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
Anesthesia is divided
into four basic categories:
DefinitionsConsciousness 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.
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.
- general anesthesia
- regional anesthesia
- local anesthesia
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
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 anesthetics produce an
unconscious state. In this state a person is:
- unaware of what is happening
- 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
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.
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
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.
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 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.
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
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.
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
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