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How Anesthesia Works
by Eugenie Heitmiller, Ph.D.

If you are faced with the possibility of needing surgery in the future, chances are you will need some type of anesthesia to go along with it. There are many different types of anesthesia. Which one you will need depends on a variety of factors such as the type of surgery you are having and your state of health. Some surgical procedures require only an injection of local anesthesia into the incision area. Other procedures cannot be performed unless you are completely anesthetized -- unconscious and unaware of pain. Your anesthesiologist will tell you which type of anesthesia you need.

In this edition of How Stuff Works, we will look at the many types of anesthesia so that you can understand what it is, how it works, and how the anesthesiologist determines which type of anesthesia to use for your particular situation. By addressing this topic, we hope that you will have a better understanding of what happens to you while you're anesthetized and perhaps make it a little less mysterious.

The Basics


Consciousness is being clearly aware of yourself and your environment.

Unconsciousness is when you are completely or partially unaware of yourself and your environment, or you don't respond to sensory stimuli.

Conscious sedation is caused when anesthesiologist administers depressant drugs and/or analgesics in addition to anesthesia during surgery. Consciousness is depressed and you may fall asleep, but are not unconscious.

Sleep is a state of reduced consciousness, depressed metabolism, and little activity of the skeletal muscles. Strong stimuli such as a loud noise, bright light or shaking can arouse the sleeper.

Anesthesia is divided into four basic categories:

  • general anesthesia
  • regional anesthesia
  • local anesthesia
  • sedation

Each type of anesthesia has an effect on a part of the nervous system, which results in a depression or numbing of nerve pathways. General anesthesia affects the brain cells, which causes you to lose consciousness. Regional anesthesia has an effect on a large bundle of nerves to a particular area of the body, which results in losing sensation to that area without affecting your level of consciousness. Local anesthesia causes you to lose sensation in a very specific area.

Some of the drugs that produce general anesthesia in large doses can be used to produce sedation, or "twilight sleep" in lower doses. Sedation can be given in many ways. A common example of an anesthetic gas that is used for sedation is nitrous oxide or laughing gas.

If you are scheduled to have surgery, you may be told not to eat anything for eight hours. It is very important that you follow whatever instructions you are given for not eating or drinking anything prior to surgery. Why? Because when you are given anesthesia, you lose the ability to protect your lungs from inhaling something you're not supposed to inhale. When you are awake, you can usually swallow saliva and food without choking because part of the swallowing mechanism involves a reflex that results in covering the opening into the lungs. When you are anesthetized, you lose that reflex. So, if you have any solids or liquids in your stomach, they could come up into your mouth and be inhaled into your lungs. The result could be very serious lung damage.

General Anesthesia
General anesthetics produce an unconscious state. In this state a person is:

  1. unaware of what is happening
  2. pain-free
  3. immobile
  4. free from any memory of the period of time during which he or she is anesthetized

It is not completely clear exactly how general anesthetics work at a cellular level, but it is speculated that general anesthetics affect the spinal cord (resulting in immobility), the brain-stem reticular activating system (resulting in unconsciousness) and the cerebral cortex (seen as changes in electrical activity on an electroencephalogram).

General anesthesia can be administered as an inhaled gas or as an injected liquid. There are several drugs and gases that can be combined or used alone to produce general anesthesia. The potency of a given anesthetic is measured as minimum alveolar concentration (MAC). This term describes the potency of anesthetic gases. (Aveolar is the area in the lung where gases enter and exit the bloodstream via the capillary system). Technically, MAC is the alveolar partial pressure of a gas at which 50 percent of humans will not move to a painful stimulus (e.g. skin incision). Injected liquid anesthetics have a "MAC equivalent" which is the blood concentration of the liquid anesthetic that provides the same effect. Using MAC as a guideline, the amount of anesthetic given to a patient depends on that particular patient's needs.

When anesthetics reach the bloodstream, the drugs that affect the brain pass through other blood vessels and organs so they are often affected too. Therefore, patients must be carefully monitored. The anesthesiologist continuously monitors the patient's heart rate, heart rhythm, blood pressure, respiratory rate, and oxygen saturation. Some patients may have even more extensive monitoring depending on their health and which type of procedure or surgery they are having.

