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.

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, general

Definition

General anesthesia is the induction of a balanced state of unconsciousness, accompanied by the absence of pain sensation and the paralysis of skeletal muscle over the entire body. It is induced through the administration of anesthetic drugs and is used during major surgery and other invasive surgical procedures.

Purpose

General anesthesia is intended to bring about five distinct states during surgery:

• analgesia, or pain relief
• amnesia, or loss of memory of the procedure
• loss of consciousness
• motionlessness
• weakening of autonomic responses

Precautions

A complete medical history, including a history of allergies in family members, is an important precaution. Patients may have a potentially fatal allergic response to anesthesia known as malignant hyperthermia, even if there is no previous personal history of reaction.

General anesthetics should be administered only by board-certified medical professionals. Anesthesia providers consider many factors, including a patient's age, weight, allergies to medications, medical history, and general health when deciding which anesthetic or combination of anesthetics to use. The American Society of Anesthesiologists has compiled guidelines for classifying patients according to risk levels as follows:

• I: healthy patient
• II: patient with mild systemic disease without functional limitations
• III: patient with severe systemic disease with definite functional limitations
• IV: patient with severe systemic disease that is life-threatening
• V: dying patient not expected to survive for 24 hours without an operation

Equipment for general anesthesia should be thoroughly checked before the operation; all items that might be needed, such as extra tubes or laryngoscope blades, should be available. Staff members should be knowledgeable about the problems that might arise with the specific anesthetic being used, and be able to recognize them and respond appropriately. General anesthetics cause a lowering of the blood pressure (hypotension), a response that requires close monitoring and special drugs to reverse it in emergency situations.

Description

General anesthetics may be gases or volatile liquids that evaporate as they are inhaled through a mask along with oxygen. Other general anesthetics are given intravenously. The amount of anesthesia produced by inhaling a general anesthetic can be adjusted rapidly, if necessary, by adjusting the anesthetic-to-oxygen ratio that is inhaled by the patient. The degree of anesthesia produced by an intravenously injected anesthetic cannot be changed as rapidly and must be reversed by administration of another drug.

The precise mechanism of general anesthesia is not yet fully understood. There are, however, several hypotheses that have been advanced to explain why general anesthesia occurs. The first, the so-called Meyer-Overton theory, suggests that anesthesia occurs when a sufficient number of molecules of an inhalation anesthetic dissolve in the lipid cell membrane. The second theory maintains that protein receptors in the central nervous system are involved, in that inhalation anesthetics inhibit the enzyme activity of proteins. A third hypothesis, proposed by Linus Pauling in 1961, suggests that anesthetic molecules interact with water molecules to form clathrates (hydrated microcrystals), which in turn inhibit receptor function.

Stages of anesthesia

There are four stages of general anesthesia that help providers to better predict the course of events, from anesthesia induction to emergence.

• Stage I begins with the induction of anesthesia and ends with the patient's loss of consciousness. The patient still feels pain in Stage I.
• Stage II, or REM stage, includes uninhibited and sometimes dangerous responses to stimuli, including vomiting and uncontrolled movement. This stage is typically shortened by administering a barbiturate, such as sodium pentothal, before the anesthetic agent.
• Stage III, or surgical anesthesia, is the stage in which the patient's pupillary gaze is central and the pupils are constricted. This is the target depth of surgical anesthesia. During this stage, the skeletal muscles relax, the patient's breathing becomes regular, and eye movements stop.
• Stage IV, or overdose, is marked by hypotension or circulatory failure. Death may result if the patient cannot be revived quickly.

Types of anesthetic agents

There are two major types of anesthetics used for general anesthesia, inhalation and intravenous anesthetics. Inhalation anesthetics, which are sometimes called volatile anesthetics, are compounds that enter the body through the lungs and are carried by the blood to body tissues. Inhalation anesthetics are less often used alone in recent clinical practice; they are usually used together with intravenous anesthetics. A combination of inhalation and intravenous anesthetics, often with opioids added for pain relief and neuromuscular blockers for muscle paralysis, is called balanced anesthesia.

inhalation anesthetics. The following are the most commonly used inhalation anesthetics:

• Halothane causes unconsciousness but provides little pain relief; often administered with analgesics . It may be toxic to the liver in adults. Halothane, however, has a pleasant smell and is therefore often the anesthetic of choice when mask induction is used with children.
• Enflurane is less potent, but produces a rapid onset of anesthesia and possibly a faster recovery. Enflurane is not used in patients with kidney failure.
• Isoflurane is not toxic to the liver but can induce irregular heart rhythms.
• Nitrous oxide (laughing gas) is used with other such drugs as thiopental to produce surgical anesthesia. It has the fastest induction and recovery time. It is regarded as the safest inhalation anesthetic because it does not slow respiration or blood flow to the brain. However, because nitrous oxide is a relatively weak anesthetic, it is not suited for use in major surgery. Although it may be used alone for dental anesthesia, it should not be used as a primary agent in more extensive procedures.
• Sevoflurane works quickly and can be administered through a mask since it does not irritate the airway. On the other hand, one of the breakdown products of sevoflurane can cause renal damage.
• Desflurane, a second-generation version of isoflurane, is irritating to the airway and therefore cannot be used for mask (inhalation) inductions, especially not in children. Desflurane causes an increase in heart rate, and so should be avoided for patients with heart problems. Its advantage is that it provides a rapid awakening with few adverse effects.

intravenous anesthetics. Commonly administered intravenous general anesthetics include ketamine, thiopental (a barbiturate), methohexital (Brevital), etomidate, and propofol (Diprivan). Ketamine produces a different set of reactions from other intravenous anesthetics. It resembles phencyclidine, which is a street drug that may cause hallucinations. Because patients who have been anesthetized with ketamine often have sensory illusions and vivid dreams during post-operative recovery, ketamine is not often given to adult patients. It is, however, useful in anesthetizing children, patients in shock, and trauma casualties in war zones where anesthesia equipment may be difficult to obtain.

