anesthesia

Anesthesia, or anaesthesia (see spelling differences; from Greek αν-, an-, "without"; and αἲσθησις, aisthēsis, "sensation"), has traditionally meant the condition of having sensation (including the feeling of pain) blocked or temporarily taken away. 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.^[1] Another definition is a "reversible lack of awareness", whether this is a total lack of awareness (e.g. a general anaesthetic) or a lack of awareness of a part of the body such as a spinal anaesthetic or another nerve block would cause. Anesthesia is a pharmacologically induced reversible state of amnesia, analgesia, loss of consciousness, loss of skeletal muscle reflexes and decreased stress response.

Today, the term general anesthesia in its most general form can include:^[2]

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

Patients undergoing anesthesia 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 can often neither maintain their own airway nor breathe on their own. While usually administered with inhalational agents, general anesthesia can be achieved with intravenous agents, such as propofol.^[3]
• 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.^[3]
• 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.^[3]
• 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.^[3]

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 the 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 anesthesia (a herbal remedy) was administered in prehistory. Opium poppy capsules were collected in 4200 BC, 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 BC. By 1100 BC poppies were scored for opium collection in Cyprus by methods similar to those used in the present day, and simple apparatus for smoking of opium were found in a Minoan temple. Opium was not introduced to India and China until 330 BC and 600–1200 AD respectively, 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. In 13th century Italy Theodoric Borgognoni used similar mixtures along with opiates to induce unconsciousness, 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 had 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 famous 10th century Persian work, the Shahnameh, the author, Ferdowsi, describes a caesarean section performed on Rudabeh when giving birth, in which a special wine agent was prepared as an anesthetic^[4] by a Zoroastrian priest in Persia, 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.

The use of herbal anaesthesia had a crucial drawback compared to modern practice—as lamented by Fallopius, "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 19th 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^{[citation needed]}, and a medicine containing willow leaves (salicylate, the predecessor of aspirin) would then be applied directly to the source of inflammation^{[citation needed]}.

In 1804, the Japanese surgeon Seishū Hanaoka 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 named Kan Aiya.^[5] He used a compound he called Tsusensan, based on the plants Datura metel, Aconitum and others.

Non-pharmacological methods

Hypnotism have a long history of use as anesthetic techniques. 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

The works of Greek authors such as Dioscorides were well-known among physicians in the Islamic Empire, and Arab and Persian physicians such as Muhammad ibn Zakarīya Rāzi (Rhazes), Avicenna (Ibn Sina) and Abu al-Qasim al-Zahrawi wrote medical textbooks of great importance in the development of medicine in Europe and the Middle East. Arabic and Iranian anesthesiologists were the first to utilize oral as well as inhalant anesthetics. In Islamic Spain, Abulcasis and Ibn Zuhr (Avenzoar), among other Muslim surgeons, performed hundreds of surgeries under inhalant anesthesia with the use of narcotic-soaked sponges. Abulcasis and Avicenna wrote about anesthesia in their influential medical encyclopedias, the Al-Tasrif and The Canon of Medicine.^[6][7] These were the precursors to the true narcotic derivatives, now known as general anesthesia or general anesthetics, which were not produced until Dr. Janssen developed narcotics, except morphine, in the past 50 years.

Contemporary re-enactment of Morton's October 16, 1846, ether operation; daguerrotype by Southworth & Hawes.

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 anesthetic qualities of nitrous oxide (discovered in 1769 by Joseph Priestley^[8]) were discovered by the British chemist Humphry Davy in 1799^[8] 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 of Connecticut, a traveling 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 colorful 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 sulfuric 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 his friend, who was also a school teacher (James M. Venable) before excising a cyst from his neck. Long got the idea to do this from his observations at ether frolics. He noted that participants experienced bumps and bruises but afterward had no recall of what had happened. He did not publicize this information until 1849.

On October 16, 1846, 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 tumor from his neck. In a letter to Morton shortly thereafter, Oliver Wendell Holmes, Sr. proposed naming the procedure anæsthesia.

Anesthesia pioneer Crawford W. Long

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 traveled 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-anesthetist. On the same day, 19 December 1846 in Dumfries Royal Infirmary, Scotland, a Dr. Scott used ether for a surgical procedure. The first use of anesthesia 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 anesthesia 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 anesthetics at this time). This led to many deaths from the use of chloroform that (with hindsight) might have been preventable. The first fatality directly attributed to chloroform anesthesia (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 anesthesia.

