For several centuries, opiates and alcohol were the mainstays of anesthetics (substances used to reduce sensation of pain) in the control of pain. Th ese substances had limited success, but were probably better than nothing. It was not until the 1840s that surgical anesthesia (reduction or elimination of pain) became possible, with the introduction of three agents: chloroform, ether, and nitrous oxide. Th ese three substances, upon inhalation, quickly lead to a state of unconsciousness in which pain is not felt. Nitrous oxide is still one of the most widely used gaseous anesthetics, and diethyl ether is still occasionally used. Chloroform is rarely used today because of its toxicity, but other, newer halogenated hydrocarbons, such as halothane, are extremely common.

Gaseous anesthetics are the principal agents used in the maintenance of anesthesia, but agents given by other routes are still used in the induction of anesthesia. Anesthesia is basically characterized by four reversible actions: unconsciousness, analgesia, immobility, and amnesia. Th e critical factor is that there should be no signifi cant impairment of cardiovascular or respiratory functions, especially those supplying the brain and other vital organs with adequate blood, nutrients, and gases.

General anesthetics are used to produce loss of consciousness before and during surgery. Local anesthetics numb small areas of the body tissue where a minor procedure is to be done, and are commonly used in dentistry for minor surgery. Regional anesthesia aff ects a larger (but still limited) part of the body, but does not make the person unconscious. Spinal and epidural anesthesia are examples of regional anesthesia.