Development of an Adequate Propofol Sedation Model in Elderly Patients for Elective Surgery under Regional Anesthesia

Background: Propofol, an ultrashort-acting anesthetic agent, is being increasingly used for sedation during regional anesthesia. The goal of this study was to characterize the pharmacodynamic relation between the effect site concentration of propofol and the occurrence of loss of consciousness (LOC) and apnea in elderly patients undergoing regional anesthesia. Methods: Twenty patients aged 65 years or older presenting for elective surgery requiring regional anesthesia were enrolled. After performing spinal anesthesia, the target effect site concentration of propofol was set at 1.0 g/mL. Effect site concentration was increased by 0.2 or 0.3 g/mL until LOC and apnea were observed. LOC was determined by the loss of response to verbal command ("open your eyes") and apnea as the loss of spontaneous breathing for 20 seconds. The pharmacodynamic relation between effect site concentrations of propofol and LOC or apnea was characterized by population analysis using nonlinear mixed effects model. Approximate entropy (ApEn) as a surrogate measure of central nervous effect of propofol was calculated from raw electroencephalogram, retrospectively. The correlation between effect site concentration of propofol and ApEn was tested. Results: The estimates (standard error) of Ce 50 for LOC and apnea (the effect site concentration of propofol associated with 50% probability of LOC and apnea) were 1.74 (0.09) and 2.35 (0.11) g/mL, respectively. The Spearman correlation coefficient between effect site concentration of propofol and ApEn was -0.8164 (p<0.0001). Conclusion: These results provide an adequate guidance for sedation in elderly patients for elective surgery under regional anesthesia.