general anesthesia

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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.

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.

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 high-risk 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, neuro-surgery, 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.

— Jennifer Sisk

Anesthesia characterized by unconsciousness, muscle relaxation, and loss of sensation over the entire body, and resulting from the administration of a general anesthetic.

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 DrugReference. 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 theDental Office. 5, no. 10 (April 22–24, 1985): 1–18.

— Lisette Hilton; Sam Uretsky, PharmD

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.

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2009)

In modern medical practice, general anaesthesia (AmE: anesthesia) is a state of total unconsciousness resulting from general anaesthetic drugs. A variety of drugs are given to the patient that have different effects with the overall aim of ensuring unconsciousness, amnesia and analgesia. The anaesthetist (AmE: anesthesiologist) selects the optimal technique for any given patient and procedure.

Contents 1 Overview 2 Preanaesthetic evaluation 2.1 Premedication 3 General anaesthesia 3.1 Induction of anaesthesia 3.2 Maintenance 3.3 Muscle relaxation / Neuromuscular blockade 3.4 Airway management 3.5 Monitoring 4 Stages of anaesthesia 4.1 Stage 1 4.2 Stage 2 4.3 Stage 3 4.4 Stage 4 5 Postoperative care 6 Mortality rates 7 See also 8 Notes 9 External links

Overview

General anaesthesia is a complex procedure involving:

• Preanaesthetic assessment
• Administration of general anaesthetic drugs
• Cardiorespiratory monitoring
• Analgesia
• Airway management
• Fluid management
• Postoperative pain relief

Preanaesthetic evaluation

Prior to surgery, the anaesthetist interviews the patient to determine the best combination of drugs and dosages and the degree to which monitoring is required to ensure a safe and effective procedure. Key factors of this determination are the patient's age, weight, medical history, current medications, previous anaesthetics, and fasting time. Patients are typically required to fill out this information on a separate form during the pre-operative evaluation. Depending on the existing medical conditions reported, the anaesthetist will review this information with the patient either during the pre-operative evaluation or on the day of the surgery.

Truthful and accurate answering of the questions is important so that the anaesthetist can select the proper anaesthetic drugs and procedures. For example, a heavy drinker or drug user who does not disclose their chemical uses could be undermedicated, which could then lead to anaesthesia awareness or dangerously high blood pressure. Commonly used medications such as Viagra can interact with anaesthesia drugs; failure to disclose such usage can endanger the patient.

An important aspect of this assessment is that of the patient's airway, involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx. The condition of teeth and location of dental crowns and caps are checked, neck flexibility and head extension observed. If an endotracheal tube is indicated and airway management is deemed difficult, then alternative placement methods such as fiberoptic intubation may be required, after induction of anaesthesia.

Premedication

Anesthesiologist may prescribe or administer a sedative pre-medication by injection or by mouth anywhere from a couple of hours to a couple of minutes before induction,

The most common drugs used for pre-medication are narcotics (opioids such as fentanyl) and sedatives (most commonly benzodiazepines such as midazolam).

General anaesthesia

General Anaesthesia implies loss of consciousness and of protective reflexes. General Anaesthesia is traditionally described as comprising of 3 components; Hypnosis,Relaxation and Analgesia

1 Hypnosis or sleep refers to being deeply asleep, unconscious, and totally unaware of events. 2 Relaxation implies abolition of reflex muscle tone, or specific block of nerve/muscle function, causing immobility and allowing easy surgical access. 3 Analgesia refers to use of one or more of a wide range of pain reducing drugs from paracetamol to morphine, and perhaps local anaesthetics to block pain impulse transmission along nerves, in the hope of reducing heart rate and blood pressure responses to surgery.

Induction of anaesthesia

The general anaesthetic is administered in either the operating theatre itself or a special ante-room.

