ALTERATIONS IN ANESTHETIC REQUIREMENT PERTINENT TO THEORIES OF NARCOSIS

Effects of Temperature

In mammals, the MAC decreases with decreasing body temperatures for all anesthetics, but the reduction (about 2% to 5%) per degree decrease in body temperature varies slightly from agent to agent.^[5] Although the gas-phase potencies of inhaled agents increase with decreasing temperature, there is an associated increase in anesthetic solubility with a decrease in temperature so that the aqueous-phase potencies remain relatively constant with changes in temperature ( Fig. 4-3 ).^[12] For in vitro experiments involving clinical

Figure 4-3 Plot of the halothane minimum alveolar concentration (MAC) in dogs versus temperature. When measured as a function of the gas-phase partial pressure, MAC decreases with decreasing temperature. When measured as a function of calculated aqueous concentrations, MAC remains relatively constant with changes in temperature from 28°C to 43°C. (From Franks NP, Lieb WR: Temperature dependence of the potency of volatile general anesthetics. Anesthesiology 84:716, 1996.)