[Decrease in serum propofol concentrations after acute autologous blood letting]

A change in serum propofol concentrations associated with acute autologous blood letting during anesthesia was investigated in seven scheduled surgical patients. Anesthesia was induced with propofol 2 mg.kg-1 and maintained with infusion of propofol 6 mg.kg-1.hr-1 at a constant rate. After achieving a stable anesthesia, about 10 g.kg-1 of autologous blood was withdrawn in about 15 minutes and 20 ml.kg-1 of acetated Ringer's solution was infused to manage the hypotension caused by withdrawal. A blood sample each 4 ml was taken before and 0, 5, 15, 30 minutes after blood withdrawing. Another 7 patients were anesthetized with the same procedure without blood letting to distinguishing the effect of blood letting from rapid infusion therapy of crystalloid. Assay of serum concentration of propofol was performed with HPLC-spectrofluorometry. Concentrations of propofol were significantly decreased from 2.8 micrograms.ml-1 to 2.3 micrograms.ml-1 just after blood letting, and remained at 2.3 micrograms.ml-1 after 30 minutes from letting. Rapid infusion therapy also decreased the concentrations of propofol from 2.4 micrograms.ml-1 to 1.7 micrograms.ml-1. Continuous infusions of propofol may become a major method of general anesthesia with target controlled infusion techniques (TCI) in clinical settings for the accuracy and reliability of prediction of blood concentrations. However, this study demonstrated unexpected decreases of concentration of propofol during acute autologous blood letting similar to surgical mass bleeding, which might be mainly caused by rapid infusion therapy. The rate of infusion of anesthetic should be readjusted to counteract the effect of acute blood loss or volume replacement.