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Anesthesia, General

Definition

General anesthesia is the induction of a state of unconsciousness with the absence of pain sensation over the entire body, through the administration of anesthetic drugs. It is used during certain medical and surgical procedures.

Purpose

General anesthesia has many purposes including:

pain relief (analgesia)
blocking memory of the procedure (amnesia)
producing unconsciousness
inhibiting normal body reflexes to make surgery safe and easier to perform
relaxing the muscles of the body

Description

Anesthesia performed with general anesthetics occurs in four stages which may or may not be observable because they can occur very rapidly:

Stage One: Analgesia. The patient experiences analgesia or a loss of pain sensation but remains conscious and can carry on a conversation.
Stage Two: Excitement. The patient may experience delirium or become violent. Blood pressure rises and becomes irregular, and breathing rate increases. This stage is typically bypassed by administering a barbiturate, such as sodium pentothal, before the anesthesia.
Stage Three: Surgical Anesthesia. During this stage, the skeletal muscles relax, and the patient's breathing becomes regular. Eye movements slow, then stop, and surgery can begin.
Stage Four: Medullary Paralysis. This stage occurs if the respiratory centers in the medulla oblongata of the brain that control breathing and other vital functions cease to function. Death can result if the patient cannot be revived quickly. This stage should never be reached. Careful control of the amounts of anesthetics administered prevent this occurrence.

ANESTHETICS: HOW THEY WORK
Type	Name(s)	Administered	Affect
General	Halothane, Enflurane Isoflurane, Ketamine, Nitrous Oxide, Thiopental	Intravenously, Inhalation	Produces total unconsciousness affecting the entire body
Regional	Mepivacaine, Chloroprocaine, Lidocaine	Intravenously	Temporarily inter- rupts transmission of nerve impulses (temperature, touch, pain) and motor functions in a large area to be treated; does not produce unconsciousness
Local	Procaine, Lidocaine, Tetracaine, Bupivacaine	Intravenously	Temporarily blocks transmission of nerve impulses and motor functions in a specific area; does not produce unconsciousness
Topical	Benzocaine, Lidocaine Dibucaine, Pramoxine, Butamben, Tetracaine	Demal (Sprays, Drope, Ointments, Creams, Gels)	Temporarily blocks nerve endings in skin and mucous membranes; does not produce unconsciousness

Agents used for general anesthesia may be either gases or volatile liquids that are vaporized and inhaled with oxygen, or drugs delivered intravenously. A combination of inhaled anesthetic gases and intravenous drugs are usually delivered during general anesthesia; this practice is called balanced anesthesia and is used because it takes advantage of the beneficial effects of each anesthetic agent to reach surgical anesthesia. If necessary, the extent of the anesthesia produced by inhaling a general anesthetic can be rapidly modified by adjusting the concentration of the anesthetic in the oxygen that is breathed by the patient. The degree of anesthesia produced by an intravenously injected anesthesic is fixed and cannot be changed as rapidly. Most commonly, intravenous anesthetic agents are used for induction of anesthesia and then followed by inhaled anesthetic agents.

Key terms

Amnesia — The loss of memory.

Analgesia — A state of insensitivity to pain even though the person remains fully conscious.

Anesthesiologist — A medical specialist who administers an anesthetic to a patient before he is treated.

Anesthetic — A drug that causes unconsciousness or a loss of general sensation.

Arrhythmia — Abnormal heart beat.

Barbiturate — A drug with hypnotic and sedative effects.

Catatonia — Psychomotor disturbance characterized by muscular rigidity, excitement or stupor.

Hypnotic agent — A drug capable of inducing a hypnotic state.

Hypnotic state — A state of heightened awareness that can be used to modulate the perception of pain.

Hypoxia — Reduction of oxygen supply to the tissues.

Malignant hyperthermia — A type of reaction (probably with a genetic origin) that can occur during general anesthesia and in which the patient experiences a high fever, muscle rigidity, and irregular heart rate and blood pressure.

Medulla oblongata — The lowest section of the brainstem, located next to the spinal cord. The medulla is the site of important cardiac and respiratory regulatory centers.

Opioid — Any morphine-like synthetic narcotic that produces the same effects as drugs derived from the opium poppy (opiates), such as pain relief, sedation, constipation and respiratory depression.

Pneumothorax — A collapse of the lung.

Stenosis — A narrowing or constriction of the diameter of a passage or orifice, such as a blood vessel.

