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General anesthesia is a state characterized by unconsciousness, analgesia, amnesia, skeletal muscle relaxation, and loss of reflexes. Drugs used as general anesthetics are CNS depressants with actions that can be induced and terminated more rapidly than those of conventional sedative-hypnotics.

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Balanced anesthesiaAnesthesia produced by a mixture of drugs, often including both inhaled and intravenous agents
Inhalation anesthesiaAnesthesia induced by inhalation of drug
Minimum alveolar anesthetic concentration (MAC)The alveolar concentration of an anesthetic that is required to prevent a response to a standardized painful stimulus in 50% of patients
AnalgesiaA state of decreased awareness of pain, sometimes with amnesia
General anesthesiaA state of unconsciousness, analgesia, and amnesia, with skeletal muscle relaxation and loss of reflexes
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Modern anesthetics act very rapidly and achieve deep anesthesia quickly. With older and more slowly acting anesthetics, the progressively greater depth of central depression associated with increasing dose or time of exposure is traditionally described as stages of anesthesia.

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Stage 1: Analgesia

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In stage 1, the patient has decreased awareness of pain, sometimes with amnesia. Consciousness may be impaired but is not lost.

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Stage 2: Disinhibition

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In stage 2, the patient appears to be delirious and excited. Amnesia occurs, reflexes are enhanced, and respiration is typically irregular; retching and incontinence may occur.

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Stage 3: Surgical Anesthesia

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In stage 3, the patient is unconscious and has no pain reflexes; respiration is very regular, and blood pressure is maintained.

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Stage 4: Medullary Depression

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In stage 4, the patient develops severe respiratory and cardiovascular depression that requires mechanical and pharmacologic support.

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Anesthesia protocols vary according to the proposed type of diagnostic, therapeutic, or surgical intervention. For minor procedures, conscious sedation techniques that combine intravenous agents with local anesthetics (see Chapter 26) are often used. These can provide profound analgesia, with retention of the patient's ability to maintain a patent airway and respond to verbal commands. For more extensive surgical procedures, anesthesia protocols commonly include intravenous drugs to induce the anesthetic state, inhaled anesthetics (with or without intravenous agents) to maintain an anesthetic state, and neuromuscular blocking agents to effect muscle relaxation (see Chapter 27). Vital sign monitoring remains the standard method of assessing "depth of anesthesia" during surgery. Cerebral monitoring, automated techniques based on quantification of anesthetic effects on the electroencephalograph (EEG), is also useful.

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The mechanisms of action of general anesthetics are varied. As CNS depressants, these drugs usually increase the threshold for firing of CNS neurons. The potency of inhaled anesthetics is roughly proportional to their lipid solubility. Mechanisms of action include effects on ion channels by interactions of anesthetic drugs with membrane lipids or proteins with subsequent effects on central neurotransmitter mechanisms. Inhaled ...

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