General anesthesia can be induced by intravenous (IV) injection, or breathing a volatile anaesthetic through a facemask (inhalational induction). Onset of anaesthesia is faster with IV injection than with inhalation, taking about 10-20 seconds to induce total unconsciousness.^{[citation needed]} This has the advantage of avoiding the excitatory phase of anaesthesia (see below), and thus reduces complications related to induction of anaesthesia. An inhalational induction may be chosen by the anesthesiologist where IV access is difficult to obtain, where difficulty maintaining the airway is anticipated, or due to patient preference (e.g. children). Commonly used IV induction agents include propofol, sodium thiopental, etomidate, and ketamine. The most commonly-used agent for inhalational induction is sevoflurane because it causes less irritation than other inhaled gases.

Maintenance

The duration of action of IV induction agents is generally 5 to 10 minutes,^{[citation needed]} after which time spontaneous recovery of consciousness will occur. In order to prolong anaesthesia for the required duration (usually the duration of surgery), anaesthesia must be maintained. Usually this is achieved by allowing the patient to breathe a carefully controlled mixture of oxygen, nitrous oxide, and a volatile anaesthetic agent or by having a carefully controlled infusion of medication, usually propofol, through an IV. The inhalation agents are transferred to the patient's brain via the lungs and the bloodstream, and the patient remains unconscious. Inhaled agents are frequently supplemented by intravenous anaesthetics, such as opioids (usually fentanyl or a fentanyl derivative) and sedative-hypnotics (usually propofol or midazolam). Though for a propofol-based anaesthetic, supplementation by inhalation agents is not required. At the end of surgery the volatile or intravenous anaesthetic is discontinued. Recovery of consciousness occurs when the concentration of anaesthetic in the brain drops below a certain level (usually within 1 to 30 minutes depending upon the duration of surgery).

In the 1990s a novel method of maintaining anaesthesia was developed in Glasgow, UK. Called TCI (target controlled infusion), this involves using a computer controlled syringe driver (pump) to infuse propofol throughout the duration of surgery, removing the need for a volatile anaesthetic, and allowing pharmacologic principles to more precisely guide amount of infusion of the drug. Purported advantages include faster recovery from anaesthesia, reduced incidence of post-operative nausea and vomiting, and absence of a trigger for malignant hyperthermia. At present, TCI is not permitted in the United States.

Other medications will occasionally be given to anaesthetized patients to treat side effects or prevent complications. These medications include antihypertensives to treat high blood pressure, drugs like ephedrine and phenylephrine to treat low blood pressure, drugs like albuterol to treat asthma or laryngospasm/bronchospasm, and drugs like epinephrine or diphenhydramine to treat allergic reactions. Sometimes glucocorticoids or antibiotics are given to prevent inflammation and infection, respectively.

Muscle relaxation / Neuromuscular blockade

"Paralysis" or temporary muscle relaxation with a neuromuscular blocker is an integral part of modern anaesthesia. The first drug used for this purpose was curare, introduced in the 1940s, which has now been superseded by drugs with fewer side effects and generally shorter duration of action.

Muscle relaxation allows surgery within major body cavities, eg. abdomen and thorax without the need for very deep anaesthesia, and is also used to facilitate endotracheal intubation.

Acetylcholine, the natural neurotransmitter substance at the neuromuscular junction, causes muscles to contract when it is released from nerve endings. Muscle relaxants work by preventing acetylcholine from attaching to its receptor.

Paralysis of the muscles of respiration, ie. the diaphragm and intercostal muscles of the chest requires that some form of artificial respiration be implemented. As the muscles of the larynx are also paralysed, the airway usually needs to be protected by means of an endotracheal tube.

Monitoring of paralysis is most easily provided by means of a peripheral nerve stimulator. This device intermittently sends short electrical pulses through the skin over a peripheral nerve while the contraction of a muscle supplied by that nerve is observed.

The effects of muscle relaxants are commonly reversed at the termination of surgery by anticholinesterase drugs.