General anesthesia in dental procedures

The use of general anesthesia in dental and oral surgery patients differs from its use in major surgery because the patient's level of fear is usually a more important factor than the nature of the procedure. In 1985, an NIH Consensus Statement reported that high levels of preoperative anxiety, lengthy and complex procedures, and the need for a pain-free operative period may be indications for general anesthesia in healthy adults and very young children. The NIH statement specified that at least three professionals are required when general anesthesia is used during dental procedures: one is the operating dentist; the second is a professional responsible for observing and monitoring the patient; the third person assists the operating dentist.

Although the United States allows general anesthesia for dental procedures to be administered outside hospitals (provided that the facility has the appropriate equipment and emergency drugs), Scotland banned the use of general anesthesia outside hospitals in 2000, after a ten-year-old boy died during a procedure to have a tooth removed.

Preparation

Preparation for general anesthesia includes the taking of a complete medical history and the evaluation of all factors—especially a family history of allergic responses to anesthetics—that might influence the patient's response to specific anesthetic agents.

Patients should not eat or drink before general anesthesia because of the risk of regurgitating food and liquid or aspirating vomitus into the lungs.

Informed consent

Patients should be informed of the risks associated with general anesthesia as part of their informed consent . These risks include possible dental injuries from intubation as well as such serious complications as stroke, liver damage, or massive hemorrhage. If local anesthesia is an option for some procedures, the patient should be informed of this alternative. In all cases, patients should be given the opportunity to ask questions about the risks and benefits of the procedure requiring anesthesia as well as questions about the anesthesia itself.

Premedication

Depending on the patient's level of anxiety and the procedure to be performed, the patient may be premedicated. Most medications given before general anesthesia are either anxiolytics, usually benzodiazepines; or analgesics. Patients in severe pain prior to surgery may be given morphine or fentanyl. Anticholinergics (drugs that block impulses from the parasympathetic nervous system) may be given to patients with a known history of bronchospasm or heavy airway secretions.

Aftercare

The anesthetist and medical personnel provide supplemental oxygen and monitor patients for vital signs and monitor their airways. Vital signs include an EKG (unless the patient is hooked up to a monitor), blood pressure, pulse rate, oxygen saturation, respiratory rate, and temperature. The staff also monitors the patient's level of consciousness as well as signs of excess bleeding from the incision.

Risks

Although the risk of serious complications from general anesthesia are low, they can include heart attack, stroke, brain damage, and death. The risk of complications depends in part on the patient's age, sex, weight, allergies, general health, and history of smoking, alcohol or drug use.

The overall risk of mortality from general anesthesia is difficult to evaluate, because so many different factors are involved, ranging from the patient's overall health and the circumstances preceding surgery to the type of procedure and the skill of the physicians involved. The risk appears to be somewhere between 1:1,000 and 1:100,000, with infants younger than age one and patients older than 70 being at greater risk.

Awareness during surgery

One possible complication is the patient's "waking up" during the operation. It is estimated that about 30,000 patients per year in the United States "come to" during surgery. This development is in part the result of the widespread use of short-acting general anesthetics combined with blanket use of neuromuscular blockade. The patients are paralyzed with regard to motion, but otherwise "awake and aware." At present, special devices that measure brain wave activity are used to monitor the patient's state of consciousness. The bispectral index monitor was approved by the FDA in 1996 and the patient state analyzer in 1999.

Nausea and vomiting

Post-operative nausea and vomiting is a common problem during recovery from general anesthesia. In addition, patients may feel drowsy, weak, or tired for several days after the operation, a combination of symptoms sometimes called the hangover effect. 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.

Anesthetic toxicity

Inhalation anesthetics are sometimes toxic to the liver, the kidney, or to blood cells. Halothane may cause hepatic necrosis or hepatitis. Sevoflurane may react with the carbon dioxide absorbents in anesthesia machines to form compound A, a haloalkene that is toxic to the kidneys. The danger to red blood cells comes from carbon monoxide formed by the breakdown products of inhalation anesthetics in the circuits of anesthesia machines.

Malignant hyperthermia

Malignant hyperthermia is a rare condition caused by an allergic response to a general anesthetic. The 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 anesthetics, especially halothane.

Normal results

General anesthesia is much safer today than it was in the past, thanks 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, in part because of recent developments in equipment that reduces the problems of anesthetizing patients who are difficult to intubate. These developments include the laryngeal mask airway and the McCoy laryngoscope, which has a hinged tip on its blade that allows a better view of the patient's larynx.

Resources

books

u.s. pharmacopeia staff. consumer reports complete drug reference. yonkers, ny: consumer reports books, 2002.

periodicals

christie, bryan. "scotland to ban general anaesthesia in dental surgeries." british medical journal 320 (march 4, 2000): 55–59.

fox, andrew j. and david j. rowbotham. "recent advances in anaesthesia." british medical journal 319 (august 28, 1999): 557–560.

marcus, mary brophy. "how does anesthesia work? a state that is nothing like sleep: no memory, no fight-or-flight response, no pain." u.s. news & world report 123 (august 18, 1997): 66.

preboth, monica. "waking up under the surgeon's knife." american family physician (february 15, 1999).

wenker, olivier c., md. "review of currently used inhalation anesthetics: parts i and ii." the internet journal of anesthesiology 3, nos. 2 and 3 (1999).

organizations

american academy of anesthesiologist assistants. po box 81362, wellesley, ma 02481-0004. (800) 757-5858. <http://www.anesthetist.org>.

american association of nurse anesthetists. 222 south prospect avenue, park ridge, il 60068-4001 (847) 692-7050. <http://www.aana.com>.

american society of anesthesiologists. 520 n. northwest highway, park ridge, il 60068-2573. (847) 825-5586. <http://www.asahq.org>.

other

american medical association, office of the general counsel, division of health law. informed consent. chicago, il: ama press, 1998.

interview with harvey plosker, md, board-certified anesthesiologist. the pain center, 501 glades road, boca raton, fl 33431.

nih consensus statement. anesthesia and sedation in the dental office. 5, no. 10 (april 22–24, 1985): 1–18.

