Anesthesia
Anesthesia (American English), also anaesthesia (British English), is 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-, "without" and aesthetos, "perceptible, able to feel".
Types
There are several forms of anesthesia:- general anesthesia - with reversible loss of Consciousness
- Local anesthesia - with reversible loss of Sensation in a (small) part of the body by localized administration of anesthetic drugs at the affected site.
- regional anesthesia - with reversible loss of sensation and possibly movement in a region of the body by selective blockade of sections of the Spinal cord or nerves supplying the region.
Anaesthesiologists and the profession
Physicians specialising in the administration of anesthetics are known as anesthesiologists (AE) or anaesthetists (BE). Nurses specialising in the administration of anesthetics are known as nurse anesthetists (AE), who typically have gone to graduate school after nursing school, or have at least obtained certification as a Crna (Certified Registered Nurse Anesthetist). "Anesthetist", despite typically (in the USA) referring to nurses, can refer to a physician or a nurse. Anesthesiologist Assistants are another group of health care providers who administer anesthetics. They pursue a graduate degree in anesthesia from an accredited program and are supervised directly by an anesthesiologist.In the United Kingdom, specially trained anaesthetic personnel known as ODPs (operating department practitioner, health care workers without prior medical training) 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
History
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 with Ice can achieve local effects, while Hyperventilation can provide general effects (see Lamase).Herbal derivatives
The first herbal anaesthesia was administered in Prehistory. Opium and Hemp 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
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, sulfuric 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 the 16th of October 1846, another dentist, William Thomas Green Morton, invited to the Massachusetts General Hospital, performed the first public demonstration of 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 anęsthesia.
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 had 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.
The surgical amphitheater at Massachusetts General Hospital, or "etherdome" 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.
Anaesthetic equipment and physics
In modern anaesthesia, a wide variety of medical equipment is desirable depending on the necessity for portable field use, surgical operations or intensive care support. An anaesthesiologist has to have 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.Anaesthetic agents
Local anaesthetics
The first effective local anaesthetic was Cocaine. Anaestheics were used and are still used, in modern times to relax muscels and to help uneasy patients to sleep. Isolated in 1859 it was first used by Karl Koller, at the suggestion of Sigmund Freud, in ophthalmic surgery in 1884. Prior to 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 unconciousness. 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, levobupivicaine, ropivicaine, dibucaine) are generally heat stable with a long shelf life of 2 years, with a slower onset (longer half life) and present in a Racemic mixture. It is these type of local anaesthetic agents that are 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 epidural or dural space)
Early opioids and hypnotics
Opioids were first used by Racoviceanu-Pitesti, who reported his work in 1901.Current pharmacological agents
- Thiopental (first used in 1934)
- Intravenous (Benzodiazepine)
- Propofol (2,6-di-isopropyl-phenol)
- Etomidate (an imidazole derivative)
- Ketamine (a phencyclidine derivative, as is 'Angel Dust/Special K')
- Curare (1942) (A non-depolarizing neuromuscular blocking agent or paralyzing drug)
- Fentanyl (d 1960 Paul Janssen) fentanyl citrate
- Halothane (d 1951 Charles W. Suckling, 1956 James Raventos
- Succinylcholine (A depolarizing neuromuscular blocking agent or paralyzing drug)
- Enflurane (d 1963 u 1972), Isoflurane (d 1965 u 1971), Desflurane, Sevoflurane
- New synthetic opioids - meperidine, Alfentanil, Sufentanil (1981), Remifentanil
- Neurosteroids
Volatile agents
These are specially formulated gaseous vapors for the use of induction or maintenance of general anaesthesia. The ideal anesthetic vapor or gas should be non-flammable; non-explosive; non-lipid soluble; have no end organ (heart, liver, kidney) side effects; not be metabolized and be easy and comfortable to deliver to the patient. No anesthetic gas 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 succesful drugs in use. Ether is still used in poorer countries as it is safe, particularly when administered by untrained personel, it also very cheap. In theory any anesthetic vapor can be used for induction of general anesthesia, however some of the vapors are very irritating to the airway. All of the modern vapors can be used alone or in combination with other medications to maintain anesthesia. Currently research into the use of Xenon as an anesthetic gas is being pursued but it is very expensive and may require special equipment for delivery and recovery to be used.Volatile agents are compared in terms of potency, which is inversely proportional to the MAC :
>> For more detailed information: Minimum alveolar concentration
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. Neuraxial blockade may reduce the risk of Deep vein thrombosis, Pulmonary embolism, transfusion, Pneumonia, respiratory depression, Myocardial infarction and Renal failure.