Examples of skeletal muscle relaxants in use today are pancuronium, rocuronium, vecuronium, atracurium, mivacurium, and succinylcholine.

Airway management

With the loss of consciousness caused by general anaesthesia, there is loss of protective airway reflexes (such as coughing), loss of airway patency and sometimes loss of a regular breathing pattern due to the effect of anaesthetics, opioids, or muscle relaxants. To maintain an open airway and regulate breathing within acceptable parameters, some form of "breathing tube" is inserted in the airway after the patient is unconscious. To enable mechanical ventilation, an endotracheal tube is often used (intubation), although there are alternative devices such as face masks or laryngeal mask airways.

Monitoring

Monitoring involves the use of several technologies to allow for a controlled induction of, maintenance of and emergence from general anaesthesia.

1. Continuous Electrocardiography (ECG): The placement of electrodes which monitor heart rate and rhythm. This may also help the anaesthetist to identify early signs of heart ischemia.

2. Continuous pulse oximetry (SpO2): The placement of this device (usually on one of the fingers) allows for early detection of a fall in a patient's hemoglobin saturation with oxygen (hypoxemia).

3. Blood Pressure Monitoring (NIBP or IBP): There are two methods of measuring the patient's blood pressure. The first, and most common, is called non-invasive blood pressure (NIBP) monitoring. This involves placing a blood pressure cuff around the patient's arm, forearm or leg. A blood pressure machine takes blood pressure readings at regular, preset intervals throughout the surgery. The second method is called invasive blood pressure (IBP) monitoring. This method is reserved for patients with significant heart or lung disease, the critically ill, major surgery such as cardiac or transplant surgery, or when large blood losses are expected. The invasive blood pressure monitoring technique involves placing a special type of plastic cannula in the patient's artery - usually at the wrist or in the groin.

4. Agent concentration measurement - Common anaesthetic machines have meters to measure the percent of inhalational anaesthetic agent used (e.g. sevoflurane, isoflurane, desflurane, halothane etc).

5. Low oxygen alarm - Almost all circuits have a backup alarm in case the oxygen delivery to the patient becomes compromised. This warns if the fraction of inspired oxygen drops lower than room air (21%) and allows the anaesthetist to take immediate remedial action.

6. Circuit disconnect alarm - indicates failure of circuit to achieve a given pressure during mechanical ventilation.

7. Carbon dioxide measurement (capnography)- measures the amount of carbon dioxide expired by the patient's lungs. It allows the anaesthetist to assess the adequacy of ventilation

8. Temperature measurement to discern hypothermia or fever, and to aid early detection of malignant hyperthermia.

9. EEG or other system to verify depth of anaesthesia may also be used. This reduces the likelihood that a patient will be mentally awake, although unable to move because of the paralytic agents. It also reduces the likelihood of a patient receiving significantly more amnesic drugs than actually necessary to do the job.

Stages of anaesthesia

The 4 Stages of anaesthesia were described in 1937^[1]. Despite newer anaesthetic agents and delivery techniques, which have led to more rapid onset and recovery from anaesthesia, with greater safety margins, the principles remain.

Stage 1

Stage 1 anaesthesia, also known as the "induction", is the period between the initial administration of the induction medications and loss of consciousness. During this stage the patient progresses from analgesia without amnesia to analgesia with amnesia. Patients can carry on a conversation at the time.

Stage 2

Stage 2 anaesthesia, also known as the "excitement stage", is the period following loss of consciousness and marked by excited and delirious activity. During this stage, respirations and heart rate may become irregular. In addition, there may be uncontrolled movements, vomiting, breath holding, and pupillary dilation. Since the combination of spastic movements, vomiting, and irregular respirations may lead to airway compromise, rapidly acting drugs are used to minimize time in this stage and reach stage 3 as fast as possible.

Stage 3

Stage 3, "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.

