The General Anesthetic Potency of Propofol and Its Dependence on Hydrostatic Pressure


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Abstract

Although plasma concentrations of propofol during anesthesia are well known, the free concentration remains unknown because of uncertainties regarding plasma protein binding, interaction with other protein-bound substances, the level of binding to its lipid carrier, and the use of adjuvants. At elevated surrounding pressure, all general anesthetics require higher concentrations to reach adequate levels of anesthesia. To determine the anesthetic potency of propofol at equilibrium conditions and to study the effects of pressure on propofol-induced anesthesia, Rana pipiens tadpoles were exposed to different concentrations of pure, not emulsified, propofol in aqueous solution. Anesthesia was defined as loss of the righting reflex. Ten animals per concentration were used, and each experiment was conducted twice. Pressure experiments were performed with nonanesthetized tadpoles and urethane-anesthetized tadpoles as control groups. Propofol concentrations were measured spectrophotometrically. At 1 atmosphere absolute (atm abs), a semilogarithmic sigmoidal concentration–response curve was obtained with a half-maximal effect of propofol at 2.2 ± 0.22 µM (EC50; mean ± SE). Increased pressure shifted the concentration–response curve to the right. The EC50 increased linearly with increasing pressure up to 121 atm abs (EC50 at 121 atm abs = 4.1 ± 0.41µ M). For pressure greater than 121 atm abs, an increased excitability of the tadpoles made it difficult to distinguish the righting reflex from involuntary movements. The saturated solubility of propofol in aqueous solution was found to be 1.0 ± 0.02 mM (mean ± SD), and the octanol/water partition coefficient was 4,300 ± 280. Propofol adhered to the correlation between anesthetic potency and octanol/water partition coefficient exhibited by other general anesthetics. Thus, it behaves like a typical general anesthetic in that it obeys the Meyer-Overton rule and its effects are reversed by increased pressure.

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