ănĭsthēˈzhə [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.

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).

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