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.^[8] 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.^[9] 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 or CRNAs, and about 10% are provided by CRNAs in solo practice.^[10][11][12] -^[13] -^[14]

Anesthesiologists/Anaesthetists (medically-trained physicians)

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

In the US, 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.^[15]

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, Fellowship of the Royal College of Anaesthetists (FRCA), is conferred upon medical doctors following completion of the written and oral parts of the Royal College's examination. 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 (or of the American Osteopathic Board of Anesthesiology, for osteopathic physicians).

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 perform well as peri-operative physicians, and will involve themselves in optimizing the patient's health before surgery (colloquially called "work-up"), performing the anaesthetic,including specialized intraoperative monitoring (like^[16] transesophageal echocardiography), 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 Canada 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 39,000 CRNAs in the US administer approximately 30 million anesthetics each year, roughly two thirds of the US total.^[17] 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,^[18] and gain a masters degree in nurse anesthesia before passing the mandatory Certification Exam. Masters-level CRNA training programs range in length from 27 to 36 months.

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. CRNAs do not require Anesthesiologist supervision in any state and require surgeon/dentist/podiatrists to sign and approve the chart for medicare billing in all but 16 states. Many states place restrictions on practice, and hospitals often regulate what CRNAs and other midlevel providers can or can not do based on local laws, provider training and experience, and hospital and physician preferences.^[19]

Anaesthesia Assistants

In the US, anesthesiologist assistants (AAs) are graduate-level trained specialists who have undertaken specialized education and training to provide anesthesia care under the direction of an Anesthesiologist. AAs typically hold a masters degree and practice under Anesthesiologist supervision in 18 states through licensing, certification or physician delegation.^[20]

In the UK, a similar group of assistants are currently being evaluated. They are named Physician's Assistant (Anaesthesia) (PAAs). Their background can be nursing, Operating Department Practice or another profession allied to medicine or a science graduate. Training is in the form of a post-graduate diploma and takes 27 months to complete. Once finished, a masters degree can be undertaken. Anesthesiologist Assistant

Anesthesia technicians

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

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

Operating Department Practitioners

In the United Kingdom, personnel known as ODPs (Operating Department Practitioners) provide close assistance and support to the anaesthetist (anaesthesiologist). They can also assist with Surgical procedures alongside the Surgeon and provide Post-Operative Care to patients emerging from Anaesthesia. ODPs can be found in the Operating Department, Accident and Emergency (providing advanced airway assistance), Intensive Care Unit, High Dependency Unit and for specialist MRI scanners which require Anaesthetic cover. They also work with organ retrieval teams in transplant surgery and attend pre hospital care to injury victims in the community and will undertake advanced specialist training to carry out this work. They are state registered in the UK and their title, Operating Department Practitioner is a protected title. The ODP is not a technician but a practitioner of peri-opertive care. ODPs also work in the field of teaching as lecturers, resuscitation trainers and work in senior positions in management of operating theatre departments.

Veterinary anesthetists/anesthesiologists

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

It has been suggested that this section be split into a new article. (Discuss)

Local anesthetics

• procaine
• amethocaine
• cocaine
• lidocaine (also known as Lignocaine)
• prilocaine
• bupivacaine
• levobupivacaine
• ropivacaine
• mepivacaine
• 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 (e.g., procaine, amethocaine, cocaine) are generally unstable in solution and fast-acting, and allergic reactions are common.

Amide local anesthetics (e.g., lidocaine, prilocaine, bupivicaine, levobupivacaine, ropivacaine, mepivacaine 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. Peripheral nerve blocks are when a nerve block is a shot of anesthetic near a specific nerve or group of nerves. It blocks pain in the part of the body supplied by the nerve. Nerve blocks are most often used for procedures on the hands, arms, feet, legs, or face. Epidural and spinal anesthesia is a shot of anesthetic near the spinal cord and the nerves that connect to it. It blocks pain from an entire region of the body, such as the belly, hips, or legs.

Current inhaled general anesthetic agents

• Desflurane
• Enflurane
• Halothane
• Isoflurane
• Nitrous oxide
• Sevoflurane
• 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, and 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.^[21] Partly because of its side effects, enflurane never gained widespread popularity.^[21]

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^{[citation needed]}. 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).

