Anaesthesia, loss of sensation, especially the sensation of touch. It can be general (affecting the entire body and usually accompanied by loss of consciousness) or local (affecting limited areas of the body). The condition may be the result of damage to nerves or nerve centres (seeNervous System) by disease or injury, or it may be intentionally induced by the administration of drugs for the prevention or relief of pain.
HISTORY OF ANAESTHETICS
Of the few anaesthetic agents known to the ancients, opium and hemp were the most important. Both were taken by ingestion or by burning the drug and inhaling the smoke. Nitrous oxide, discovered by Sir Humphry Davy in about 1800, was first used as an anaesthetic in 1844 by an American dentist, Horace Wells. In 1842 an American surgeon, Crawford Long, successfully used ethyl ether as a general anaesthetic during surgery. He failed to publish his findings, however, and credit for the discovery of the anaesthetic properties of ether was given to William Morton, who in the United States in 1846 publicly demonstrated its use during a tooth extraction. In 1847 in Britain James Simpson discovered the anaesthetic properties of chloroform.
Many other general anaesthetics have since been discovered. Ether and chloroform have been largely abandoned because of their dangerous side effects and flammability. Some anaesthetics act by depressing the central nervous system (barbiturates, halothane), whereas others induce amnesia and dissociation (nitrous oxide, enflurane).
Surgical anaesthesia (complete general anaesthesia, characterized by muscular paralysis sufficient to permit surgical manipulation) is produced by inhalation anaesthetics: gases or volatile liquids such as cyclopropane, nitrous oxide, halothane, and enflurane. The anaesthetic, usually mixed with oxygen, is either inhaled or administered into the windpipe through a tube.
Modern anaesthesia almost always involves a combination of agents. Before administering the inhalation anaesthetic, the anaesthetist might give intravenously a short-acting barbiturate such as pentobarbital or sodium pentothal (more properly called thiopental sodium or thiopentone), or an anti-anxiety drug such as diazepam, to induce unconsciousness. An opiate analgesic such as meperidine or fentanyl may be used in addition.
To allow use of smaller amounts of the inhalation anaesthetic, special muscle-paralysing drugs are given. These include tubocurarine, gallamine, and succinylcholine. The combination of an opiate, a barbiturate, a muscle-paralysing drug, and nitrous oxide is called balanced anaesthesia. Because muscular activity is prevented in all these procedures, the anaesthetist must induce breathing in the patient mechanically.
Surgical anaesthesia must continue throughout the operation, but prolonged anaesthetization can kill, paralysing first the respiratory system and then the heart. The correct level is maintained by constant monitoring of the patient's condition and increasing or decreasing the dosage as needed.
Thiopental sodium in small doses is sometimes used in psychiatry because it allows patients to talk uninhibitedly. This quality has also given the drug some value in law enforcement as a "truth serum".
Some surgical procedures do not require total muscular relaxation and can be performed using local anaesthetics, which temporarily block nerve conduction without damaging nerve fibres. Local anaesthesia is produced by injecting into the tissues to be affected a solution of a natural alkaloid such as cocaine, the oldest of all local anaesthetics, or a synthetic agent such as lignocaine, or lidocaine (Xylocaine).
Block anaesthesia, a much more extensive local anaesthesia, is produced by injecting the agent into a nerve trunk, next to a nerve, or all around the operative field, thereby deadening the entire area. The best-known block anaesthesia is probably the spinal block, produced by injecting an anaesthetic into the spinal canal. A variant of this type of block is the epidural where local anaesthetic is injected to anaesthetize the nerves after they have exited the spinal canal, so that nerves below the waist are deadened; that is, the drug is not injected into the spinal fluid. Epidurals are commonly used in pregnancy or for chronic, intractable pain, such as backache.
A mild local anaesthesia, useful in many minor dental and medical procedures, can be produced by numbing the tissues with cold, either by applying ice or by spraying with a volatile liquid such as ethyl chloride.