Lisette Hilton

Sam Uretsky, PharmD

Anesthesia, local

Definition

Local, or regional, anesthesia involves the injection or application of an anesthetic drug to a specific area of the body. This is in contrast to general anesthesia, which provides anesthesia to the entire body and brain.

Purpose

Local anesthetics are used to prevent patients from feeling pain during medical, surgical, or dental procedures. Over-the-counter local anesthetics are also available to provide temporary relief from pain, irritation, and itching caused by various conditions such as cold sores, canker sores, sore throats, sunburn, insect bites, poison ivy, and minor cuts and scratches.

Precautions

People who feel strongly that they do not want to be awake and alert during certain procedures may not be good candidates for local or regional anesthesia. However, other medications that have systemic effects may be given in addition to an anesthetic to relieve anxiety and help the patient relax.

Local anesthetics should be used only for the conditions for which they are intended. For example, a topical anesthetic meant to relieve sunburn pain should not be used on cold sores. Anyone who has had an unusual reaction to any local anesthetic in the past should check with a doctor before using any type of local anesthetic again. The doctor should also be told about any allergies to foods, dyes, preservatives, or other substances.

Older people may be more sensitive to the effects of local anesthetics, especially lidocaine. Children may also be especially sensitive to some local anesthetics, and certain types should not be used at all on young children. People caring for these groups need to be aware that they are at increased risk of more severe side effects. Packages should be followed carefully so that the recommended dosage is not exceeded. A doctor or pharmacist should be consulted about any concerns.

Regional anesthetics

Serious and possibly life-threatening side effects may occur when injectable or inhaled anesthetics are given to people who use street drugs. Doctors and nurses should inform patients about the dangers of mixing anesthetics with cocaine, marijuana, amphetamines, barbiturates , phencyclidine (PCP, or angel dust), heroin, or other street drugs. Some anesthetic drugs may interact with other medicines. When this happens, the effects of one or both of the drugs may change, or the risk of side effects may be greater. In select cases, a urinalysis can help identify drug use.

Patients who have a personal or family history of malignant hyperthermia after receiving a general anesthetic must also be cautious when receiving regional or local anesthetics. Malignant hyperthermia is a serious reaction that involves a fast or irregular heartbeat, high fever, breathing problems, and muscle spasms. All patients should be asked if they are aware of such a risk in their family before receiving any kind of anesthetic.

Although problems are rare, some side effects may occur when regional anesthetics are used during labor and delivery. Anesthetics can prolong labor and increase the risk of requiring a cesarean section . Doctors should discuss the risks and benefits associated with epidural or spinal anesthesia with pregnant patients.

Regional anesthetics should be used only by an experienced anesthesiologist in a properly equipped environment with suitable resuscitative equipment. Although these anesthetics are generally safe when properly selected and administered, severe adverse reactions are still possible. If inadvertent subarachnoid injection occurs, the patient is likely to require resuscitation with oxygen and drug therapy. Careful positioning of the patient is essential to prevent leaking of cerebrospinal fluid.

Patients should not drive or operate machinery immediately following a procedure involving regional anesthesia because numbness or weakness may cause impairment. Doctors and nurses should also warn patients who have had local anesthesia, especially when combined with drugs to make patients sleep or to reduce pain, about operating any type of machinery.

Injectable local anesthetics

Until the anesthetic wears off, patients should be careful not to inadvertently injure the numbed area. If the anesthetic was used in the mouth, patients should not eat or chew gum until feeling returns.

Topical anesthetics

Unless advised by a doctor, topical anesthetics should not be used on or near any part of the body with large sores, broken or scraped skin, severe injury, or infection. They should also not be used on large areas of skin. Some topical anesthetics contain alcohol and should not be used near an open flame or while smoking.

Patients should be careful not to get topical anesthetics in the eyes, nose, or mouth. If a spray-type anesthetic is to be used on the face, it can be applied with a cotton swab or sterile gauze pad. After using a topical anesthetic on a child, the caregiver should make sure the child does not get the medicine in his or her mouth or eyes.

Topical anesthetics are intended for the temporary relief of pain and itching. They should not be used for more than a few days at a time. A doctor should be consulted if:

• Discomfort continues for more than seven days.
• The problem gets worse.
• The treated area becomes infected.
• New signs of irritation such as skin rash, burning, stinging, or swelling appear.

Dental anesthetics

Dental anesthetics should not be used if certain kinds of infections are present. Package directions should be checked or a dentist, pharmacist, or doctor should be consulted if there is any uncertainty. Dental anesthetics should be used only for temporary pain relief. Consult the dentist if problems such as toothache, mouth sores, or pain from dentures or braces continue or if signs of general illness such as fever, rash, or vomiting develop.

Patients should not eat or chew gum while the mouth is numb from a dental anesthetic to avoid accidentally biting the tongue or the inside of the mouth. In addition, nothing should be eaten or drunk for one hour after applying a dental anesthetic to the back of the mouth or throat, because the medicine may interfere with swallowing and may cause choking. If normal feeling does not return to the mouth within a few hours after receiving a dental anesthetic or if it is difficult to open the mouth, the dentist should be consulted.

Ophthalmic anesthetics

When anesthetics are used in the eye, it is important not to rub or wipe the eye until the effect of the anesthetic has worn off and feeling has returned. Rubbing the eye while it is numb could cause injury.

Description

Medical procedures and situations that regularly make use of local or regional anesthesia include the following:

• biopsies, in which skin or tissue samples are taken for diagnostic procedures
• childbirth
• scar repair
• surgery on the face (including plastic surgery), skin, arms, hands, legs, and feet
• eye surgery
• surgery involving the urinary tract or reproductive organs

Surgery involving the chest or abdomen is usually performed under general anesthesia. Laparoscopy and hernia repair, however, may be performed under local or regional anesthesia.

Local and regional anesthesia have many advantages over general anesthesia. Most importantly, the risk of unusual and sometimes fatal reactions to general anesthesia is lessened. More minor, but significant, risks of general anesthesia include longer recovery time and the psychological discomfort of losing consciousness.

