Anesthesia



Anesthesia or anaesthesia (see spelling differences), has traditionally meant the process of blocking the perception of pain and other sensations. This allows patients to undergo surgery and other procedures without the distress and pain they would otherwise experience. It comes from the Greek roots an-, "not, without" and aesthētos, "perceptible, able to feel". The word was coined by Oliver Wendell Holmes, Sr. in 1846.

Today, the term  general anesthesia in its most general form has 5 components: 1. Analgesia - blocking the conscious perception of pain 2. Hypnosis - producing unconsciousness 3. Amnesia - preventing memory formation 4. Relaxation - preventing unwanted movement or muscle tone 5. Homeostasis - preserving normal body functioning  (e.g., maintaining blood pressure within normal physiological range)

Types
There are several forms of anaesthesia:
 * general anaesthesia &mdash; with reversible loss of consciousness and memory of unpleasant events
 * local anaesthesia &mdash; with reversible loss of sensation in a part of the body by localised administration of anaesthetic drugs at the affected site.
 * regional anaesthesia &mdash; with reversible loss of sensation and possibly movement in a region of the body by selective blockage of sections of the spinal cord or nerves supplying the region.

The administration of drugs to make a patient more comfortable or less anxious, but without inducing anaesthesia, is called sedation.

History
Anesthesia was used as early back as the classical age. Dioscorides, for example, reports potions being prepared from opium and mandragora as surgical anesthetics. The biblical Book of Genesis (2:20) describes God causing Adam to fall into a deep sleep, during which one of his ribs was removed and fashioned into Eve.

In the East, in the 10th century work Shahnama, 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.

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

In modern anaesthetic practice, these techniques are seldom employed.

Herbal derivatives
The first herbal anaesthesia was administered in prehistory. Opium and Cannabis were two of the most important herbs used. They were ingested or burned and the smoke inhaled. Alcohol was also used, its vasodilatory properties being unknown. In early America preparations from datura, effectively scopolamine, were used as was coca. In Medieval Europe various preparations of mandrake were tried as was henbane (hyoscyamine).

Early gases and vapours
In 1804, the Japanese surgeon Hanaoka Seishū performed the world's first general anaesthesia for the operation of a breast cancer (mastectomy), by combining Chinese herbal medicine know-how and Western surgery techniques learnt through "Rangaku", or "Dutch studies", 40 years before the better-known Western innovations of Long, Wells and Morton. He used a compound he called Tsusensan, based on the plants Datura metel and Aconitum and others.

In the West, the development of effective anaesthetics 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 by Joseph Priestley) were discovered by the British chemist Humphry Davy about 1795 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 in December 1844 for painless tooth extraction by American dentist Horace Wells. Demonstrating it the following year, at Massachusetts General Hospital, he made a mistake and the patient suffered considerable pain. This lost Wells any support.

Another dentist, William E. Clarke, performed an extraction in January 1842 using a different chemical, diethyl ether (discovered in 1540). In March 1842 in Danielsville, Georgia, Dr. Crawford Williamson Long was the first to use anaesthesia during an operation, giving it to a boy 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 undergoing an excision of a 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.

Ether has a number of drawbacks, like its tendency to induce vomiting and its flammability. In England it was quickly replaced with chloroform. Discovered in 1831, its use in anaesthesia is usually linked to James Young Simpson, who, in a wide-ranging study of organic compounds, found chloroform's efficacy in 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 surgical amphitheater 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.

Anesthetists, Anesthesiologists and the profession
Physicians specialising in the administration of anesthetics are known as anaesthetists or, in the U.S., anesthesiologists. In the U.S., nurses specialising in the administration of anesthetics are known as nurse anesthetists. Consequently, in the U.S. "Anesthetist" typically refers to nurses, but can refer to a physician or a nurse. Anesthesiologist Assistants are another group of health care providers who administer anesthetics, however since there are only two programs training Anesthesiology Assistents ( Case Western Reserve University School of Medicine and Emery University) their numbers are limited.

In the United Kingdom, specially trained anaesthetic personnel known as  ODPs (operating department practitioner, specialised practitioners within the operating department area) or Anaesthetic nurses (nurses with prior nursing training choosing to specialize in anaesthetics) provide crucial support and aid in the administration, safety and running of the anaesthetic list. At present, all anaesthetics in the UK are administered by doctors.

As with other specialties within medicine, practitioners wishing to specialise in anaesthesia must undertake extensive training. The length of this training varies by country, but is typically several years. In the U.S., the training of a physician anesthesiologist typically consists of 4 years of college, 4 years of medical school, 1 year of internship, and 3 years of residency. The training of a nurse anesthetist in the U.S. typically consists of 4 years of college, 1 year of acute care work experience (e.g., ICU), and then 2-3 years additional training. Anesthesiogist Assistants also obtain graduate education in anesthesia administration. In the UK this training lasts a minimum of seven years after the awarding of a medical degree, 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, 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.

