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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
                     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
                                         Definitions
                               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.


              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
                                           brainstem
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 ectatic 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.
                                       Sedation
   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
                                         cherryflavored
   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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