Regional anesthesia typically affects a larger area than local anesthesia. As a result, regional anesthesia is typically used for more involved or complicated procedures. The duration of action of an anesthetic depends on the type and amount of anesthetic administered.

Regional anesthetics are injected. Local anesthesia involves the injection into the skin or application to the skin surface of an anesthetic directly where pain will occur. Local anesthesia can be divided into four groups: injectable, topical, dental (non-injectable), and regional blockade injection.

Local and regional anesthesia work by altering the flow of sodium molecules into nerve cells (neurons) through the cell membrane. The exact mechanism 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. Experts believe, however, that when the sodium molecules do not get into the neurons, nerve impulses are not generated and pain impulses are not transmitted to the brain.

Regional anesthesia

Types of regional anesthesia include:

• Spinal anesthesia, which involves the injection of a small amount of local anesthetic into the cerebrospinal fluid surrounding the spinal cord (the subarachnoid space). A drop in blood pressure is a common but easily treated side effect.
• Epidural anesthesia, which involves the injection of a large volume of local anesthetic into the space surrounding the spinal fluid sac (the epidural space), not directly into the spinal fluid. Pain relief occurs more slowly, but is less likely to produce a drop in blood pressure. The block can be maintained for long periods, even for days if necessary.
• Nerve blockades, which involve the injection of an anesthetic into the area around a sensory or motor nerve that supplies a particular region of the body, preventing the nerve from carrying nerve impulses to and from the brain.

Local and regional anesthetics may be administered with other drugs to enhance their action. Examples include vasoconstrictors such as epinephrine (adrenaline) to decrease bleeding, or sodium bicarbonate to lower acidity, which may make a drug work faster. In addition, medications may be administered to help a patient remain calm and more comfortable or to make them sleepy.

Local anesthesia

injectable local anesthetics. Injectable local anesthetics provide pain relief for some part of the body during surgery, dental procedures, or other medical procedures. They are given only by a trained health care professional and only in a doctor's office or a hospital. Some commonly used injectable local anesthetics are lidocaine (Xylocaine), bupivacaine (Marcaine), and mepivacaine (Carbocaine).

topical anesthetics. Topical anesthetics such as benzocaine, lidocaine (in smaller quantities or doses), dibucaine, and tetracaine relieve pain and itching by blocking the sensory nerve endings in the skin. They are ingredients in a variety of nonprescription products that are applied to the skin to relieve the discomfort of sunburn, insect bites or stings, poison ivy, and minor cuts, scratches, and burns. These products are sold as creams, ointments, sprays, lotions, and gels.

Topical dental anesthetics are intended for pain relief in the mouth or throat. They may be used to relieve throat pain, teething pain, painful canker sores, toothaches, or discomfort from dentures, braces, or bridgework. Some dental anesthetics are available only with a doctor's prescription. Others may be purchased over the counter, including products such as Num-Zit, Orajel, Chloraseptic lozenges, and Xylocaine.

Ophthalmic anesthetics are designed for use in the eye. Lidocaine and tetracaine are used to numb the eye before certain eye examinations. Eye doctors may also use these medicines before measuring eye pressure or removing stitches or foreign objects from the eye. These drugs are to be given only by a trained health care professional.

The recommended dosage of a topical anesthetic depends on the type of local anesthetic and the purpose for which it is being used. When using a nonprescription local anesthetic, patients are advised to follow the directions on the package. Questions concerning how to use a product should be referred to a doctor, dentist, or pharmacist.

Aftercare

Most patients can return home immediately after a local anesthetic, but some patients might require limited observation. The degree of aftercare needed depends on where the anesthetic was given, how much was given, and other individual circumstances. Patients who have had their eyes numbed should wear a patch after surgery or treatment until full feeling in the eye area has returned. If the throat was anesthetized, the patient cannot drink until the gag reflex returns. If a major extremity was anesthetized, the patient may have to wait until function returns before being discharged. Some local anesthetics can cause cardiac arrhythmias and therefore require monitoring for a time with an EKG. Patients who have had regional anesthesia or larger amounts of local anesthesia usually recover in a post-anesthesia care unit before being discharged. There, medical personnel watch for immediate postoperative problems. These patients need to be driven home after discharge.

Risks

Side effects of regional or local anesthetics vary depending on the type of anesthetic used and the way it is administered. Any unusual symptoms following the use of an anesthetic requires the immediate attention of a doctor.

Paralysis after a regional anesthetic such as an epidural, spinal, or ganglionic blockade is extremely rare, but can occur. Paralysis reportedly occurs even less frequently than deaths due to general anesthesia.

There is also a small risk of developing a severe headache called a spinal headache following a spinal or epidural block. This headache is severe when the patient is upright, even when only elevated 30°, and is hardly felt when the patient lies down. It is treated by increasing fluids to help clear the anesthetic and enhance the flow of spinal fluid.

Finally, blood clots or abscess can form at the site where an anesthetic is injected. Although they can usually be treated, antibiotic resistance is becoming increasingly common. Such infections must be regarded as potentially dangerous, particularly if they develop at the site of a spinal injection.

A physician should be notified immediately if any of the following occur:

• symptoms of an allergic reaction such as hives (urticaria), which are itchy swellings on the skin, or swelling in the mouth or throat
• severe headache
• blurred or double vision or photophobia (sensitivity to light)
• dizziness or lightheadedness
• drowsiness
• confusion
• an irregular, too slow, or rapid heartbeat
• anxiety, excitement, nervousness, or restlessness
• convulsions (seizures)
• feeling hot, cold, or numb anywhere other than the anesthetized area
• ringing or buzzing in the ears
• shivering or trembling
• sweating
• pale skin
• breathing problems
• unusual weakness or tiredness

Normal results

Local and regional anesthetics help to make many conditions and procedures more comfortable and tolerable for patients.

Resources

books

nettina, sandra. lippincott manual of nursing practice, 7th edition. philadelphia: lippincott, 2001, pp. 115-117.

organizations

american academy of anesthesiologist assistants. po box 81362, wellesley, ma 02481-0004. (800) 757-5858. <http://www.anesthetist.org>.

american society of anesthesiologists. 520 n. northwest highway, park ridge, il 60068-2573. (847) 825-5586. <http://www.asahq.org>.

other

interview with harvey plosker, md. the pain center. 501 glades road, boca raton, fl 33431.