General anesthesia works by altering the flow of sodium molecules into nerve cells (neurons) through the cell membrane. Exactly how the anesthetic does this is not understood since the drug apparently does not bind to any receptor on the cell surface and does not seem to affect the release of chemicals that transmit nerve impulses (neurotransmitters) from the nerve cells. It is known, however, that when the sodium molecules do not get into the neurons, nerve impulses are not generated and the brain becomes unconscious, does not store memories, does not register pain impulses from other areas of the body, and does not control involuntary reflexes. Although anesthesia may feel like deep sleep, it is not the same. In sleep, some parts of the brain speed up while others slow down. Under anesthesia, the loss of consciousness is more widespread.

When general anesthesia was first introduced in medical practice, ether and chloroform were inhaled with the physician manually covering the patient's mouth. Since then, general anesthesia has become much more sophisticated. During most surgical procedures, anesthetic agents are now delivered and controlled by computerized equipment that includes anesthetic gas monitoring as well as patient monitoring equipment. Anesthesiologists are the physicians that specialize in the delivery of anesthetic agents. Currently used inhaled general anesthetics include halothane, enflurane, isoflurane, desfluorane, sevofluorane, and nitrous oxide.

Halothane (Fluothane) is a powerful anesthetic and can easily be overadministered. This drug causes unconsciousness but little pain relief so it is often used with other agents to control pain. Very rarely, it can be toxic to the liver in adults, causing death. It also has the potential for causing serious cardiac dysrhythmias. Halothane has a pleasant odor, and was frequently the anesthetic of choice for use with children, but since the introduction of sevofluorane in the 1990s, halothane use has declined.
Enflurane (Ethrane) is less potent and results in a more rapid onset of anesthesia and faster awakening than halothane. In addition, it acts as an enhancer of paralyzing agents. Enflurane has been found to increase intracranial pressure and the risk of seizures; therefore, its use is contraindicated in patients with seizure disorders.
Isoflurane (Forane) is not toxic to the liver but can cause some cardiac irregularities. Isofluorane is often used in combination with intravenous anesthetics for anesthesia induction. Awakening from anesthesia is faster than it is with halothane and enfluorane.
Desfluorane (Suprane) may increase the heart rate and should not be used in patients with aortic valve stenosis; however, it does not usually cause heart arrhythmias. Desflurane may cause coughing and excitation during induction and is therefore used with intravenous anesthetics for induction. Desflurane is rapidly eliminated and awakening is therefore faster than with other inhaled agents.
Sevofluorane (Ultane) may also cause increased heart rate and should not be used in patients with narrowed aortic valve (stenosis); however, it does not usually cause heart arrhythmias. Unlike desfluorane, sevofluorane does not cause any coughing or other related side effects, and can therefore be used without intravenous agents for rapid induction. For this reason, sevofluorane is replacing halothane for induction in pediatric patients. Like desfluorane, this agent is rapidly eliminated and allows rapid awakening.
Nitrous oxide (laughing gas) is a weak anesthetic and is used with other agents, such as thiopental, to produce surgical anesthesia. It has the fastest induction and recovery and is the safest because it does not slow breathing or blood flow to the brain. However, it diffuses rapidly into air-containing cavities and can result in a collapsed lung (pneumothorax) or lower the oxygen contents of tissues (hypoxia).

Commonly administered intravenous anesthetic agents include ketamine, thiopental, opioids, and propofol.

Ketamine (Ketalar) affects the senses, and produces a dissociative anesthesia (catatonia, amnesia, analgesia) in which the patient may appear awake and reactive, but cannot respond to sensory stimuli. These properties make it especially useful for use in developing countries and during warfare medical treatment. Ketamine is frequently used in pediatric patients because anesthesia and analgesia can be achieved with an intramuscular injection. It is also used in high-risk geriatric patients and in shock cases, because it also provides cardiac stimulation.
Thiopental (Pentothal) is a barbiturate that induces a rapid hypnotic state of short duration. Because thiopental is slowly metabolized by the liver, toxic accumulation can occur; therefore, it should not be continuously infused. Side effects include nausea and vomiting upon awakening.
Opioids include fentanyl, sufentanil, and alfentanil, and are frequently used prior to anesthesia and surgery as a sedative and analgesic, as well as a continuous infusion for primary anesthesia. Because opioids rarely affect the cardiovascular system, they are particularly useful for cardiac surgery and other highrisk cases. Opioids act directly on spinal cord receptors, and are freqently used in epidurals for spinal anesthesia. Side effects may include nausea and vomiting, itching, and respiratory depression.