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 solubility 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
  • Methohexital
  • Thiopental (Previously known as Thiopentone in the UK)
• Benzodiazepines
  • Diazepam
  • Lorazepam
  • Midazolam
• Etomidate
• Ketamine
• Propofol

The two barbiturates mentioned above, thiopental and methohexital, are ultra-short-acting, and are used to induce and maintain anesthesia.^[2] However, though they produce unconsciousness, they provide no analgesia (pain relief) and must be used with other agents.^[2] Benzodiazepines can be used for sedation before or after surgery and can be used to induce and maintain general anesthesia.^[2] When benzodiazepines are used to induce general anesthesia, midazolam is preferred.^[2] Benzodiazepines are also used for sedation during procedures that do not require general anesthesia.^[2] Like barbiturates, benzodiazepines have no pain-relieving properties.^[2] Propofol is one of the most commonly used intravenous drugs employed to induce and maintain general anesthesia.^[2] It can also be used for sedation during procedures or in the ICU.^[2] Like the other agents mentioned above, it renders patients unconscious without producing pain relief.^[2] Because of its favourable physiological effects, "etomidate has been primarily used in sick patients".^[2] 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, and illusions."^[22] However, like etomidate it is frequently used in emergency settings and with sick patients because it produces fewer adverse physiological effects.^[2] Unlike the intravenous anesthetic drugs previously mentioned, ketamine produces profound pain relief, even in doses lower than those which induce general anesthesia.^[2] 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.^[2]

Current intravenous opioid analgesic agents

While opioids can produce unconsciousness, they do so unreliably and with significant side effects.^[23][24] 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.^[2] 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:

• Alfentanil
• Fentanyl
• Remifentanil
• Sufentanil (Not available in the UK)

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

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

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, "Celokurin" or "celo" for short in Europe)
• Non-depolarizing muscle relaxants
  • Short acting
    • Mivacurium
    • Rapacuronium
  • Intermediate acting
    • Atracurium
    • Cisatracurium
    • Rocuronium
    • Vecuronium
  • Long acting
    • Alcuronium
    • Doxacurium
    • Gallamine
    • Metocurine
    • Pancuronium
    • Pipecuronium
    • d-Tubocurarine

Adverse effects of muscle relaxants

• Depolarising Muscle Relaxants i.e. Suxamethonium
  • Hyperkalaemia - A small rise of 0.5 mmol/l occurs normally, this is of little consequence unless Potassium is already raised such as in Renal Failure
  • Hyperkalaemia - Exaggerated potassium release in burn patients (occurs from 24 hours after injury, lasting for up to 2 years), neuromuscular disease and paralyzed (quadraplegic, paraplegic) patients. The mechanism is reported to be through upregulation of acetylcholine receptors in those patient populations with increased efflux of potassium from inside muscle cells. May cause life threatening arrhymias
  • Muscle aches, commoner in young muscular patients who mobilise soon after surgery
  • Bradycardia, especially if repeat doses are given
  • Malignant hyperthermia, a potentially life threatening condition in susceptible patients
  • Suxamethonium Apnoea, a rare genetic condition leading to prolonged duration of neuromuscular blockade, this can range from 20 minutes to a number of hours. Not dangerous as long as it is recognised and the patient remains intubated and sedated, there is the potential for awareness if this does not occur.
  • Anaphylaxis
• Non-depolarising Muscle Relaxants
  • Histamine release e.g. Atracurium & Mivacurium
  • Anaphylaxis

Another potentially disturbing complication where neuromuscular blockade is employed is 'anesthesia awareness'. In this situation, patients paralyzed may awaken during their anesthesia, due to an inappropriate decrease in the level 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

• Flumazenil, reverses the effects of benzodiazepines
• Naloxone, reverses the effects of opioids
• Neostigmine, helps reverses the effects of non-depolarizing muscle relaxants
• Sugammadex, new agent that is designed to bind Rocuronium therefore terminating its action

Anesthetic 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. In the UK the Association of Anaesthetists (AAGBI) have set minimum monitoring guidelines for General and Regional Anaesthesia. 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.^[25] 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.

• The 1978 film Coma depicts a series of events at the Boston Memorial Hospital in which patients are intentionally put into a comatose state via anesthesia and their organs later sold.

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

• Awake, a 2007 suspense thriller, is about anaesthesia awareness^{[see below]}.

See also

• Geriatric anesthesia
• Anaesthesia awareness
• Anaesthetic Technician
• Allergic reactions during anaesthesia
• ASA score
• Sedation
• EEG measures during anesthesia
• Patient safety
• Perioperative mortality

Notes

External links

• International Anesthesia Research Society
• OpenAnesthesia - the Anesthesiology Wiki (presented by the IARS)
• International Federation of Nurse Anesthetists