Lisette Hilton Sam Uretsky, PharmD

ANESTHESIA

"My diseases are an asthma and a dropsy and, what is less curable, seventy-five."

—Samuel Johnson.

Over 40 percent of all surgical procedures in the United States are performed on patients over age sixty-five, a remarkable statistic given that those over sixty-five comprise only 13 percent of the U.S. population. Elderly patients are more likely than their younger counterparts to suffer serious medical complications such as a heart attack, pneumonia, or kidney failure during or after an operation, further compounding the impact that caring for elderly patients has on the medical system.

Basics of anesthesia

There are three broad categories of anesthesia: local anesthesia, regional anesthesia, and general anesthesia. Local and regional anesthesia involve the injection of a drug, such as lidocaine or bupivacaine, that soaks into the nerves and blocks the electrical signals from traveling down the nerves. With local anesthesia the drug is injected under the skin in the area of the surgery where the nerves are diffusely spread about in the tissue, whereas in regional anesthesia the drug is injected next to large, discrete nerves traveling to the surgical area. For example, when injected at the right location in the armpit, the arm can be made completely numb, allowing surgery to proceed without the patient feeling any pain. A spinal anesthetic involves placing the needle between the vertebrae into the spinal sac. The drug then reaches the nerves that go to the lower half of the body, making the patient numb from approximately the upper abdomen down. An epidural anesthetic is similar to a spinal, only the needle is placed outside the spinal sac, and, typically, a catheter is inserted (and the needle removed). An advantage of the catheter is it is easier to give subsequent injections.

A general anesthetic renders the patient unconscious during surgery. Most often, unconsciousness is rapidly achieved by injecting a large dose of a sedative, such as pentothal or propofol. Since the drug wears off quickly, it is immediately followed by a gas anesthetic to keep the patient asleep. During surgery, narcotic painkillers may be used to reduce the amount of gas being used, and to get a head start on the pain control that may be required after surgery. Sometimes drugs that paralyze the muscles must also be used to facilitate the operation.

During the use of any anesthetic, the patient's vital signs are watched carefully and continuously. The electrical activity of the heart (electrocardiogram) is displayed on a monitor (see Figure 1); blood pressure is measured every few minutes with an automated machine; and the oxygen level in the arterial blood is measured via a device that clips to a finger. During a general anesthetic a machine will measure the concentration of the gas anesthetic, as well as the level of carbon dioxide coming from the lungs. Careful monitoring is important because all anesthetics can lower blood pressure, depress breathing, and impair many of the body's defense mechanisms. The amount of anesthetic given the patient must therefore be continuously adjusted to match the conditions present during surgery.

The unique challenge of the elderly patient

Aging decreases the ability of every organ system in the body to withstand stress, including those associated with surgery and anesthesia. Stress begins in surgery with the combined effects of the anesthetic and surgical trauma. After surgery, the patient faces a potentially long period of recovery from that trauma, as well as the stress of pain. Chronic diseases such as stroke, heart disease, diabetes, or high blood pressure also compromise the body's ability to withstand stress and make the patient more vulnerable to complications such as a heart attack, pneumonia, kidney failure, or even death. Aging has its greatest adverse impact on older patients who also have medical illness. Among healthy people, the risk of complications from anesthesia and surgery increases only slightly with age. Among people with multiple chronic medical conditions, however, risk dramatically increases with age. The challenge to the care of elderly patients lies in tailoring the anesthetic to the patient's medical illnesses as well as taking into account the effect of age on the responses to the anesthetic. In all phases of anesthetic care, everything is done with an eye to reducing the likelihood that complications will occur.

Preoperative assessment

Before a patient has surgery, it must be determined that the expected benefits of the surgery outweigh the risks. With a healthy patient, this decision is usually straightforward; but this determination is more difficult for an elderly patient with multiple medical problems contemplating a high-risk surgery. Sometimes it is useful to get other specialists involved in order to perform more sophisticated tests that will better define the extent of the disease. Such testing may lead to therapy aimed at improving the medical status of the patient in order to decrease the risk of the surgery. For example, a patient with poorly controlled asthma might benefit from a few days of steroids to bring the asthma under control. With the current trend of performing as many surgeries as possible on an outpatient basis, many patients now go to preoperative clinics where their medical history and current condition can be assessed and further evaluation or treatment initiated well in advance of the scheduled surgery.

Intraoperative management

Virtually all anesthetic drugs have more pronounced effects on elderly patients. Drug effects typically last longer in older adults because metabolism (elimination of the drug from the body) slows with age. A given dose of a drug usually has a greater effect on older patients because higher initial blood levels are achieved than in young patients, thereby permitting more drug to enter the brain. In some cases the older brain is also more sensitive to the drug. In consequence, elderly patients usually receive small doses, and whenever possible drugs are used that possess a short duration of action.

Maintenance of a stable blood pressure is also more difficult with older patients. Blood pressure is the product of cardiac output (the amount of blood the heart pumps to the body per minute) and vascular resistance (how hard it is for blood to flow through the blood vessels). Vascular resistance is partly controlled by the brain. Aging is associated with increasing stimulation of the blood vessels by the brain and therefore vascular resistance increases with age. During anesthesia that stimulation is lost. Consequently, the vascular resistance decreases more than in a young adult and takes the blood pressure down with it. Furthermore, aging decreases the body's ability to resist changes in blood pressure, making changes in blood pressure due to external forces such as blood loss during surgery less opposed, and therefore more dramatic. Fortunately, modest swings in blood pressure, whether up or down, are usually well tolerated by almost every patient. Nevertheless, the control of blood pressure generally requires more direct manipulation by the anesthesiologist when caring for elderly patients.

