Anesthesia Definition

Anaesthesia Definition

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

Purpose

General anaesthesia 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

ANESTHETICS: HOW THEY WORKDescription

Anaesthesia performed with general anaesthetics 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 anaesthesia.

Stage Three: Surgical Anaesthesia. 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 centres 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 anaesthetics administered prevent this occurrence.

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

General Anaesthesia works by altering the flow of sodium molecules into nerve cells (neurons) through the cell membrane. Exactly how the anaesthetic 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 Anaesthesia may feel like deep sleep, it is not the same. In sleep, some parts of the brain speed up while others slow down. Under Anaesthesia, the loss of consciousness is more widespread.

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

Halothane (Fluothane) is a powerful anaesthetic and can easily be over administered. 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 odour, and was frequently the anaesthetic 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 Anaesthesia 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 anaesthetics for anaesthesia induction. Awakening from anaesthesia 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 anaesthetics 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 paediatric patients. Like desfluorane, this agent is rapidly eliminated and allows rapid awakening.

Nitrous oxide (laughing gas) is a weak anaesthetic and is used with other agents, such as thiopental, to produce surgical Anaesthesia. 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 anaesthetic agents include ketamine, thiopental, opioids, and propofol.

Ketamine (Ketalar) affects the senses, and produces a dissociative Anaesthesia (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 paediatric patients because Anaesthesia 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 Anaesthesia and surgery as a sedative and analgesic, as well as a continuous infusion for primary Anaesthesia. Because opioids rarely affect the cardiovascular system, they are particularly useful for cardiac surgery and other high-risk cases. Opioids act directly on spinal cord receptors, and are frequently used in epidurals for spinal Anaesthesia. Side effects may include nausea and vomiting, itching, and respiratory depression.

Propofol (Diprivan) is a non-barbiturate hypnotic agent and the most recently developed intravenous anaesthetic. 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 Anaesthesia, unlike thiopental. Hence, propofol is rapidly replacing thiopental as an intravenous induction agent. It is used for general surgery, cardiac surgery, neurosurgery, and paediatric surgery.

General anaesthetics are given only by anaesthesiologists, 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 anaesthetic or combination of anaesthetics to use. General anaesthetics are usually inhaled through a mask or a breathing tube or injected into a vein, but are also sometimes given rectally.

General Anaesthesia is much safer today than it was in the past. This progress is due to faster-acting anaesthetics, 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 anaesthetic, 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 Anaesthesia are very low, they can include heart attack, stroke, brain damage, and death. Anyone scheduled to undergo general Anaesthesia 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 anaesthesiologist 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 anaesthetics and may be more likely to experience side effects from these drugs.

Patients who have had general Anaesthesia 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 anaesthetics. Before undergoing general Anaesthesia, anyone with the following conditions should absolutely inform their doctor.

ALLERGIES

Anyone who has had allergic or other unusual reactions to barbiturates or general anaesthetics in the past should notify the doctor before having general Anaesthesia. In particular, people who have had malignant hyperthermia or whose family members have had malignant hyperthermia during or after being given an anaesthetic 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 Anaesthesia 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 anaesthetics on pregnant women and foetuses vary, depending on the type of drug. In general, giving large amounts of general anaesthetics to the mother during labour and delivery may make the baby sluggish after delivery. Pregnant women should discuss the use of anaesthetics during labour and delivery with their doctors. Pregnant women who may be given general Anaesthesia for other medical procedures should ensure that the treating physician is informed about the pregnancy.

BREASTFEEDING

Some general anaesthetics 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 anaesthetic, 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 oesophagus, 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 anaesthetics affect the central nervous system, patients may feel drowsy, weak, or tired for as long as a few days after having general Anaesthesia. Fuzzy thinking, blurred vision, and coordination problems are also possible. For these reasons, anyone who has had general Anaesthesia 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 anaesthetic 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, light-headedness, 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 Anaesthesia:

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 anaesthetics 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 Anaesthesia should inform the doctor about all other medication that he or she is taking. This includes prescription drugs, non-prescription medicines, and street drugs. Serious and possibly life-threatening reactions may occur when general anaesthetics 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.

Key Terms

Amnesia

The loss of memory.

Analgesia

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

AnaesthesiologistA medical specialist who administers an anaesthetic to a patient before he is treated.

Anaesthetic

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 Anaesthesia 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 centres.

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.