It has been divided into 4 planes:

1. roving eye balls, ending with fixed eyeballs
2. loss of corneal and laryngeal reflexes
3. pupils dilate and light reflex is lost
4. intercostal paralysis, shallow abdominal respiration, dilated pupils

Stage 4

Stage 4 anaesthesia, also known as "overdose", is the stage where too much medication has been given and the patient has severe brain stem or medullary depression. This results in a cessation of respiration and potential cardiovascular collapse. This stage is lethal without cardiovascular and respiratory support.

Postoperative care

Post-operative Analgesia

The anaesthesia should conclude with a pain-free awakening and a management plan for postoperative pain relief. This may be in the form of regional analgesia, oral, transdermal or parenteral medication. Minor surgical procedures are amenable to oral pain relief medications such as paracetamol and NSAIDs such as ibuprofen. Moderate levels of pain require the addition of mild opiates such as tramadol.

Major surgical procedures may require a combination of modalities to confer adequate pain relief. Parenteral methods include Patient Controlled Analgesia involving a strong opiate such as morphine, fentanyl or oxycodone. Here,to activate a syringe device,the patient presses a button and receives a preset dose or "bolus" of the drug e.g.one milligram of morphine. The PCA device then "locks out" for e.g.5 minutes, to allow the drug to take effect. if the patient becomes too sleepy or sedated, they make no more morphine requests. This confers a fail safe aspect which is lacking in continuous opiate infusion techniques.

Shivering

Shivering is a frequent occurrence in the post-operative period. Apart from causing discomfort and exacerbating post-operative pain, shivering has been shown to increase oxygen consumption, catecholamine release, cardiac output, heart rate, blood pressure and intra-ocular pressure. There are a number of techniques used to reduce this occurrence, such as increasing the ambient temperature in theatre, using conventional or forced warm air blankets and using warmed intravenous fluids.

[2]

Mortality rates

This article contains weasel words, vague phrasing that often accompanies biased or unverifiable information. Such statements should be clarified or removed. (February 2009)

Overall, the mortality rate for general anaesthesia is about three to five deaths per million anaesthetic administrations.^[3] Death during anaesthesia is most commonly related to surgical factors or pre-existing medical conditions. These include major haemorrhage, sepsis, and organ failure (eg. heart, lungs, kidneys, liver). Common causes of death directly related to anaesthesia include:

• aspiration of stomach contents
• suffocation (due to inadequate airway management)
• allergic reactions to anaesthesia (specifically and not limited to anti-nausea agents) and other deadly genetic predispositions
• human error
• equipment failure

In the U.S., up until about 1980, anaesthesia held significant risk, with at least one death per 10,000 times administered. [1] After becoming something of a public scandal, a careful effort was made to understand the causes and improve the results. [2] It is generally believed that anaesthesia is now at least ten times safer than it was then.[3] However, there is some controversy about this.[4] In the U.S., the data is not made public (in fact, the data is not even collected), so the truth is uncertain.[5] The death rate for dental anaesthesia is reported to be one out of 350,000. [6]

See also

• Anaesthetic equipment
• Anaesthesia awareness
• Seishū Hanaoka - first physician to use general anaesthesia

Notes

1. ^ PubMed Central.
2. ^ English, William (2002). "Post anaesthesia shivering (PAS)" (in English). World Anaesthetia. World Federation of Societies of Anaesthesiologists. http://www.nda.ox.ac.uk/wfsa/html/u15/u1503_01.htm. Retrieved on 2008-11-01. 
3. ^ Henry Rosenberg. "Mortality Associated with Anesthesia". ExpertPages.com. http://expertpages.com/news/mortality_anesthesia.htm. Retrieved on 2006-07-11. 

External links

• Chloroform: The molecular lifesaverAn article at University of Bristol providing interesting facts about chloroform.
• Australian & New Zealand College of Anaesthetists Monitoring Standard
• Royal College of Anaesthetists Patient Information page

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