The lungs are another area of great concern. Aging diminishes the transfer of oxygen to the blood, and anesthesia worsens this problem. Elderly patients are therefore likely to need extra oxygen for a longer period of time after surgery to prevent the risk of having periods of low blood-oxygen levels. Aging also increases the likelihood that portions of the lungs will compress and make the lungs more prone to pneumonia. The reflexes in the mouth and upper windpipe protect against regurgitated stomach contents from entering the trachea and damaging the lungs. These protective reflexes also diminish with age, again making the older patient at higher risk of low blood-oxygen levels or pneumonia. Deep breathing and coughing out secretions that accumulate in the lungs are important maneuvers done by the patient to help prevent low blood oxygen or pneumonia.

Although it is a controversial area, there is suspicion that surgery somehow causes blood to clot more easily. This tendency might be a good thing at the site of the surgery, but it may also lead to clots forming at diseased areas of the arteries that supply blood to the heart or brain. If so, such clots could lead to a heart attack or to a stroke. Prevention of such complications is a major area of current research.

Postoperative care

Surgery, especially operations where the chest or abdomen is opened, creates a significant stress to the patient that continues for at least several days after the surgery. Besides problems such as pneumonia or a heart attack, older patients are particularly prone to becoming confused within a day or two of surgery. Although the confusion almost always goes away, the condition may leave the patient in a more debilitated state for a long time thereafter, and thus requiring longer hospitalization and perhaps even nursing home care on discharge from the hospital. Patients may also suffer a potentially permanent decline in mental abilities in association with surgery. Prevention of these phenomena is an important area of current research.

Anesthesiologists have been particularly involved with preventing complications by helping to provide better pain control after surgery. A popular method of pain control is the administration of morphine via a pump controlled by the patient. Within certain safety limits, a small dose of morphine is given each time the patient pushes a button. Narcotics such as morphine have side effects, however, such as itching, nausea, and sedation. In part to avoid these problems, non-narcotic drugs have been gaining popularity. For surgery on the arms or legs, the use of long-lasting local anesthetics can safely extend the anesthetic for up to a day after surgery. Through mechanisms not yet fully understood, this technique may reduce the amount of pain experienced even after the local anesthetic has worn off.

Another option for pain relief after surgery is provided by the epidural catheter described previously. By administering a low concentration of both a local anesthetic and a narcotic through the catheter, excellent pain control can be achieved without affecting the patient's brain, allowing the patient to breathe more deeply and cough more easily, thereby helping to prevent pneumonia. Good pain control may also diminish the risk of other problems, such as a heart attack. The exact role of pain control with epidural catheters is still unclear, but it appears that complications can be reduced in high-risk (often elderly) patients.

Conclusion

The anesthetic care of the elderly patient is complex and demanding because of the effects of aging on organ function, plus the greater likelihood of chronic disease with increased age. Greater attention must be afforded such patients, beginning with the evaluation and optimization of the patient's medical status. The anesthetic requires close attention to detail, and, in selected patients, special techniques may be useful in lowering the risk of complications.

G. Alec Rooke

See also Pain Management; Revascularization: Bypass Surgery and Angioplasty; Surgery in Elderly People.

BIBLIOGRAPHY

Liu, S.; Carpenter, R. L.; and Neal, J. M. "Epidural Anesthesia and Analgesia—Their Role in Postoperative Outcome." Anesthesiology 85 (1995): 1474–1506.

Mcleskey, C. H., ed. Geriatric Anesthesia. Baltimore, Md.: Williams & Wilkins, 1997.

Moller, J. T.; Cluitmans, P.; Rasmussen, L. S.; et al. "Long-Term Postoperative Cognitive Dysfunction in the Elderly: ISPOCD1 Study." Lancet 351 (1998): 857–861.

Morgan, G. E., and Mikhail, M. S. Clinical Anesthesia, 2d ed. New York: Lange Medical Books/McGraw-Hill, 1996.

Muravchick, S. Geroanesthesia. St. Louis, Mo.: Mosby, 1997.

Rooke, G. A. "Autonomic and Cardiovascular Function in the Geriatric Patient." Anesth Clin NA 18 (2000): 31–46.

Tiret, L.; Desmonts, J. M.; Hatton, F.; and Vourc'h, G. "Complications Associated with Anesthesia—A Prospective Survey in France." Canadian Anaesthetists' Society Journal 33 (1986): 336–344.

ANEURYSM, ABDOMINAL AORTIC

See Vascular disease

ANNUAL CHECK-UP

See Periodic health examination

Ozone

The name ozone comes from the Greek Ozon meaning smell. At atmospheric temperatures, ozone is a colorless gas with an odor similar to chlorine that can usually be detected at a level of about 0.01 parts per million.

High in the atmosphere, ozone plays an important protective role by diminishing the amount of potentially damaging ultraviolet radiation reaching Earth. In sufficient concentration, however, ozone is a poison that at lower atmospheric levels, is a pollutant that can be damaging to health. Ozone is also a strong oxidizing agent used in many industrial processes for bleaching and sterilization. Although ozone is often used in water treatment, the largest commercial application of ozone is in the production of pharmaceuticals, synthetic lubricants, and other commercially useful organic compounds.

In the atmosphere, ozone is formed predominantly by electric discharges (e.g., lightning ). In the laboratory, ozone can be extracted form a mixture of oxygen and ozone by fractionation.

Ozone can also be formed by ultraviolet light. Ultraviolet light is energetic, and when it strikes the atmosphere it can break down some oxygen molecules producing highly energized oxygen atoms (free radicals). These free radicals can then react with molecular oxygen to produce ozone. The absorption of energetic light radiation also triggers the decomposition of ozone. As a result, ozone is an unstable molecule that exists in a dynamic equilibrium of formation and destruction. Consequently, the protective ozone layer is also in dynamic equilibrium.

The area where ozone is formed at the fastest rate is in the atmosphere at a height of approximately 164,042 ft (50 km). At this height, the number of free radicals made by ultraviolet light and electric discharge is balanced by the concentration of diatomic oxygen, which is sufficiently high to ensure that reactive collisions occur.