Propofol (Diprivan) is a nonbarbiturate hypnotic agent and the most recently developed intravenous anesthetic. Its rapid induction and short duration of action are identical to thiopental, but recovery occurs more quickly and with much less nausea and vomiting. Also, propofol is rapidly metabolized in the liver and excreted in the urine, so it can be used for long durations of anesthesia, unlike thiopental. Hence, propofol is rapidly replacing thiopental as an intravenous induction agent. It is used for general surgery, cardiac surgery, neurosurgery, and pediatric surgery.

General anesthetics are given only by anesthesiologists, the medical professionals trained to use them. These specialists consider many factors, including a patient's age, weight, medication allergies, medical history, and general health, when deciding which anesthetic or combination of anesthetics to use. General anesthetics are usually inhaled through a mask or a breathing tube or injected into a vein, but are also sometimes given rectally.

General anesthesia is much safer today than it was in the past. This progress is due to faster-acting anesthetics, improved safety standards in the equipment used to deliver the drugs, and better devices to monitor breathing, heart rate, blood pressure, and brain activity during surgery. Unpleasant side effects are also less common.

Recommended dosage

The dosage depends on the type of anesthetic, the patient's age and physical condition, the type of surgery or medical procedure being done, and other medication the patient takes before, during, or after surgery.

Precautions

Although the risks of serious complications from general anesthesia are very low, they can include heart attack, stroke, brain damage, and death. Anyone scheduled to undergo general anesthesia should thoroughly discuss the benefits and risks with a physician. The risks of complications depend, in part, on a patient's age, sex, weight, allergies, general health, and history of smoking, drinking alcohol, or drug use. Some of these risks can be minimized by ensuring that the physician and anesthesiologist are fully informed of the detailed health condition of the patient, including any drugs that he or she may be using. Older people are especially sensitive to the effects of certain anesthetics and may be more likely to experience side effects from these drugs.

Patients who have had general anesthesia should not drink alcoholic beverages or take medication that slow down the central nervous system (such as antihistamines, sedatives, tranquilizers, sleep aids, certain pain relievers, muscle relaxants, and anti-seizure medication) for at least 24 hours, except under a doctor's care.

Special conditions

People with certain medical conditions are at greater risk of developing problems with anesthetics. Before undergoing general anesthesia, anyone with the following conditions should absolutely inform their doctor.

ALLERGIES. Anyone who has had allergic or other unusual reactions to barbiturates or general anesthetics in the past should notify the doctor before having general anesthesia. In particular, people who have had malignant hyperthermia or whose family members have had malignant hyperthermia during or after being given an anesthetic should inform the physician. Signs of malignant hyperthermia include rapid, irregular heartbeat, breathing problems, very high fever, and muscle tightness or spasms. These symptoms can occur following the administration of general anesthesia using inhaled agents, especially halothane. In addition, the doctor should also be told about any allergies to foods, dyes, preservatives, or other substances.

PREGNANCY. The effects of anesthetics on pregnant women and fetuses vary, depending on the type of drug. In general, giving large amounts of general anesthetics to the mother during labor and delivery may make the baby sluggish after delivery. Pregnant women should discuss the use of anesthetics during labor and delivery with their doctors. Pregnant women who may be given general anesthesia for other medical procedures should ensure that the treating physician is informed about the pregnancy.

BREASTFEEDING. Some general anesthetics pass into breast milk, but they have not been reported to cause problems in nursing babies whose mothers were given the drugs.

OTHER MEDICAL CONDITIONS. Before being given a general anesthetic, a patient who has any of the following conditions should inform his or her doctor:

neurological conditions, such as epilepsy or stroke
problems with the stomach or esophagus, such as ulcers or heartburn
eating disorders
loose teeth, dentures, bridgework
heart disease or family history of heart problems
lung diseases, such as emphysema or asthma
history of smoking
immune system diseases

arthritis or any other conditions that affect movement
diseases of the endocrine system, such as diabetes or thyroid problems

Side effects

Because general anesthetics affect the central nervous system, patients may feel drowsy, weak, or tired for as long as a few days after having general anesthesia. Fuzzy thinking, blurred vision, and coordination problems are also possible. For these reasons, anyone who has had general anesthesia should not drive, operate machinery, or perform other activities that could endanger themselves or others for at least 24 hours, or longer if necessary.