These colleges typically set rigorous examinations, which must be passed before training is complete. These examinations encompass the whole field of anaesthetic practice, and are usually split into several parts. 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 anesthesiolgist allows one to be called "Board Certified". Passing a written licensing exam is mandatory for CRNAs.

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 optimising 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.

Anaesthetic equipment and physics
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.

Local anaesthetics
The first effective local anaesthetic 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. Cocaine soon produced a number of derivatives and safer replacements, including procaine (1905), Eucaine (1900), Stovaine (1904), and lidocaine (1943).

Local anaesthetics 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).

Classification: Local anaesthetics can be either ester or amide based.

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

- Amide local anaesthetics (eg. lidocaine, prilocaine, bupivicaine, levobupivacaine, ropivacaine, dibucaine) are generally heat stable with a long shelf life of 2 years, with a slower onset (longer half life) and are usually a racemic mixture (with the exceptions being levobupivacaine which is S(-)-bupivacaine, and ropivacaine, which is actually S(-)-ropivacaine). It is this type of local anaesthetic agent that is generally used within regional and epidural/spinal techniques namely due to their longer duration of action providing adequate analgesia suitable for surgery, labour and symptomatic relief.

NB: Only local anaesthetic agents that are preservative free may be injected intrathecally (i.e within the subarachnoid space).

Adverse Effects Of Local Anaesthesia
Local anesthetic drugs are toxic to the heart (where they cause arrhythmia) and brain (where they 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, metallic taste in mouth, and nausea that can quickly progress to seizure and cardiovascular collapse.

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

Toxicity can occur with any local anesthetic.

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

Current inhaled general anesthetic agents

 * Nitrous Oxide
 * Halothane
 * Enflurane
 * Isoflurane
 * Sevoflurane
 * Desflurane
 * Xenon (rarely used)

Current IV general or sedative agents

 * Thiopental
 * Methohexital
 * Propofol
 * Etomidate
 * Ketamine
 * Diazepam
 * Midazolam

Muscle relaxants

 * Succinylcholine (also known as suxamethonium in the UK, New Zealand, Australia and other countries)
 * Vecuronium
 * Rocuronium
 * Pancuronium
 * Pipecuronium
 * Rapacuronium
 * Mivacurium
 * Atracurium
 * Cisatracurium
 * Curare, the active ingredient of which is tubocurarine
 * Metocurine
 * Gallamine

Adverse effects of muscle relaxants
If insufficient anaesthetic is administered, a patient can regain consciousness during the operation. Although muscle relaxants are essential for many types of surgery, their use can (very rarely) lead to this occurence being missed by the anaesthetist. This can be very traumatic to the patient.

Opioid analgesics

 * Morphine
 * Diamorphine, (diacetyl morphine, also known as heroin)
 * Codeine, (methyl morphine)
 * Fentanyl
 * Alfentanil
 * Sufentanil
 * Remifentanil
 * Meperidine, also called pethidine in the UK, New Zealand, Australia and other countries
 * Methadone
 * Oxycodone


 * Naloxone, although chemically similar to some analgesics, is not a painkiller and reverses the effects of morphine-like agents.

Volatile agents
These are specially formulated organic liquids, which evaporate readily into vapors, which are given by inhalation for induction and/or maintenance of general anaesthesia. The ideal anesthetic vapor or gas should be non-flammable; non-explosive; lipid soluble; possess low blood gas solubility; have no end organ (heart, liver, kidney) side effects; not be metabolized and be non-irritant when breathed by patients.

No anesthetic vapour currently in use meets all of these requirements. The vapors in current use are halothane, isoflurane, desflurane and sevoflurane. Nitrous oxide is still in widespread use, making it one of the most long lived and successful drugs in use. Ether is still used in poorer countries as it is cheap to manufacture and safe, particularly when administered by untrained personnel.

In theory, any anesthetic vapor can be used for induction of general anesthesia. However, most of the vapors are irritating to the airway, resulting in coughing, laryngospasm and overall difficult inductions. Commonly used agents for inhalational induction include sevoflurane and halothane. All of the modern vapors 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 gas is being pursued but it is very expensive to produce, and requires special equipment for delivery, monitoring and scavenging of unused 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 their versatility for a faster emergence from anesthesia, thanks to their lower blood:gas partition coefficient.

Choice of anesthetic technique
The choice of anesthetic technique is a complex one, requiring consideration of both patient and surgical factors.

In certain patient populations, however, regional anesthesia may be safer than general anesthesia, but there is no conclusive scientific evidence favoring one technique over the other. Neuraxial blockade may reduce the risk of deep vein thrombosis, pulmonary embolism, blood transfusion, pneumonia, respiratory depression, myocardial infarction and renal failure.