anesthesia

 

(ănĭsthē'zhə) [Gr.,=insensibility], loss of sensation, especially that of pain, induced by drugs, especially as a means of facilitating safe surgical procedures. Early modern medical anesthesia dates to experiments with nitrous oxide (laughing gas) by Sir Humphry Davy of England and the dentist Horace Wells of the United States. Ether came into general use as an anesthetic after a demonstration at the Massachusetts General Hospital in Boston by William T. G. Morton in 1846.

General anesthetics, administered by inhalation or intravenous injection, cause unconsciousness as well as insensibility to pain, and are used for major surgical procedures. In the past, ether was the most commonly used general anesthetic. Today, safer anesthetics include Halothane and Isoflurane, both of which are administered through inhalation. Short-acting anesthetic agents, such as pentothal, Diprivan, and Midazolam, are generally given through intravenous or intramuscular routes. Inhaled nitrous oxide is used for light anesthesia in minor surgical procedures and in dentistry. Ultra-short-acting analgesics can also be given intranasally for pre-medication prior to the induction of general anesthesia. Anesthetics such as Brevital may be administered rectally, primarily among children.

Local anesthetics affect sensation only in the region where they are injected, and are used regularly in dentistry and minor surgery. Spinal and epidural anesthesia involves the injection of an anesthetic agent into a space adjacent to the spinal cord, a technique frequently employed for surgical procedures below the waist (e.g., obstetrics) where total unconsciousness is not necessary. Such anesthetics are known as regional blocks. Muscle relaxants may be used in conjunction with general anesthetics, particularly to reduce the amount of anesthetic required. Body temperatures are generally lowered in conjunction with the use of anesthetics in heart and brain surgery, reducing the body's metabolic rate so that cells are not damaged by the lack of circulating blood and reduced oxygenation. Several forms of anesthesia may be used in combination. Safer and more efficient anesthetics are constantly researched, in the hopes of perfecting new ways of combining and administering them.

See also acupuncture, analgesic, anesthesiology, and surgery.

Bibliography

See J. Rupreht et al., ed., Anesthesia: Essays on Its History (1985); J. Tolmie and A. Birch, Anesthesia for the Uninterested (2d ed. 1986); J. M. Fenster, Ether Day: The Strange Tale of America's Greatest Medical Discovery and the Haunted Men Who Made It (2001).

Anesthesia or anaesthesia (see spelling differences; from Greek αν- an- “without” + αἲσθησις aisthesis “sensation”) has traditionally meant the condition of having the feeling of pain and other sensations blocked. This allows patients to undergo surgery and other procedures without the distress and pain they would otherwise experience. The word was coined by Oliver Wendell Holmes, Sr. in 1846. Another definition is a "reversible lack of awareness", whether this is a total lack of awareness (e.g. a general anaesthestic) or a lack of awareness of a part of a the body such as a spinal anaesthetic or another nerve block would cause.

Today, the term general anesthesia in its most general form can include:

• Analgesia: blocking the conscious sensation of pain;
• Hypnosis: producing unconsciousness;
• Amnesia: preventing memory formation;
• Relaxation: preventing unwanted movement or muscle tone;
• Obtundation of reflexes, preventing exaggerated autonomic reflexes.

Patients undergoing surgery usually undergo preoperative evaluation. It includes gathering history of previous anesthetics, and any other medical problems, physical examination, ordering required blood work and consultations prior to surgery.

There are several forms of anesthesia. The following forms refer to states achieved by anesthetics working on the brain:

• General anesthesia: "Drug-induced loss of consciousness during which patients are not arousable, even by painful stimulation." Patients undergoing general anesthesia often cannot maintain their own airway and breathe on their own. While usually administered with inhalational agents, general anesthesia can be achieved with intravenous agents, such as propofol.^[1]
• Deep sedation/analgesia: "Drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully following repeated or painful stimulation." Patients may sometimes be unable to maintain their airway and breathe on their own.^[1]
• Moderate sedation/analgesia or conscious sedation: "Drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation." In this state, patients can breathe on their own and need no help maintaining an airway.^[1]
• Minimal sedation or anxiolysis: "Drug-induced state during which patients respond normally to verbal commands." Though concentration, memory, and coordination may be impaired, patients need no help breathing or maintaining an airway.^[1]

The level of anesthesia achieved ranges on a continuum of depth of consciousness from minimal sedation to general anesthesia. The depth of consciousness of a patient may change from one minute to the next.

