Changing Concepts of Anesthesia Depth

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GUEDAL’S1 description of the planes of surgical anesthesia with ether has led to the wide use of his chart in interpreting the depth of surgical anesthesia. Surgical anesthesia is, in these terms, characterized by unconsciousness, insensitivity to pain, moderate muscular relaxation and impairment of certain reflexes; indeed, loss of the eyelid, the swallowing and the vomiting reflexes is indicative of passage from the second to the third stage.2 As new anesthetic agents have come into use, notably thiopental sodium, it has been determined that it is both dangerbus and impossible to define surgical anesthesia in general in terms of the stages practical with ether.3 Such an interpretation however is still implicit in most publications, so that many clinicians believe that an adequate anesthetic level is not obtained until the patient is totally unconscious and unresponsive to external stimuli. Such levels of “surgical anesthesia” may be dangerous; they are sometimes associated with serious cardiac arrhythmias, and circulatory and respiratory depressions.

The gas-oxygen-ether (GOE) of yesterday has largely given way to an anesthesia accomplished by the judicious combination of several agents, each of which performs its special function. It is therefore necessary to examine and to re-evaluate the planes of anesthesia and to identify their components.

The benefits of light anesthesia were described in 1914 by Crile4 when he outlined his theory of anoci-association. If local anesthetic is used to block all painful somatic stimuli originating at the site of the surgical incision, and light cortical suppression is accomplished with another anesthetic . . .



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