Most adults are first anesthetized with liquid intravenous anesthetics followed by anesthetic gases after they are asleep. Children, however, may not like having an injection or intravenous catheter placed in them while they are awake. Therefore, they often breathe themselves to sleep with anesthetic gases given through a mask.

Inhaled Anesthetics
Many adults may remember having ether for their anesthetic when they were young. Ether is a flammable anesthetic that is no longer used in the United States. Today, the commonly used inhaled anesthetics are nitrous oxide (also known as laughing gas), sevoflurane, desflurane, isoflurane and halothane.

Why do we have so many different kinds of gases? Because each gas has its own special properties. For example, sevoflurane and halothane are easy to inhale while desflurane is very irritating to inhale and has a shorter duration of action. If you need to breathe yourself to sleep, halothane or sevoflurane would be easiest to inhale. If a very short-acting anesthetic is needed, the anesthesiologist can switch to desflurane after you fall asleep. Nitrous oxide is easy to inhale, but when used alone is not potent enough to be a complete general anesthetic. However, it can be used alone for sedation, or combined with one of the other inhaled anesthetics or injected liquid anesthetics for general anesthesia.

These gases have different effects on other organs as well. For example, halothane may cause the heart rate to slow down and the blood pressure to decrease while desflurane may cause the heart rate to speed up and the blood pressure to increase. How do these inhaled anesthetics reach the brain? When an anesthetic gas is inhaled into the lungs, the blood that travels through the lungs carries the anesthetic gas to central nervous system cells. The rate at which the bloodstream takes up the anesthetic is dependent on many factors including the concentration of the inspired gas, the rate of flow of the gas from the anesthesia machine, the solubility of the gas in blood, the rate and depth of breathing, and the amount of blood the heart pumps each minute in the person breathing the gas.

An important property of anesthetics is reversibility. When the surgery is over, the anesthesiologist wants to shut off the anesthetic and have the patient wake up from the anesthetic-induced sleep. Once the anesthetic gas is turned off, the blood stream brings the gas back to the lungs where it is eliminated. The more soluble the gas is in blood, the longer it takes to eliminate. Nitrous oxide and desflurane are the shortest-acting anesthetic gases because they are the least soluble in blood.

Injected Anesthetics
A liquid anesthetic drug is delivered to the brain by injecting it directly into the bloodstream usually through an intravenous catheter. Examples of injected drugs are barbiturates, propofol, ketamine, and etomidate, as well as larger doses of narcotics (such as morphine) and benzodiazepines (Valium-like drugs). These drugs quickly reach the brain and their effect is dependent on several factors including the volume in which the drug is distributed in the body, the fat-solubility of the drug, and how quickly the body eliminates the drug.

A commonly used injected barbiturate anesthetic is sodium thiopental, also known as Pentothal. This drug is fat-soluble and acts very quickly. If you receive sodium thiopental and then you are asked to count backward from 100 after the drug is injected, you probably won't remember counting past 95. Some injected anesthetics are used in low doses for sedation. A small dose of a narcotic or a benzodiazepine like Valium or Versed can significantly decrease anxiety. These drugs are used in these doses either as a premedication prior to general anesthesia or as "twilight sleep" or sedation when used in conjunction with local or regional anesthesia.

Regional Anesthesia
Regional anesthesia is so named because a "region" of the body is anesthetized without making the person unconscious. One example of this is spinal anesthesia, which is often used on woman during childbirth. A local anesthetic is injected into the spinal fluid and causes a loss of sensation of the lower body. Spinal anesthesia can be used for surgery on the legs or lower abdomen (below the bellybutton).

Epidural anesthesia is similar to spinal anesthesia in that a patient loses sensation in the legs and lower abdomen, but instead of injecting the local anesthetic into the spinal fluid, the anesthetic is injected into a space outside the spinal canal called the epidural space. A small tube or catheter can be placed into this space and a local anesthetic can be infused (fed) through the tube for hours, days, or even weeks. This type of anesthesia can be used for surgery with larger doses of anesthetic, or for chronic pain relief with lower doses of anesthetic. Regional anesthesia techniques can be used to block very specific areas such as one foot, one leg, one arm, or one side of the neck. In these cases, a smaller group of nerves is blocked by injection of the local anesthetic into a specific area. For spinals and epidurals, narcotic painkillers such as morphine and fentanyl can be used in addition to a local anesthetic.