The following refer to states achieved by anesthetics working outside of the brain:

• Regional anesthesia: Loss of pain sensation, with varying degrees of muscle relaxation, in certain regions of the body. Administered with local anesthesia to peripheral nerve bundles, such as the brachial plexus in the neck. Examples include the interscalene block for shoulder surgery, axillary block for wrist surgery, and femoral nerve block for leg surgery. While traditionally administered as a single injection, newer techniques involve placement of indwelling catheters for continuous or intermittent administration of local anesthetics.
  • Spinal anesthesia: also known as subarachnoid block. Refers to a Regional block resulting from a small volume of local anesthetics being injected into the spinal canal. The spinal canal is covered by the dura mater, through which the spinal needle enters. The spinal canal contains cerebrospinal fluid and the spinal cord. The sub arachnoid block is usually injected between the 4th and 5th lumbar vertebrae, because the spinal cord usually stops at the 1st lumbar vertebra, while the canal continues to the sacral vertebrae. It results in a loss of pain sensation and muscle strength, usually up to the level of the chest (nipple line or 4th thoracic dermatome).
  • Epidural anesthesia: Regional block resulting from an injection of a large volume of local anesthetic into the epidural space. The epidural space is a potential space that lies underneath the ligamenta flava, and outside the dura mater (outside layer of the spinal canal). This is basically an injection around the spinal canal.
• Local anesthesia is similar to regional anesthesia, but exerts its effect on a smaller area of the body.

History

Herbal derivatives

The first herbal anesthesia was administered in prehistory. Opium poppy capsules were collected in 4200 B.C., and opium poppies were farmed in Sumeria and succeeding empires. The use of opium-like preparations in anaesthesia is recorded in the Ebers Papyrus of 1500 B.C. By 1100 B.C. in Cyprus poppies were scored for collection of opium by methods similar to those used in the present day, and simple apparatus for smoking of opium was found in a Minoan temple. Opium was not introduced to India and China until 330 B.C. and 600-1200 A.D., but these nations pioneered the use of cannabis incense and aconitum. In the second century, according to the Book of Later Han, the physician Hua Tuo performed abdominal surgery using an anesthetic substance called mafeisan (麻沸散 "cannabis boil powder") dissolved in wine. Throughout Europe, Asia, and the Americas a variety of Solanum species containing potent tropane alkaloids were used, such as mandrake, henbane, Datura metel, and Datura inoxia. Classic Greek and Roman medical texts by Hippocrates, Theophrastus, Aulus Cornelius Celsus, Pedanius Dioscorides, and Pliny the Elder discussed the use of opium and Solanum species, and treatment with the combined alkaloids proved a mainstay of anaesthesia until the nineteenth century. In the Americas coca was also an important anaesthetic used in trephining operations. Incan shamans chewed coca leaves and performed operations on the skull while spitting into the wounds they'd inflicted to anaesthetize the site.^{[citation needed]} Alcohol was also used, its vasodilatory properties being unknown. Ancient herbal anaesthetics have variously been called soporifics, anodynes, and narcotics, depending on whether the emphasis is on producing unconsciousness or relieving pain.

In the East, in the 10th century work Shahnameh, the author describes a caesarean section performed on Rudaba when giving birth, in which a special wine agent was prepared by a Zoroastrian priest, and used to produce unconsciousness for the operation. Although largely mythical in content, the passage does at least illustrate knowledge of anesthesia in ancient Persia. Dioscorides and other Greek authors were well-known in the Islamic Empire, and physicians such as al-Razi, Avicenna, and Abu al-Qasim wrote textbooks of great importance in the development of medicine in Europe and the Middle East. Muslim anesthesiologists were the first to utilize oral as well as inhalant anesthetics. In Islamic Spain, Abu al-Qasim (Abulcasis) and Ibn Zuhr (Avenzoar), among other Muslim surgeons, performed hundreds of surgeries under inhalant anesthesia with the use of narcotic-soaked sponges which were placed over the face. Abulcasis and Ibn Sina (Avicenna) wrote about anasthesia in their influential medical encyclopedias, the al-Tasrif and The Canon of Medicine.^[2]

