Propofol Concentrations--An Alternative Explanation

Abstract

In Response: We thank Dr. Eger for his comments. He proposed another explanation for our finding about concentration differences between plasma and whole blood samples [1]. His suggestion, delay in propofol release from the lipid vesicle used as the vehicle, is a rational explanation for the results observed from constant infusion of propofol. However, such a mechanism cannot explain the results after a bolus injection of propofol. We extracted all propofol by CHCl3 from the plasma samples, regardless of the bound of vehicle lipid [2]. The apparent plasma or blood concentration composes total (released and vehicle entrapped) propofol. If the delay in propofol release from vehicle lipid is true, the immediate separation of plasma would result in higher propofol concentrations than those from storage in plasma. Propofol in its vehicle prevents release to red blood cells (RBCs) when the lipid is cleared from the plasma. However, our results show the opposite. The plasma concentration is higher in the sample with storage for 1 h in the group receiving a bolus injection. From the rapid distribution phase of propofol, the effect of delay in propofol release from the lipid vesicles, used as vehicle, on the distribution of propofol seems negligible. The high lipid solubility of propofol and its distribution in RBCs must be localized mainly in the RBC membrane [3]. Perhaps it would be more appropriate to change the wording from "across" blood cell membrane to "from" blood cell membrane. Shou-Zen Fan, MD, PhD Hsiu-Ying Yu, PhD Yung-Liang Chen, PhD Chien-Chiang Lui, MD, PhD Department of Anesthesiology National Taiwan University Hospital Taipei, Taiwan 10016, Republic of China