Local Anesthesia
Local anesthesia involves numbing a small area by injecting a local anesthetic under the skin just where an incision is to be made. When used alone, this type of anesthesia has the least number of risks. Local anesthetics are thought to block nerve impulses by decreasing the permeability of nerve membranes to sodium ions. There are many different local anesthetics that differ in absorption, toxicity, and duration of action.

One of the most commonly used local anesthetics is lidocaine (Xylocaine). Lidocaine can be administered as an injection or placed topically on mucous membranes. Another topical anesthetic is cocaine. Cocaine is primarily used to anesthetize the nasal passages for surgical procedures. A topical anesthetic that is gaining popularity for anesthetizing the skin prior to painful procedures, such as injections, is known as eutectic mixture of local anesthetics (EMLA) cream which contains lidocaine and prilocaine. This white cream is placed on the skin and then covered with an occlusive dressing for approximately one hour to obtain a good numbing effect. In addition, EMLA can be used to numb the skin prior to giving injections or pulling superficial splinters.

Some of the drugs that produce general anesthesia in large doses can be used to produce sedation or "twilight sleep" in lower doses. Sedation can be given in many ways. A common example of an anesthetic gas that is used for sedation is nitrous oxide or laughing gas. Liquid sedating drugs are usually given by injection but some can also be given by mouth. Ketamine and Versed are examples of sedating drugs that can be given by injection or by mouth. The oral route is particularly useful for sedating children who do not like injections.

Children who refuse to drink medications may also receive sedation through the rectum via a small, lubricated tube or via the nasal route by spraying it into the nose. Regional and local anesthesia can be combined with sedation to make patients more comfortable during a procedure in which general anesthesia is not necessary, or when general anesthesia may be too large a risk for the patient.

The Anesthesiologist
To become an anesthesiologist, a person must complete college, medical school, an internship and a three-year anesthesiology residency (see How Becoming a Doctor Works). An anesthesiologist is a physician who specializes in anesthetic administration, pain relief, and the care of patients before, during, and after surgery. The anesthesiologist monitors vital organ functions including heart rate, blood pressure, breathing, and blood-oxygen saturation during surgery.

Prior to the day of surgery, you may meet your anesthesiologist. Most of the time, you meet on the day of surgery. Your anesthesiologist may ask several questions about your medical history. This information is important in determining which type of anesthesia you should receive. Your anesthesiologist can answer any questions you have about the side effects caused by anesthesia. Children receive the same anesthetics for surgery as adults, but the doses of the injected anesthetics are determined by the child's weight.

My patients are often small children who are not happy about seeing a doctor. Their parents or guardians are usually very anxious about the anesthesia. A large part of my job before surgery is explaining the anesthetic plan and reassuring the parents or guardians by answering questions about what is about to happen to their child. Children may be given the sedative, midazolon, before they are anesthetized. This is often in the form of a cherry-flavored drink. After about 10 minutes, the children are smiling and don't mind coming to the operating room with one of their parents or guardians. They usually don't remember it either. In the operating room, the child receives the anesthetic either by breathing a gas through a mask or by an injection. After the child is asleep, the parents or guardians leave and the child's safety is in the anesthesiologist's hands.

While the child is anesthetized, blood pressure is closely monitored with a blood pressure cuff, the heart rate and rhythm with an electrocardiogram (also known as an EKG), the child's breathing, temperature, and the saturation of oxygen in the blood with a special machine called a pulse oximeter. These vital signs are used to help determine the depth of anesthesia. In addition, there is a monitor that can measure the depth of anesthesia using brain waves. However, the most important part of the monitoring is done by the anesthesiologist watching the patient.

After the surgery, the patient goes to the recovery room for constant monitoring until he or she awakens. Then the patient is ready to be discharged from the recovery room. The doctor assigns the patient to a hospital bed or discharges the patient if it is outpatient surgery. Sometimes, the patient goes to an intensive care unit after surgery. Anesthesiologists, who specialize in intensive care, often take care of patients in the intensive care unit as well.

About the Author
Eugenie Heitmiller, M.D. is an associate professor of Anesthesia and Pediatrics at Johns Hopkins Medical School. She has been an anesthesiologist since 1982 and specializes in anesthesia for children. She finds anesthesiology an exciting and challenging field because of the continuous development of new drugs, new types of operations, and equipment. Most of all, with each new patient comes a new possibility for making what is for most people a difficult and frightening experience a little safer and easier.


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