Most side effects usually disappear as the anesthetic wears off. A nurse or doctor should be notified if these or other side effects persist or cause problems, such as:

Headache
vision problems, including blurred or double vision
shivering or trembling
muscle pain
dizziness, lightheadedness, or faintness
drowsiness
mood or mental changes
nausea or vomiting
sore throat
nightmares or unusual dreams

A doctor should be notified as soon as possible if any of the following side effects occur within two weeks of having general anesthesia:

severe headache
pain in the stomach or abdomen
back or leg pain
severe nausea
black or bloody vomit
unusual tiredness or weakness
weakness in the wrist and fingers
weight loss or loss of appetite
increase or decrease in amount of urine
pale skin
yellow eyes or skin

Interactions

General anesthetics may interact with other medicines. When this happens, the effects of one or both of the drugs may be altered or the risk of side effects may be greater. Anyone scheduled to undergo general anesthesia should inform the doctor about all other medication that he or she is taking. This includes prescription drugs, nonprescription medicines, and street drugs. Serious and possibly life-threatening reactions may occur when general anesthetics are given to people who use street drugs, such as cocaine, marijuana, phencyclidine (PCP or angel dust), amphetamines (uppers), barbiturates (downers), heroin, or other narcotics. Anyone who uses these drugs should make sure their doctor or dentist knows what they have taken.

Resources

Books

Dobson, Michael B. Anaesthesia at the District Hospital. 2nd ed. World Health Organization, 2000.

Periodicals

Adachi, Y.U., K. Watanabe, H. Higuchi, and T. Satoh. "The Determinants of Propofol Induction of Anesthesia Dose." Anesthesia and Analgesia 92 (2001): 656-661.

Other

Wenker, O. "Review of Currently Used Inhalation Anesthetics Part I." "The Internet Journal of Anesthesiology." 1999. http://www.ispub.com/journals/IJA/Vol3N2/inhal1.htm.

anesthesia /an·es·the·sia/ (an″es-the´zhah)

1. loss of sensation, usually by damage to a nerve or receptor.

2. loss of the ability to feel pain, caused by administration of a drug or other medical intervention.

basal anesthesia narcosis produced by preliminary medication so that the inhalation of anesthetic necessary to produce surgical anesthesia is greatly reduced.

block anesthesia regional a.

bulbar anesthesia that due to a lesion of the pons.

caudal anesthesia see under block.

closed circuit anesthesia that produced by continuous rebreathing of a small amount of anesthetic gas in a closed system with an apparatus for removing carbon dioxide.

crossed anesthesia see under hemianesthesia.

anesthesia doloro´sa pain in an area or region that is anesthetic.

electric anesthesia that induced by passage of an electric current.

endotracheal anesthesia that produced by introduction of a gaseous mixture through a tube inserted into the trachea.

epidural anesthesia that produced by injection of the anesthetic into the extradural space, either between the vertebral spines or into the sacral hiatus (caudal block).

general anesthesia a state of unconsciousness and insusceptibility to pain, produced by administration of anesthetic agents by inhalation, intravenously, intramuscularly, rectally, or via the gastrointestinal tract.

infiltration anesthesia local anesthesia produced by injection of the anesthetic solution in the area of terminal nerve endings.

inhalation anesthesia that produced by the inhalation of vapors of a volatile liquid or gaseous anesthetic agent.

insufflation anesthesia that produced by blowing a mixture of gases or vapors into the respiratory tract through a tube.

local anesthesia that produced in a limited area, as by injection of a local anesthetic or by freezing with ethyl chloride.

lumbar epidural anesthesia that produced by injection of the anesthetic into the epidural space at the second or third lumbar interspace.

muscular anesthesia loss or lack of muscle sense.

open anesthesia general inhalation anesthesia using a cone, without significant rebreathing of exhaled gases.

peripheral anesthesia that due to changes in the peripheral nerves.

regional anesthesia insensibility of a part induced by interrupting the sensory nerve conductivity of that region of the body; it may be produced by either field block or nerve block (see under block ).

sacral anesthesia spinal anesthesia by injection of anesthetic into the sacral canal and about the sacral nerves.

saddle block anesthesia see under block.

spinal anesthesia

1. regional anesthesia by injection of a local anesthetic into the subarachnoid space around the spinal cord.

