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The discovery and development of anesthetic agents has been one of the most significant contributions in the advancement of surgical technique. Before the use of anesthesia, surgery was used only as a last resort and was often performed with the patient conscious but physically restrained by several large assistants. During the past century, anesthetic drugs have allowed surgeons to perform surgery in a manner that is safer and much less traumatic to the patient and that permits lengthier and more sophisticated surgical procedures.

Anesthetics are categorized as general or local, depending on whether the patient remains conscious when the anesthetic is administered. General anesthetics are usually administered for more extensive surgical procedures. Local anesthetics are given when analgesia is needed in a relatively small, well-defined area, or when the patient needs to remain conscious during surgery. The use of general anesthesia and general anesthetic agents is presented in this chapter; local anesthetics are addressed in Chapter 12.

Most physical therapists and other rehabilitation specialists are usually not involved with patients who are under general anesthesia. However, knowledge of how these agents work will help the therapist understand some of the residual effects of anesthesia. These effects may directly influence any therapy sessions that take place during the first few days after the procedure.


General anesthesia is a reversible state of unconsciousness. During major surgery (e.g., laparotomy, thora-cotomy, joint replacement, amputation), the patient is unconscious throughout the procedure and upon awakening has no recollection of what occurred during the surgery.1,2 An ideal anesthetic agent must be able to produce each of the following conditions:

  • Rapid onset of anesthesia (loss of consciousness and sensation)

  • Skeletal muscle relaxation (this requirement is currently met with the aid of skeletal muscle blockers used in conjunction with the anesthetic [see “Neuromuscular Blockers,” below])

  • Inhibition of sensory and autonomic reflexes

  • Easy adjustment of the anesthetic dosage during the procedure

  • A minimum of toxic side effects (i.e., be relatively safe)

  • Rapid, uneventful recovery after administration is terminated

  • Amnesia (i.e., no recollection of what occurred during the surgery)

Current general anesthetics meet these criteria quite well, providing that the dose is high enough to produce an adequate level of anesthesia but not so high that problems occur. The relationship between dosage and level, or plane, of anesthesia is discussed in the next section


During general anesthesia induction, the patient goes through a series of stages as the anesthetic dosage and amount of anesthesia reaching the brain progressively increase. These four stages of anesthesia are commonly identified accordingly:1,3

  • Stage I: Analgesia. The patient begins to lose somatic sensation but is still conscious and somewhat aware of what is happening.

  • Stage II: Excitement (Delirium). The patient is unconscious ...

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