General Anesthetics

Description: Nitrous oxide (dinitrogen monoxide) is a colorless, odorless, tasteless, nonflammable, nonirritating, inorganic gas used for general anesthesia. Nitrous oxide is a powerful analgesic, but it is a relatively weak inhalational anesthetic and usually must be supplemented by other agents if general anesthesia is desired. It is used in low doses to provide analgesia in obstetrics and during procedures that do not require unconsciousness.

Mechanism of Action: The exact mechanism by which inhalational anesthetics function is not known. There appears to be a correlation between anesthetic potency and lipid solubility (Meyer-Overton theory), suggesting that these anesthetics likely affect the lipid matrix of nerve cell membranes in the brain. Furthermore, NMR and electron spin resonance studies indicate that anesthetics cause a local disordering of the lipid membrane matrix, possibly decreasing the number of molecules that alternate simultaneously between the gel and crystalline states and thereby altering membrane function.

Nitrous oxide has no muscle relaxation properties and causes surgical anesthesia only when administered under hyperbaric conditions. At a concentration of 20% (in oxygen), analgesia equivalent to that of morphine is produced. For unconsciousness, 80% nitrous oxide is needed for most patients, but this carries the prohibitive risk of inducing hypoxemia. When used as an adjunct to anesthesia, nitrous oxide significantly reduces the MAC (defined as the minimal alveolar concentration that prevents movements in 50% of patients subjected to a painful stimulus) for halogenated anesthetics.

Pharmacokinetics: Nitrous oxide is readily absorbed into the blood via the pulmonary capillary system. It has a relatively low solubility in blood, with a blood-to-gas partition coefficient (37 degrees C) of 0.47, an oil-to-gas partition coefficient (37 degrees C) of 1.4, and a MAC of 100%. Nitrous oxide is rapidly eliminated in the expired breath essentially unchanged, with minimal diffusion through the skin.