2. loss of sensation due to a spinal lesion.

surgical anesthesia that degree of anesthesia at which operation may safely be performed.

tactile anesthesia loss or impairment of the sense of touch.

topical anesthesia that produced by application of a local anesthetic directly to the area involved, as to the oral mucosa or the cornea.

transsacral anesthesia sacral a.

general anesthesia

Loss of the ability to perceive pain associated with loss of consciousness, produced by anesthetic agents.

general anesthesia,

the absence of sensation and consciousness as induced by various anesthetic medications, given by inhalation or IV injection. The components of general anesthesia are analgesia, amnesia, muscle relaxation, control of vital signs, and unconsciousness. The depth of anesthesia is planned to allow the surgical procedure to be performed without the patient's experiencing pain, moving, or having any recall of the procedure. Endotracheal intubation or insertion of another artificial airway device and respiratory support are often necessary. General anesthesia may be administered only by an anesthesiologist with or without an anesthesia assistant or a Certified Registered Nurse Anesthetist. See also anesthesia. Compare local anesthesia, regional anesthesia, topical anesthesia.

anesthesia (an´esthē´zē

, an´esthē´zh

n the loss of feeling or sensation, especially loss of tactile sensibility, with or without loss of consciousness, resulting from the use of certain drugs or gases that serve as inhibitory neurotransmitters.

anesthesia, basal,

n a state of narcosis, induced before the administration of a general anesthetic, that permits the production of states of surgical anesthesia with greatly reduced amounts of general anesthetic agents.

anesthesia, block,

n a local anesthesia induced by injecting the local anesthetic drug close to the nerve trunk, at some distance from the operative field. See also anesthesia, infiltration, and block.

anesthesia, conduction,

n a local anesthesia induced by injecting the local anesthetic agent close to the nerve trunk, at some distance from the operative field.

anesthesia, general,

n an irregular, reversible depression of the cells of the higher centers of the central nervous system that makes the patient unconscious and insensible to pain.

anesthesia, glove,

n an anesthesia with a distribution corresponding to the part of the skin covered by a glove.

anesthesia, infiltration,

n a local anesthesia induced by injecting the anesthetic agent directly into or around the tissues to be anesthetized; used for operative procedures on the maxillary premolar, anterior teeth, and
mandibular incisors. Also called field block. See also anesthesia, block.

Infiltration anesthesia.

anesthesia, intraosseous,

n the local anesthesia produced by the injection of a local anesthetic agent into the cancellous portion of a bone.

anesthesia, intrapulpal,

n the injection of a local anesthetic agent directly into pulpal tissue under pressure.

anesthesia, local,

n (regional anesthesia), the loss of pain sensation over a specific area of the anatomy without loss of consciousness.

anesthesia, regional,

n a term used for local anesthesia. See also anesthesia, local.

anesthesia, topical,

n a form of local anesthetic agent with which the surface free nerve endings in accessible structures are rendered incapable of stimulation by applying a suitable solution directly to the surface of the area. Used on the surface soft tissue before a local anesthetic injection to anesthetize surface soft tissues for minor operative procedures.

anesthesia

loss of feeling or sensation. Artificial anesthesia may be produced by a number of agents capable of bringing about partial or complete loss of sensation. It is induced to permit the performance of surgery or other painful procedures. See also anesthetic.

balanced anesthesia

anesthesia that balances the depressing effects on the motor, sensory, reflex and mental aspects of nervous system function by the anesthetic agents. The philosophy encourages the use of several agents, each designed to affect one of the functions.

basal anesthesia

narcosis produced by preliminary medication so that the inhalation of anesthetic necessary to produce surgical anesthesia is greatly reduced.

block anesthesia

regional anesthesia. See also block.

caudal anesthesia

injection of an anesthetic into the sacral canal. See also caudal anesthesia.

central anesthesia

lack of sensation caused by disease of the nerve centers.

closed anesthesia

that produced by continuous rebreathing of a small amount of anesthetic gas in a closed system with an apparatus for removing carbon dioxide.

crossed anesthesia

loss of sensation on one side of the face and loss of pain and temperature sense on the opposite side of the body.

dissociated anesthesia, dissociation anesthesia

loss of perception of certain stimuli while that of others remains intact.

electric anesthesia

anesthesia induced by passage of an electric current.

endotracheal anesthesia

anesthesia produced by introduction of a gaseous mixture through a tube inserted into the trachea.

epidural anesthesia

see epidural anesthesia.

field block anesthesia

the anesthetic agent is injected around the boundaries of the area to be anesthetized, with no attempt to locate specific nerves.

frost anesthesia

abolition of feeling or sensation as a result of topical refrigeration produced by a jet of a highly volatile liquid.

