General anesthesia is used for operative procedures requiring unconscious sedation and muscle relaxation involving the torso and head or in those procedures to the perineum and extremities not amenable to regional anesthesia. There are four phases of general anesthesia including:
Induction is produced initially by administering an agent to cause sedation such as thiopental, midazolam, or propofol until loss of consciousness occurs noted by the loss of a blink reflex.
Induction also occurs with inhalational agents, often through combinations of nitrous oxide with halothane or enflourane. Loss of consciousness with these agents is noted by a loss of nystagmus and divergent gaze, midsized pupils, and a regular deep breathing pattern. Once unconscious, the patient is given a dose of muscle relaxant and intubated.
Maintenance of the unconscious state can be performed through the use of inhalational agents or with intravenous agents. During procedures on the head or extremities the patient is often allowed spontaneous ventilation with the use of a mixture of nitrous oxide, oxygen and inhalational agent to maintain unconsciousness. The neuromuscular blocker used for maintenance of anesthesia is determined by the length of relaxation required for the procedure.
Emergence from anesthesia in a patient who is breathing spontaneously occurs as soon as the inhalational agent is discontinued. Emergence occurs when muscle relaxation reverses through the self limiting effects of depolarizing agents or the reversal of nondepolarizing agents with the administration of an anticholinesterase.
Recovery from anesthesia involves adequate analgesia, the residual sedation from anesthetic agents, and control of postoperative nausea and vomiting.
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