Volatile Anesthetics

Volatile anesthetics induce unconsciousness and loss of reflexes for surgical procedures. This CNS depression can be induced by a wide variety of different chemicals; those used in clinical medicine are selected for reasons that include low toxicity, ease of maintaining and adjusting a given depth of anesthesia, and freedom from adverse effects upon recovery. Many compounds were examined in the search for modern anesthetic agents.

The depth of anesthesia depends on how much of the medication is present in the CNS. This in turn depends on how much is in the air, to what extent the anesthetic passes between air and blood, and how much passes from blood to brain (or fat, since the brain is largely fat). An agent that is highly insoluble in blood achieves a plateau, or saturation, concentration very rapidly; an example is nitrous oxide. More soluble agents take a longer time to come to plateau, and take a longer time to be exhaled as well, so recovery from them takes longer. Nitrous oxide and cyclopropane have the same solubility in blood, and take the same amount of time to come to a steady concentration in blood; cyclopropane is more soluble in brain and fat, however, so it takes a much lower concentration to achieve the same effect. (Cycloproane is explosive, and therefore is not used in the operating room.) The way an anesthetic functions in a given individual depends on a number of variables, including the amount of fat in the individual's body, the volume of air inspired per minute, the amount of blood pumped through the lungs per minute, and various preexisting medical conditions.

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