The use of herbal anaesthesia had a crucial drawback compared to modern practice - as lamented by Fallopus, "when soporifics are weak they are useless, and when strong, they kill." To overcome this, production was typically standardized as much as feasible, with production occurring from specific famous locations (such as opium from the fields of Thebes in ancient Egypt). Anaesthetics were sometimes administered in the spongia somnifera, a sponge into which a large quantity of drug was allowed to dry, from which a saturated solution could be trickled into the nose of the patient. At least in more recent centuries, trade was often highly standardized, with the drying and packing of opium in standard chests, for example. In the nineteenth century, varying aconitum alkaloids from a variety of species were standardized by testing with guinea pigs. Despite these refinements, the discovery of morphine, a purified alkaloid that soon afterward could be injected by hypodermic for a consistent dosage, was enthusiastically received and led to the foundation of the modern pharmaceutical industry.

Another factor affecting ancient anaesthesia is that drugs used systemically in modern times were often administered locally, reducing the risk to the patient. Opium used directly in a wound acts on peripheral opioid receptors to serve as an analgesic, and a medicine containing willow leaves (salicylate, the predecessor of aspirin) would then be applied directly to the source of inflammation.

In 1804, the Japanese surgeon Hanaoka Seishū performed general anaesthesia for the operation of a breast cancer (mastectomy), by combining Chinese herbal medicine know-how and Western surgery techniques learned through "Rangaku", or "Dutch studies". His patient was a 60-year-old woman called Kan Aiya.^[3] He used a compound he called Tsusensan, based on the plants Datura metel, Aconitum and others.

Non-pharmacological methods

Hypnotism and acupuncture have a long history of use as anesthetic techniques. In China, Taoist medical practitioners developed anesthesia by means of acupuncture. Chilling tissue (e.g. with ice) can temporarily cause nerve fibers (axons) to stop conducting sensation, while hyperventilation can cause brief alteration in conscious perception of stimuli including pain (see Lamaze).

In modern anesthetic practice, these techniques are seldom employed.

Early gases and vapours

In the West, the development of effective anesthetics in the 19th century was, with Listerian techniques, one of the keys to successful surgery. Henry Hill Hickman experimented with carbon dioxide in the 1820s. The anaesthetic qualities of nitrous oxide (isolated in 1773 by Joseph Priestley) were discovered by the British chemist Humphry Davy about 1799 when he was an assistant to Thomas Beddoes, and reported in a paper in 1800. But initially the medical uses of this so-called "laughing gas" were limited - its main role was in entertainment. It was used on 30 September 1846 for painless tooth extraction upon patient Eben Frost by American dentist William Thomas Green Morton. Horace Wells Connecticut, a travelling dentist, had demonstrated it the previous year 1845, at Massachusetts General Hospital. Wells made a mistake, in choosing a particularly sturdy male volunteer, and the patient suffered considerable pain. This lost the colourful Wells any support. Later the patient told Wells he screamed in shock and not in pain. A subsequently drunk Wells died in jail, by cutting his femoral artery, after allegedly assaulting a prostitute with sulphuric acid.

Another dentist, William E. Clarke, performed an extraction in January 1842 using a different chemical, diethyl ether (discovered by Valerius Cordus in 1540). In March 1842 in Danielsville, Georgia, Dr. Crawford Long was the first to use anaesthesia during an operation, giving it to a boy (John Venables) before excising a cyst from his neck; however, he did not publicize this information until later.

On October 16, 1846, another dentist, William Thomas Green Morton, invited to the Massachusetts General Hospital, performed the first public demonstration of diethyl ether (then called sulfuric ether) as an anesthetic agent, for a patient (Edward Gilbert Abbott) undergoing an excision of a vascular tumour from his neck. In a letter to Morton shortly thereafter, Oliver Wendell Holmes, Sr. proposed naming the procedure anæsthesia.