general anesthesia

a state of unconsciousness produced by anesthestic agents, with absence of pain sensation over the entire body and a greater or lesser degree of muscular relaxation; the drugs producing this state can be administered by inhalation, intravenously, intramuscularly, or rectally, or via the gastrointestinal tract.

infiltration anesthesia

local anesthesia produced by injection of the anesthetic solution directly into the area of terminal nerve endings.

inhalation anesthesia

anesthesia produced by the respiration of a volatile liquid or gaseous anesthetic agent. Halothane, methoxyflurane, isoflurane, and a combination of nitrous oxide and oxygen are the common agents in veterinary use.

insufflation anesthesia

anesthesia produced by introduction of a gaseous mixture into the trachea through a slender tube.

intrasynovial anesthesia

injection of a local anesthetic agent into a joint or tendon sheath.

intrathecal anesthesia

introduction of local anesthetic agent into the spinal fluid by penetration of the spinal dura. Causes anesthesia in the tissues supplied by the nerves in the spinal cord zone that has been anesthetized. There is danger of injury to the cord and the technique is litte used in veterinary surgery. Called also subarachnoid, subdural or intradural anesthesia/analgesia.

intravenous anesthesia

the anesthetic agent, e.g. a barbiturate, is administered intravenously to effect. If an intravenous catheter is used, 'topping-up' amounts can also be administered as required.

intravenous regional anesthesia

see bier technique.

irreversible anesthesia

the loss of sensory and motor function of the part is permanent. The local injection of isopropyl alcohol has this effect.

local anesthesia

that produced in a limited area, as by injection of a local anesthetic or by freezing with ethyl chloride. Includes infiltration, nerve block, field block, surface, regional, retrograde regional, spinal, epidural.

mixed anesthesia

that produced by use of more than one anesthetic agent.

nerve block anesthesia

the anesthetic agent is deposited from a syringe and needle as close to the target nerve as possible. Several injections are often made if the landmarks for the location of the nerve are not outstanding.

obstetrical anesthesia

see obstetrical anesthesia.

open anesthesia

general inhalation anesthesia in which there is no rebreathing of the expired gases.

parasacral anesthesia

regional anesthesia produced by injection of a local anesthetic around the sacral nerves as they emerge from the sacral foramina.

paravertebral anesthesia

regional anesthesia produced by the injection of a local anesthetic around the spinal nerves at their exit from the spinal column, and outside the spinal dura.

parenteral anesthesia

anesthesia induced by the injection of the agent, either intravenously, intraperitoneally, subcutaneously or intramuscularly.

peripheral anesthesia

lack of sensation due to changes in the peripheral nerves.

permeation anesthesia

analgesia of a body surface produced by application of a local anesthetic, most commonly to the mucous membranes. Called also surface anesthesia.

rectal anesthesia

anesthesia produced by introduction of the anesthetic agent into the rectum.

refrigeration anesthesia

local anesthesia produced by applying a tourniquet and chilling the part to near freezing temperature. Called also cryoanesthesia.

regional anesthesia

insensibility caused by interrupting the sensory nerve conductivity of any region of the body: produced by (1) field block, encircling the operative field by means of injections of a local anesthetic; or (2) nerve block, making injections in close proximity to the nerves supplying the area.

saddle block anesthesia

the production of anesthesia in the region of the body corresponding roughly with the areas of the buttocks, perineum and inner aspects of the thighs, by introducing the anesthetic agent low in the dural sac.

segmental anesthesia

loss of sensation in a segment of the body due to a lesion of a nerve root.

spinal anesthesia

1. anesthesia due to a spinal lesion.

2. anesthesia produced by injection of the agent beneath the membrane of the spinal cord.

splanchnic anesthesia

block anesthesia for visceral operation by injection of the anesthetic agent into the region of the celiac ganglia.

subarachnoid anesthesia

see intrathecal anesthesia (above).

surface anesthesia

the application of a local anesthetic agent in solution, as in eye drops, or as a jelly, cream or ointment. The use of cold materials which freeze the superficial layers of skin is not much used in veterinary surgery. See also permeation anesthesia (above).

surgical anesthesia

that degree of anesthesia at which operation may safely be performed. There is muscular relaxation, and coordinated movements, consciousness and pain sensations disappear; many of the spinal neuromuscular reflexes are abolished.

topical anesthesia

that produced by application of a local anesthetic directly to the area involved.