Spinal blocks, which are injected into the cerebrospinal fluid, are often used for other procedures below the waist, such as Cesarean sections or hernia surgery. They tend to paralyze further than epidurals. While patients are usually awake for a spinal block during a C-section, they may be sedated for other procedures.

Regional anesthesia carries more risks than local anesthesia, such as seizures and heart attacks, because of the increased involvement of the central nervous system. Sometimes regional anesthesia fails to provide enough pain relief or paralysis, and switching to general anesthesia is necessary.

Patients under regional anesthesia need strict monitoring, which we'll look at in the next section.

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.

General Anesthesia Administration and Recovery

So how does a person "go under"? General anesthesia may be administered via gas, an IV line or a combination of both. Often, patients are first given an IV injection to induce unconsciousness, then gas to prolong and maintain the effect. (Some injected anesthetics can maintain the correct level of anesthesia without the use of gas.) The gas is usually isoflurane or desflurane combined with nitrous oxide.

Ketamine, sedatives (such as Valium) and depressants like Sodium Pentothal may be injected into an IV that was inserted prior to surgery. In addition, the person administering anesthesia might give you a muscle relaxant to ensure deeper paralysis, especially if your operation involves major organs.


As surgery ends, the gases are turned off and the IV anesthetic is discontinued. Patients go to a PACU (post-anesthesia care unit) where they are closely monitored. Often, you'll be given warm IV fluids to counteract both the dehydration that results from anesthesia as well as shivering from changes in body temperature. As the analgesic effect of the anesthetic wears off, you'd also receive some sort of pain relief -- an oral medication or even morphine, depending on the surgery. Some people recover within an hour, while others take longer to completely awaken.

After waking up, it's possible you'll deal with lasting side effects: vomiting, nausea and numbness in the area where surgery was performed. You'll probably feel disoriented and require assistance to get around.

It's also important to mention that there are serious risks associated with general anesthesia, including suffocation, allergic reaction, organ failure, stroke and death. These are concerns you should discuss with your doctor before your surgery.

Anesthesia has come a long way since its earliest days. Next, we'll take a brief look at the history of anesthesia.

The Shadowy History of Anesthesia

A nurse regulates the flow of anesthetic during an operation at a London hospital, October 1938.
A nurse regulates the flow of anesthetic during an operation at a London hospital, October 1938.
Felix Man/Picture Post/Getty Images

Although it hasn't always been called anesthesia, the concept has been around for as long as people have been performing surgery -- no matter how primitive or crude. Early anesthetics were soporifics (which dull the senses and induce sleep) or narcotics. These included opium, mandrake, jimsonweed, marijuana, alcohol and belladonna. Native American societies such as the Incas chewed coca leaves (from which cocaine is derived). While all of these substances can provide some level of pain relief, sedation or amnesia, there were no guarantees. There is also a history of using nonmedical methods such as hypnosis, ice (to numb the area) and acupuncture.

By the mid-1840s, the only two anesthetic agents regularly used in industrialized countries were opium and alcohol. Both had many negative side effects, such as addiction, and neither could typically dull the pain completely by themselves. Doses large enough to provide the desired effect could just as easily result in death. Sometimes, patients were knocked unconscious by a blow to the head. But without good anesthesia, surgery usually resulted in the tortured screams of patients.


Everything changed in 1846. A dentist named Dr. William Morton put on a demonstration at Massachusetts General Hospital when he removed a tumor from the jaw of a patient. Prior to the operation, he used a sponge soaked with ether to render his patient unconscious. Afterward, the patient claimed that he had no memory of the operation or any pain. The doctors witnessing this were highly skeptical, but Morton was soon hailed as the conqueror of pain.

The problem was that Morton wasn't the first to claim the discovery of modern surgical pain relief. A few years after medical journals published articles about Morton, Dr. Crawford Long stated that he had first used ether in operations in 1841 after observing its effects upon recreational users. In addition, Dr. Charles Jackson claimed that his work had influenced Morton. He went to Congress in attempt to gain recognition with the support of Oliver Wendell Holmes (credited with suggesting the use of the word "anesthesia").

The American Medical Association as well as the American Dental Association endorsed Dr. Horace Wells, a dentist, as the first person to use nitrous oxide to pull teeth in 1845. Chloroform was first used as an anesthetic by Dr. James Simpson in the mid-1840s as well. Because it's highly toxic, chloroform ceased to be used in favor of ether in the early 1900s.

Now, there's a wide variety of anesthetics available, but we still use derivatives of some of these early substances, such as morphine, the "-aine" drugs chemically related to cocaine, and nitrous oxide. Anesthesia continues to evolve and become safer, enabling doctors to perform necessary and life-saving operations. The next time you need anesthesia, you'll have a better understanding of the process.

For lots more information on anesthesia and related topics, check out the links on the next page.

Lots More Information

Related Articles

More Great Links

  • Albin, Maurice S. and Patrick Sim. "Oliver Wendell Holmes, M.D., 1809-1984, Poet, Physician and Anesthesia Advocate." ASA Newsletter. October 2004.
  • American Association of Nurse Anesthetists. "Conscious Sedation: What Patients Should Expect." AANA. 2009.
  • American Dental Association. "Anesthesia." ADA. 2009.
  • American Pregnancy Association. "Epidural Anesthesia." APA. October 2007.
  • American Society of Anesthesiologists. "Anesthesia and You." ASA. 1994.
  • BCMA Medical Museum. "Anaesthesiology Collection." British Columbia Medical Association. 2008.
  • Hewer, C. Langton. "The Stages and Signs of General Anesthesia." British Journal of Medicine. August 7, 1937.
  • Mayo Clinic Staff. "General Anesthesia." June 27, 2008. Mayo Foundation for Medical Education and Research (MFMER).
  • Massachusetts General Hospital Neurosurgical Service, Harvard Medical School. "'We Have Conquered Pain': A Celebration of Ether 1846-1996." MSH Neurosurgical Service. May 11, 2005.
  • Miller, Ron D. "Miller's Anesthesia." New York : Elsevier/Churchill Livingstone. 2005.
  • National Institute of General Medical Sciences. "Understanding Anesthesia." National Institutes of Health. December 2007.
  • Oysten, Dr. John. "A Patient's Guide to Local and Regional Anesthesia." Department of Anesthesia at Orillia Soldiers' Memorial Hospital. 1998.