Despite Morton's efforts to keep "his" compound a secret, which he named "Letheon" and for which he received a US patent, the news of the discovery and the nature of the compound spread very quickly to Europe in late 1846. Here, respected surgeons, including Liston, Dieffenbach, Pirogoff, and Syme undertook numerous operations with ether. An American born physician, Boott who had travelled to London, encouraged a leading dentist, Mr James Robinson, to perform a dental procedure on a Miss Lonsdale. This was the first case of an operator-anaesthetist. On the same day, Saturday 19 December 1846 in Dumfries Royal Infirmary, Scotland, A Dr Scott used ether for a surgical procedure. The first use of anaesthesia in the southern hemisphere took place in Launceston, Tasmania, that same year. Ether has a number of drawbacks, such as its tendency to induce vomiting and its flammability. In England it was quickly replaced with chloroform.

Discovered in 1831, the use of chloroform in anaesthesia is usually linked to James Young Simpson, who, in a wide-ranging study of organic compounds, found chloroform's efficacy on 4 November 1847. Its use spread quickly and gained royal approval in 1853 when John Snow gave it to Queen Victoria during the birth of Prince Leopold. Unfortunately, chloroform is not as safe an agent as ether, especially when administered by an untrained practitioner (medical students, nurses and occasionally members of the public were often pressed into giving anaesthetics at this time). This led to many deaths from the use of chloroform which (with hindsight) might have been preventable. The first fatality directly attributed to chloroform anaesthesia (Hannah Greener) was recorded on 28 January 1848.

John Snow of London published articles from May 1848 onwards 'On Narcotism by the Inhalation of vapours' in the London Medical Gazette. Snow also involved himself in the production of equipment needed for inhalational anaesthesia.

The surgical amphitheatre at Massachusetts General Hospital, or "ether dome" still exists today, although it is used for lectures and not surgery. The public can visit the amphitheater on weekdays when it is not in use.

Early local anesthetics

The first effective local anesthetic was cocaine. Isolated in 1859, it was first used by Karl Koller, at the suggestion of Sigmund Freud, in ophthalmic surgery in 1884. Before that doctors had used a salt and ice mix for the numbing effects of cold, which could only have limited application. Similar numbing was also induced by a spray of ether or ethyl chloride. A number of cocaine derivatives and safer replacements were soon produced, including procaine (1905), Eucaine (1900), Stovaine (1904), and lidocaine (1943).

Opioids were first used by Racoviceanu-Piteşti, who reported his work in 1901.

Anesthesia Providers

Physicians specialising in peri-operative care, development of an anesthetic plan, and the administration of anesthetics are known in the United States as anesthesiologists and in the UK and Canada as anaesthetists or anaesthesiologists. All anaesthetics in the UK, Australia, New Zealand and Japan are administered by physicians. Nurse anesthetists also administer anesthesia in 109 nations.^[4] In the US, 35% of anesthetics are provided by physicians in solo practice, about 55% are provided by ACTs with anesthesiologists medically directing Anesthesiologist Assistants, CRNAs, and about 10% are provided by CRNAs in solo practice. ^{[5] [6] [7]} - ^[8] - ^[9]

Physician Anesthesiologists / Anaesthetists/ Anaesthesiologists

In the US, medical doctors who specialize in anesthesiology are called anesthesiologists. Such physicians in the UK and Canada are called anaesthetists or anaesthesiologists.

In the U.S., a physician specializing in anesthesiology completes 4 years of college, 4 years of medical school, 1 year of internship, and 3 years of residency. According to the American Society of Anesthesiologists, Anesthesiologists provide or participate in more than 90 percent of the 40 million anesthetics delivered annually.^[10]

In the UK this training lasts a minimum of seven years after the awarding of a medical degree and two years of basic residency, and takes place under the supervision of the Royal College of Anaesthetists. In Australia and New Zealand, it lasts five years after the awarding of a medical degree and two years of basic residency, under the supervision of the Australian and New Zealand College of Anaesthetists. Other countries have similar systems, including Ireland (the Faculty of Anaesthetists of the Royal College of Surgeons in Ireland), Canada and South Africa (the College of Anaesthetists of South Africa).