The protective ozone layer is found in the upper reaches of the atmosphere (between 98,000–295,000 ft [30–90 km]) where it absorbs ultraviolet radiation that, in excess, can be harmful to biological organisms. The potential detrimental effects of increased exposure to ultraviolet light due to a lessening of atmospheric ozone are of great concern. Holes in the ozone layer, or a global breakdown of stratospheric ozone would lead to increasing doses of ultraviolet radiation at Earth's surface. Scientists fear that significant increases exposure to ultraviolet light will increase risks of cancer in animal skin, eyes, and immune systems. Studies have shown that high ultraviolet radiation doses can supply the needed energy for chemical reactions that produce highly reactive radicals that have the potential to damage DNA and other cell regulating chemicals and structures.

There are several atmospheric trace elements, including ozone, that are important in the regulation of the global climate . Although the atmosphere consists of mainly of nitrogen and oxygen, approximately one percent of Earth's atmosphere is made of small amounts of other gases. Trace gases include water vapor, carbon dioxide , nitrous oxide, methane, chlorofluorocarbons (CFCs), and ozone. Because the amount of trace gases in the atmosphere is small, human activities can significantly affect the proportions of atmospheric trace gases.

Chloroflourocarbons (CFCs) easily react with ozone, which has the effect of breaking down an already unstable molecule. Until recently, CFCs were commonly used in refrigeration and in aerosol propellants (a pressurized gas used to propel substances out of a container). After evidence indicating that the use of CFCs was tipping the ozone equilibrium toward overall ozone layer depletion , many industrialized countries opted to enforce restrictions on the use of CFCs. Consumer aerosol products in the United States have not used ozone-depleting substances such as CFCs since the late 1970s. Federal regulations, including the Clean Air Act and Environmental Protection Agency (EPA) regulations restrict the use of ozone-depleting substances.

Ozone played a critical role in the development of life on Earth. Once primitive plants evolved, oxygen started to accumulate in the atmosphere. Some of this oxygen was converted into ozone and the developing ozone layer gave needed protection from disruptively energetic ultraviolet radiation. As a consequence, complex organic molecules which would otherwise have been destroyed began to accumulate.

As well as being found high in the atmosphere, ozone can be found at ground level. At these locations it is regarded as a pollutant. Ozone at ground level can be manufactured as part of photochemical smog . This is brought about by the disassociation of oxides of nitrogen that produce oxygen free radicals. These free radicals can react with diatomic oxygen to produce ozone. Pollutant ozone can also be a by-product of the action of photocopiers and computer printers. Low level ozone is usually found at a concentration of less than 0.01 parts per million, whereas in photochemical smog, it can be encountered at levels as high as 0.5 parts per million. Levels of ozone exposure between 0.1 and 1 part per million cause headaches, burning eyes, and irritation to the respiratory passages in humans. Elderly people, asthma sufferers, and those exercising in photochemical smog suffer the greatest adverse effects.

Some plant species (e.g., the tobacco plant) are particularly sensitive to low-lying ozone. The presence of excessive ozone causes a characteristic spotting of the leaves. High ozone levels are also known to damage structural material such as rubber.

Replacing more dangerous chlorine gas, ozone is used in many waste treatment facilities to purify water. Ozone is responsible for disinfecting the water and the efficient removal of trace elements such as pesticides. Ozone kills bacteria and other small life forms and it reacts with organic compounds. During the process, the ozone is transformed to molecular oxygen.

See also Atmospheric pollution; Ozone layer and ozone hole dynamics

anaesthesia State of insensibility or loss of sensation produced by disease or by various anaesthetic drugs used during surgical procedures. During general, or total, anaesthesia the entire body becomes insensible and the individual sleeps; in local anaesthesia only a specific part of the body is rendered insensible and the patient remains conscious. A general anaesthetic may be either an injected drug, such as the barbiturate thiopentone, used to induce unconsciousness, or an inhalation agent such as halothane, which is used to maintain anaesthesia for surgery. Local anaesthetics, such as lignocaine, numb the relevant part of the body by blocking the transmission of impulses through the sensory nerves which supply it.

anaesthesia (anis-theez-iă) n. loss of feeling or sensation in a part or all of the body, especially when induced by drugs. general a. total unconsciousness, usually achieved by administering a combination of injections and gases. local a. loss of feeling in a limited area of the body, induced for minor operations, particularly many dental procedures. It may be achieved by injections of substances such as lidocaine close to a local nerve, which deadens the tissues supplied by that nerve. regional a. anaesthesia (usually of a limb) achieved by encircling local anaesthetic solutions or by direct application of anaesthetic to one or more peripheral nerves. See also epidural, spinal anaesthesia.

an·es·the·sia / ˌanəsˈ[unvoicedth]ēzhə/ (Brit. an·aes·the·sia) • n. insensitivity to pain, esp. as artificially induced by the administration of gases or the injection of drugs before surgical operations. ∎  the induction of this state, or the branch of medicine concerned with it.

anesthesiology , branch of medicine concerned primarily with procedures for rendering patients insensitive to pain, and for supporting life systems under the strains of anesthesia and surgery. The anesthesiologist will induce unconsciousness for various clinical purposes and will perform cardiac and respiratory resuscitation when necessary.

an·es·thet·ic / ˌanəsˈ[unvoicedth]etik/ (Brit. an·aes·thet·ic) • n. 1. a substance that induces insensitivity to pain. 2. (anesthetics) [treated as sing.] the study or practice of anesthesia. • adj. inducing or relating to insensitivity to pain.

anaesthesia loss of feeling or sensation. XIX. — modL. — Gr. anaisthēsíā, f. AN-2 + aísthēsis sensation.
So anaesthetic XIX. f. Gr. anaisthētós. Hence anaesthetist, anaesthetize XIX.

