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Chapter 25: General Anesthetics

A new halogenated gas anesthetic has a blood:gas partition coefficient of 0.5 and a MAC value of 1%. Which prediction about this agent is most accurate? (Refer to Table 25–1 for comparison of agents.)

(A) Equilibrium between arterial and venous gas tension will be achieved very slowly

(B) It will be metabolized by the liver to release fluoride ions

(C) It will be more soluble in the blood than isoflurane

(D) Speed of onset will be similar to that of nitrous oxide

(E) The new agent will be more potent than halothane

The partition coefficient of an inhaled anesthetic is a determinant of its kinetic characteristics. Agents with low blood:gas solubility have a fast onset of action and a short duration of recovery. The new agent described here resembles nitrous oxide but is more potent, as indicated by its low MAC value. Not all halogenated anesthetics undergo significant hepatic metabolism or release fluoride ions. The answer is D.

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TABLE 25–1: Properties of inhalation anesthetics.
Anesthetic Blood:Gas Partition Coefficient Minimum Alveolar Concentration (%)a Metabolism
Nitrous oxide 0.47 >100 None
Desflurane 0.42 6.5 <0.1%
Sevoflurane 0.69 2.0 2–5% (fluoride)
Isoflurane 1.40 1.4 <2%
Enflurane 1.80 1.7 8%
Halothane 2.30 0.75 >40%
Methoxyflurane 12 0.16 >70% (fluoride)

a Minimum alveolar concentration (MAC) is the anesthetic concentration that eliminates the response in 50% of patients exposed to a standardized painful stimulus. In this table, MAC is expressed as a percentage of the inspired gas mixture.

Modified and reproduced, with permission, from Katzung BG, editor: Basic & Clinical Pharmacology, 10th ed. McGraw-Hill, 2007.

Which statement concerning the effects of anesthetic agents is false?

(A) Bronchiolar smooth muscle relaxation occurs during halothane anesthesia

(B) Chest muscle rigidity often follows the administration of fentanyl

(C) Mild, generalized muscle twitching occurs at high doses of enflurane

(D) Severe hepatitis has been reported after the use of methoxyflurane

(E) The use of midazolam with inhalation anesthetics may prolong postanesthesia recovery

Hepatitis after general anesthesia has been linked to use of halothane, although the incidence is very low (1 in 20,00–35,000). Hepatotoxicity has not been reported after administration of methoxyflurane or other inhaled anesthetics. However, fluoride release from prolonged use of methoxyflurane has caused renal insufficiency. The answer is D.

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