In the UK, completion of the examinations set by the Royal College of Anaesthetists leads to award of the Diploma of Fellowship of the Royal College of Anaesthetists (FRCA). In the US, completion of the written and oral Board examinations by a physician anesthesiologist allows one to be called "Board Certified" or a "Diplomate" of the American Board of Anesthesiology.

Other specialties within medicine are closely affiliated to anaesthetics. These include intensive care medicine and pain medicine. Specialists in these disciplines have usually done some training in anaesthetics. The role of the anaesthetist is changing. It is no longer limited to the operation itself. Many anaesthetists consider themselves to be peri-operative physicians, and will involve themselves in optimizing the patient's health before surgery (colloquially called "work-up"), performing the anaesthetic, following up the patient in the post anesthesia care unit and post-operative wards, and ensuring optimal analgesia throughout.

It is important to note that the term "anesthetist" in the United States usually refers to registered nurses who have completed specialized education and training in nurse anesthesia to become certified registered nurse anesthetists (CRNAs). As noted above, the term "anaesthetist" in the UK and Cananda refers to medical doctors who specialize in anesthesiology.

Nurse Anesthetists

In the United States, advance practice nurses specializing in the provision of anesthesia care are known as Certified Registered Nurse Anesthetists (CRNAs). According to the American Association of Nurse Anesthetists, the 36,000 CRNAs in the US administer approximately 27 million anesthetics each year.[[1]] CRNAs are the sole providers of anesthesia in more than 70 percent of rural area hospitals. Thirty-four percent of nurse anesthetists practice in communities of less than 50,000. CRNAs start school with a bachelors degree and at least 1 year of acute care nursing experience[[2]], and gain a masters degree in nurse anesthesia before passing the mandatory Certification Exam. The average CRNA student has 5-7 years of nursing experience before entering a 27-36 month masters level anesthesia program.^[11]

CRNAs may work with podiatrists, dentists, anesthesiologists, surgeons, obstetricians and other professionals requiring their services. CRNAs administer anesthesia in all types of surgical cases, and are able to apply all the accepted anesthetic techniques -- general, regional, local, or sedation. The degree of physician supervision, direction, or collaboration required is determined by State law and local institution policy.

Anesthesiology Assistants

In the US, Anesthesiologist Assistants (AAs) are physician assistants who have undertaken specialized education and training to provide anesthesia care. AAs typically hold a masters degree and practice under physician supervision in sixteen states through licensing, certification or physician delegation.^[12]

In the UK, a similar group of assistants are currently being evaluated. In Scotland they are named Physician's Assistant - Anaesthesia and in the rest of the UK, they are called Anaesthesia Practitioners. Their background can be nursing, operating department professional or another profession allied to medicine or a science graduate. Training takes 27 months and to date, the first five have graduated in England.

Anesthesiology Assistants should be distinguished from Anesthesia Technicians.

Anesthesia Technicians

Anesthesia Technicians are specially trained biomedical technicians who assist anesthesiologists, nurse anesthetists, and anesthesiology assistants with monitoring equipment, supplies, and patient care procedures in the operating room.

In New Zealand, anaesthetic technicians complete a course of study recognized by the New Zealand Association of Anaesthetic Technicians and Nurses.

In the United Kingdom, personnel known as ODPs (operating department practitioners) or Anaesthetic nurses provide support to the physician anaesthetist (anaesthesiologist).

Veterinary Anesthetists / Anesthesiologists

Main article: Veterinary anesthesia

Veterinary anesthetists utilize much the same equipment and drugs as those who provide anesthesia to human patients. In the case of animals, the anesthesia must be tailored to fit the species ranging from large land animals like horses or elephants to birds to aquatic animals like fish. For each species there are ideal, or at least less problematic, methods of safely inducing anesthesia. For wild animals, anesthetic drugs must often be delivered from a distance by means of remote projector systems ("dart guns") before the animal can even be approached. Large domestic animals, like cattle, can often be anesthetized for standing surgery using only local anesthetics and sedative drugs. While most clinical veterinarians and veterinary technicians routinely function as anesthetists in the course of their professional duties, veterinary anesthesiologists in the U.S. are veterinarians who have completed a two-year residency in anesthesia and have qualified for certification by the American College of Veterinary Anesthesiologists.