anaesthesia •astrantia • Bastia •Dei gratia, hamartia •poinsettia •in absentia, Parmentier •Izvestia •meteor, wheatear •Whittier • cottier • Ostia •consortia, courtier •protea • Yakutia • frontier • Althea •Anthea • Parthia •Pythia, stichomythia •Carinthia, Cynthia •forsythia • Scythia • clothier • salvia •Latvia • Yugoslavia • envier •Flavia, Moldavia, Moravia, Octavia, paviour (US pavior), Scandinavia, Xavier •Bolivia, Livia, Olivia, trivia •Sylvia • Guinevere • Elzevir •Monrovia, Segovia •Retrovir • effluvia • colloquia •Goodyear • yesteryear • brassiere •Abkhazia •Anastasia, aphasia, brazier, dysphasia, dysplasia, euthanasia, fantasia, Frazier, glazier, grazier, gymnasia, Malaysia •amnesia, anaesthesia (US anesthesia), analgesia, freesia, Indonesia, Silesia, synaesthesia •artemisia, Kirghizia, Tunisia •ambrosia, crozier, hosier, osier, symposia

anesthesia •astrantia • Bastia •Dei gratia, hamartia •poinsettia •in absentia, Parmentier •Izvestia •meteor, wheatear •Whittier • cottier • Ostia •consortia, courtier •protea • Yakutia • frontier • Althea •Anthea • Parthia •Pythia, stichomythia •Carinthia, Cynthia •forsythia • Scythia • clothier • salvia •Latvia • Yugoslavia • envier •Flavia, Moldavia, Moravia, Octavia, paviour (US pavior), Scandinavia, Xavier •Bolivia, Livia, Olivia, trivia •Sylvia • Guinevere • Elzevir •Monrovia, Segovia •Retrovir • effluvia • colloquia •Goodyear • yesteryear • brassiere •Abkhazia •Anastasia, aphasia, brazier, dysphasia, dysplasia, euthanasia, fantasia, Frazier, glazier, grazier, gymnasia, Malaysia •amnesia, anaesthesia (US anesthesia), analgesia, freesia, Indonesia, Silesia, synaesthesia •artemisia, Kirghizia, Tunisia •ambrosia, crozier, hosier, osier, symposia

anesthetic •achromatic, acrobatic, Adriatic, aerobatic, anagrammatic, aquatic, aristocratic, aromatic, Asiatic, asthmatic, athematic, attic, autocratic, automatic, axiomatic, bureaucratic, charismatic, chromatic, cinematic, climatic, dalmatic, democratic, diagrammatic, diaphragmatic, diplomatic, dogmatic, dramatic, ecstatic, emblematic, emphatic, enigmatic, epigrammatic, erratic, fanatic, hepatic, hieratic, hydrostatic, hypostatic, idiomatic, idiosyncratic, isochromatic, lymphatic, melodramatic, meritocratic, miasmatic, monochromatic, monocratic, monogrammatic, numismatic, operatic, panchromatic, pancreatic, paradigmatic, phlegmatic, photostatic, piratic, plutocratic, pneumatic, polychromatic, pragmatic, prelatic, prismatic, problematic, programmatic, psychosomatic, quadratic, rheumatic, schematic, schismatic, sciatic, semi-automatic, Socratic, somatic, static, stigmatic, sub-aquatic, sylvatic, symptomatic, systematic, technocratic, thematic, theocratic, thermostatic, traumatic •anaphylactic, ataractic, autodidactic, chiropractic, climactic, didactic, galactic, lactic, prophylactic, syntactic, tactic •asphaltic •antic, Atlantic, corybantic, frantic, geomantic, gigantic, mantic, necromantic, pedantic, romantic, semantic, sycophantic, transatlantic •synaptic •bombastic, drastic, dynastic, ecclesiastic, elastic, encomiastic, enthusiastic, fantastic, gymnastic, iconoclastic, mastic, monastic, neoplastic, orgastic, orgiastic, pederastic, periphrastic, plastic, pleonastic, sarcastic, scholastic, scholiastic, spastic •matchstick • candlestick • panstick •slapstick • cathartic •Antarctic, arctic, subantarctic, subarctic •Vedantic • yardstick •aesthetic (US esthetic), alphabetic, anaesthetic (US anesthetic), antithetic, apathetic, apologetic, arithmetic, ascetic, athletic, balletic, bathetic, cosmetic, cybernetic, diabetic, dietetic, diuretic, electromagnetic, emetic, energetic, exegetic, frenetic, genetic, Helvetic, hermetic, homiletic, kinetic, magnetic, metic, mimetic, parenthetic, pathetic, peripatetic, phonetic, photosynthetic, poetic, prophetic, prothetic, psychokinetic, splenetic, sympathetic, syncretic, syndetic, synthetic, telekinetic, theoretic, zetetic •apoplectic, catalectic, dialectic, eclectic, hectic •Celtic •authentic, crescentic •aseptic, dyspeptic, epileptic, nympholeptic, peptic, proleptic, sceptic (US skeptic), septic •domestic, majestic •cretic •analytic, anchoritic, anthracitic, arthritic, bauxitic, calcitic, catalytic, critic, cryptanalytic, Cushitic, dendritic, diacritic, dioritic, dolomitic, enclitic, eremitic, hermitic, lignitic, mephitic, paralytic, parasitic, psychoanalytic, pyritic, Sanskritic, saprophytic, Semitic, sybaritic, syenitic, syphilitic, troglodytic •apocalyptic, cryptic, diptych, elliptic, glyptic, styptic, triptych •aoristic, artistic, autistic, cystic, deistic, distich, egoistic, fistic, holistic, juristic, logistic, monistic, mystic, puristic, sadistic, Taoistic, theistic, truistic, veristic •fiddlestick •dipstick, lipstick •impolitic, politic •polyptych • hemistich • heretic •nightstick •abiotic, amniotic, antibiotic, autoerotic, chaotic, demotic, despotic, erotic, exotic, homoerotic, hypnotic, idiotic, macrobiotic, meiotic, narcotic, neurotic, osmotic, patriotic, psychotic, quixotic, robotic, sclerotic, semiotic, symbiotic, zygotic, zymotic •Coptic, optic, panoptic, synoptic •acrostic, agnostic, diagnostic, gnostic, prognostic •knobstick • chopstick • aeronautic •Baltic, basaltic, cobaltic •caustic • swordstick • photic • joystick •psychotherapeutic, therapeutic •acoustic • broomstick • cultic •fustic, rustic •drumstick • gearstick • lunatic