How Anesthesia Works

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.

Read on for more details about different types of anesthetic drugs.

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.

For nearly 200 years, scientists weren't entirely clear on how general anesthesia worked, but a 2019 study found that these anesthetic drugs tap into a part of the brain that regulates several bodily functions, including sleep.

When Is General Anesthesia Necessary?

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.

What to Know Before You Go Under

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. They'll need to know about any preexisting medical conditions, as well as any prescription medications (whether or not you use them for pain management), herbal supplements or over-the-counter medicines you may take.

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 as it's possible for someone under general anesthesia to aspirate, or breathe in, the contents of the stomach.

Monitoring Patients Under General Anesthesia

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.

The 4 Stages of General Anesthesia

Anesthesia Awareness

Anesthesia awareness happens when patients can recall aspects of their surgery after being under general anesthesia. They may feel pressure, pain or just be aware of what's going on. This can happen if one or more of the drugs given during general anesthesia is inadequately administered, the patient is improperly monitored or one or more machines used to monitor the patient has a malfunction.

In the most extreme cases, patients have reported being completely paralyzed but feeling all the pain of their surgery, which results in severe emotional trauma. While anesthesia awareness is a horrifying thing to contemplate, the phenomenon is believed to be incredibly rare (happening to one or two people per 1,000 surgeries) and is no reason to put off having necessary surgery [source: Mayo Clinic].

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

Under conscious 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. 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.

Conscious Sedation vs. General Anesthesia

Conscious 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 entails a sedative such as ketamine or nitrous oxide, which depresses the central nervous system. Sometimes an anesthesiologist will instead use a dissociative, such as diazepam (more commonly known as Valium) or midazolam, which keeps nerve sensations from reaching the brain.

For conscious sedation, an anesthesiologist will likely use one of these types of drugs 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, a patient would inhale nitrous oxide and other sedative gases, but a certified registered nurse anesthetist (CRNA) would inject ketamine or Valium into an IV line.

How Long Does Conscious Sedation Last?

How long the conscious 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.

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.

Regional Anesthetic Types and Techniques

While a local anesthetic 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 and uses larger dosages of some of the same drugs used as local anesthetics, resulting in 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.

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 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 common in 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.

Administration and Monitoring

Sometimes doctors or nurses administer regional anesthetics with a single injection, but they can also do so 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. (This option is only viable with a relatively short procedure, though.)

Patients need strict monitoring as it carries more risks than local anesthetics, 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 becomes necessary.

This is another form of anesthesia that you've likely experienced before. For example, in addition to procedural sedation during major dental work, dentists also use a local anesthetic when they inject your gum to numb the area. Local anesthesia makes 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 Used in the Past and Present

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. 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 scientists developed synthetic alternatives.

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.

How Long Does Local Anesthesia Last?

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.

So how does a person "go under"? A CRNA or another member of your anesthesia team may administer general anesthesia via gas, an IV line or a combination of both. Often, patients first receive 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.

After Surgery

As surgery ends, the gases are turned off and the IV anesthetic is discontinued. Patients go to a PACU (post-anesthesia care unit) — sometimes called a recovery room — 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.

Side Effects and Risks

After waking up, it's possible you'll deal with side effects such as 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.

Not all anesthesia is administered by an anesthesiologist — it really depends on the situation and the type of anesthesia being administered. For example, if you get a deep gash on your head and the ER doctor wants to numb the area before stitching it up, he or she doesn't need to call in an anesthesiologist to inject a local anesthetic.

An anesthesiologist gets involved if the procedure calls for regional or general anesthesia. This type of doctor specializes in anesthetic administration, pain relief and the care of patients before, during and after surgery. To become an anesthesiologist, a person must complete an internship and a three-year anesthesiology residency after medical school. They often have an anesthesia care team (ACT) that also includes nurse anesthetists and anesthesiologist assistants.

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. 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 dull the pain completely. 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 an 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 fell out of favor while ether rose in popularity 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.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.