Anesthetic agents

Local anesthetics

Main article: Local anesthetic

• procaine
• amethocaine
• cocaine
• lidocaine
• prilocaine
• bupivicaine
• levobupivacaine
• ropivacaine
• dibucaine

Local anesthetics are agents which prevent transmission of nerve impulses without causing unconsciousness. They act by binding to fast sodium channels from within (in an open state). Local anesthetics can be either ester or amide based.

Ester local anesthetics (eg. procaine, amethocaine, cocaine) are generally unstable in solution and fast acting, and allergic reactions are common.

Amide local anesthetics (eg. lidocaine, prilocaine, bupivicaine, levobupivacaine, ropivacaine and dibucaine) are generally heat stable, with a long shelf life (around 2 years). They have a slower onset and longer half life than ester anaesthetics, and are usually racemic mixtures, with the exception of levobupivacaine (which is S(-) -bupivacaine) and ropivacaine (S(-)-ropivacaine). These agents are generally used within regional and epidural or spinal techniques, due to their longer duration of action which provides adequate analgesia for surgery, labor and symptomatic relief.

Only preservative-free local anesthetic agents may be injected intrathecally.

Adverse effects of local anaesthesia

Adverse effects of local anesthesia are generally referred to as Local Anesthetic Toxicity.

Effects may be localized or systemic.

Examples of systemic effects of local anesthesia:

Local anesthetic drugs are toxic to the heart (where they cause arrhythmia) and brain (where they may cause unconsciousness and seizures). Arrhythmias may be resistant to defibrillation and other standard treatments, and may lead to loss of heart function and death.

The first evidence of local anesthetic toxicity involves the nervous system, including agitation, confusion, dizziness, blurred vision, tinnitus, a metallic taste in the mouth, and nausea that can quickly progress to seizures and cardiovascular collapse.

Toxicity can occur with any local anesthetic as an individual reaction by that patient. Possible toxicity can be tested with pre-operative procedures to avoid toxic reactions during surgery.

An example of localized effect of local anesthesia:

Direct infiltration of local anesthetic into skeletal muscle will cause temporary paralysis of the muscle.

Current inhaled general anesthetic agents

Main article: General anaesthesia

• Nitrous oxide
• Halothane
• Enflurane
• Isoflurane
• Sevoflurane
• Desflurane
• Xenon (rarely used)

Volatile agents are specially formulated organic liquids that evaporate readily into vapors, and are given by inhalation for induction and/or maintenance of general anesthesia. Nitrous oxide and xenon are gases at room temperature rather than liquids, so they are not considered volatile agents. The ideal anesthetic vapor or gas should be non-flammable, non-explosive, lipid soluble, should possess low blood gas solubility, have no end organ (heart, liver, kidney) toxicity or side effects, should not be metabolized and should be non-irritant when inhaled by patients.

No anesthetic agent currently in use meets all these requirements. The agents in widespread current use are isoflurane, desflurane, sevoflurane, and nitrous oxide. Nitrous oxide is a common adjuvant gas, making it one of the most long-lived drugs still in current use. Because of its low potency, it cannot produce anesthesia on its own but is frequently combined with other agents. Halothane, an agent introduced in the 1950s, has been almost completely replaced in modern anesthesia practice by newer agents because of its shortcomings.^[13] Partly because of its side effects, enflurane never gained widespread popularity. ^[13]

In theory, any inhaled anesthetic agent can be used for induction of general anesthesia. However, most of the halogenated anesthetics are irritating to the airway, perhaps leading to coughing, laryngospasm and overall difficult inductions. For this reason, the most frequently-used agent for inhalational induction is sevoflurane. All of the volatile agents can be used alone or in combination with other medications to maintain anesthesia (nitrous oxide is not potent enough to be used as a sole agent).

Currently research into the use of xenon as an anesthetic is underway, but the gas is very expensive to produce and requires special equipment for delivery, as well as special monitoring and scavenging of waste gas.

Volatile agents are frequently compared in terms of potency, which is inversely proportional to the minimum alveolar concentration. Potency is directly related to lipid solubility. This is known as the Meyer-Overton hypothesis. However, certain pharmacokinetic properties of volatile agents have become another point of comparison. Most important of those properties is known as the blood:gas partition coefficient. This concept refers to the relative solubilty of a given agent in blood. Those agents with a lower blood solubility (i.e. a lower blood:gas partition coefficient, e.g. desflurane) give the anesthesia provider greater rapidity in titrating the depth of anesthesia, and permit a more rapid emergence from the anesthetic state upon discontinuing their administration. In fact, newer volatile agents (e.g. sevoflurane, desflurane) have been popular not due to their potency [minimum alveolar concentration], but due to their versatility for a faster emergence from anesthesia, thanks to their lower blood:gas partition coefficient.

Current intravenous anesthetic agents (non-opioid)

While there are many drugs that can be used intravenously to produce anesthesia or sedation, the most common are:

• Barbiturates
  • Thiopental
  • Methohexital
• Benzodiazepines
  • Midazolam
  • Lorazepam
  • Diazepam
• Propofol
• Etomidate
• Ketamine

The two barbiturates mentioned above, thiopental and methohexital, are ultra-short acting and are used to induce and maintain anesthesia.^[14] However, though they produce unconsciousness, they provide no analgesia (pain relief) and must be used with other agents.^[14] Benzodiazepines can be used for sedation before or after surgery and can be used to induce and maintain general anesthesia.^[14] When benzodiazepines are used to induce general anesthesia, midazolam is preferred.^[14] Benzodiazepines are also used for sedation during procedures that do not require general anesthesia.^[14] Like barbiturates, benzodiazepines have no pain-relieving properties.^[14] Propofol is one of the most commonly-used intravenous drugs employed to induce and maintain general anesthesia.^[14] It can also be used for sedation during procedures or in the ICU.^[14] Like the other agents mentioned above, it renders patients unconscious without producing pain relief.^[14] Because of its favorable physiological effects, "etomidate has been primarily used in sick patients."^[14] Ketamine is infrequently used in anesthesia practice because of the unpleasant experiences which sometimes occur upon emergence from anesthesia, which include "vivid dreaming, extracorporeal experiences (sense of floating out of one's body), and illusions (misinterpretation of a real, external sensory experience"^[15] However, like etomidate it is frequently used in emergency settings and with sick patients because it produces fewer adverse physiological effects.^[14] Unlike the intravenous anesthetic drugs previously mentioned, ketamine produces profound pain relief, even in doses lower than those which induce general anesthesia.^[14] Also unlike the other anesthetic agents in this section, "patients who receive ketamine alone appear to be in a cataleptic state, unlike other states of anesthesia that resemble normal sleep. Ketamine-anesthetized patients have profound analgesia but keep their eyes open and maintain many reflexes."^[14]

Current intravenous opioid analgesic agents

While opioids can produce unconsciousness, they do so unreliably and with significant side effects.^[16][17] So, while they are rarely used to induce anesthesia, they are frequently used along with other agents such as intravenous non-opioid anesthetics or inhalational anesthetics.^[14] Furthermore, they are used to relieve pain of patients before, during, or after surgery. The following opioids have short onset and duration of action and are frequently used during general anesthesia:

• Fentanyl
• Alfentanil
• Sufentanil
• Remifentanil

The following agents have longer onset and duration of action and are frequently used for post-operative pain relief:

• Morphine
• Meperidine, also called pethidine in the UK, New Zealand, Australia and other countries
• Methadone
• Diamorphine, (diacetyl morphine, also known as heroin, not available in U.S.)
• Oxycodone, (not available intravenously in U.S.)

Current muscle relaxants

Muscle relaxants do not render patients unconscious or relieve pain. Instead, they are sometimes used after a patient is rendered unconscious (induction of anesthesia) to facilitate intubation or surgery by paralyzing skeletal muscle.

• Depolarizing muscle relaxants
  • Succinylcholine (also known as suxamethonium in the UK, New Zealand, Australia and other countries)
• Non-depolarizing muscle relxants
  • Short acting
    • Mivacurium
    • Rapacuronium
  • Intermediate acting
    • Atracurium
    • Cisatracurium
    • Vecuronium
    • Rocuronium
  • Long acting
    • Pancuronium
    • Metocurine
    • d-Tubocurarine
    • Gallamine
    • Alcuronium
    • Doxacurium
    • Pipecuronium

Adverse effects of muscle relaxants

Succinylcholine may cause hyperkalemia if given to burn patients, or paralyzed (quadraplegic, paraplegic) patients. The mechanism is reported to be through upregulation of acetylcholine receptors in those patient populations. Succinylcholine may also trigger malignant hyperthermia in susceptible patients.

Another potentially disturbing complication can be 'anesthesia awareness'. In this situation, patients paralyzed with muscle relaxants may awaken during their anesthesia, due to decrease in the levels of drugs providing sedation and/or pain relief. If this fact is missed by the anaesthesia provider, the patient may be aware of his surroundings, but be incapable of moving or communicating that fact. Neurological monitors are becoming increasingly available which may help decrease the incidence of awareness. Most of these monitors use proprietary algorithms monitoring brain activity via evoked potentials. Despite the widespread marketing of these devices many case reports exist in which awareness under anesthesia has occurred despite apparently adequate anesthesia as measured by the neurologic monitor. ^{[citation needed]}

Current intravenous reversal agents

• Naloxone, reverses the effects of opioids
• Flumazenil, reverses the effects of benzodiazepines
• Neostigmine, reverses the effects of non-depolarizing muscle relaxants

Anesthetic equipment

Main article: Anaesthetic equipment

In modern anesthesia, a wide variety of medical equipment is desirable depending on the necessity for portable field use, surgical operations or intensive care support. Anesthesia practitioners must possess a comprehensive and intricate knowledge of the production and use of various medical gases, anaesthetic agents and vapours, medical breathing circuits and the variety of anaesthetic machines (including vaporizers, ventilators and pressure gauges) and their corresponding safety features, hazards and limitations of each piece of equipment, for the safe, clinical competence and practical application for day to day practice.

Anesthetic monitoring

Patients being treated under general anesthetics must be monitored continuously to ensure the patient's safety. For minor surgery, this generally includes monitoring of heart rate (via ECG or pulse oximetry), oxygen saturation (via pulse oximetry), non-invasive blood pressure, inspired and expired gases (for oxygen, carbon dioxide, nitrous oxide, and volatile agents). For moderate to major surgery, monitoring may also include temperature, urine output, invasive blood measurements (arterial blood pressure, central venous pressure), pulmonary artery pressure and pulmonary artery occlusion pressure, cerebral activity (via EEG analysis), neuromuscular function (via peripheral nerve stimulation monitoring), and cardiac output. In addition, the operating room's environment must be monitored for temperature and humidity and for buildup of exhaled inhalational anesthetics which might impair the health of operating room personnel.

Anesthesia Record

The Anesthesia Record is the medical and legal documentation of events during an anesthetic.^[18] It reflects a detailed and continuous account of drugs, fluids, and blood products administered and procedures undertaken, and also includes the observation of cardiovascular responses, estimated blood loss, urinary body fluids and data from physiologic monitors (Anesthetic monitoring, see above) during the course of an anesthetic. The Anesthesia Record may be written manually on paper; however, the paper record is increasingly replaced by an electronic record as part of an Anesthesia Information Management System (AIMS).

Anesthesia Information Management System (AIMS)

An AIMS refers to any information system that is used as an automated electronic anesthesia record keeper (i.e., connection to patient physiologic monitors and/or the Anaesthetic machine) and which also may allow the collection and analysis of anesthesia-related perioperative patient data.

Anesthesia in popular culture

The 1958 film Corridors of Blood starring Boris Karloff depicts an 1840's surgeon who experiments with anesthetic gases in an effort to make surgery pain free.

A bestseller by Robin Cook "Harmful Intent" has an anesthesiologist as the main character, who uncovers a legal conspiracy aimed at his specialty.

In the 2005 song "Twisted Transistor", by nu metal group Korn, lead man Jonathan Davis says, "This won't hurt a bit... anesthetize this bitch".

Metallica has a song from their album Kill 'Em All called (Anesthesia) Pulling Teeth.

See also

• ASA score
• Patient safety
• Perioperative mortality
• Anaesthetic Technician
• Anesthesia awareness

References