Predicted Propofol Effect-site Concentration for Induction and Emergence of Anesthesia During Early Pregnancy

BACKGROUND: Pregnancy is associated with decreased hypnotic requirement, allegedly related to progesterone. However, the effects of pregnancy and progesterone on propofol requirement have not been thoroughly investigated. We conducted this study to determine whether propofol dose and predicted effect-site concentration for loss of consciousness (LOC) during induction of anesthesia, and eye opening during emergence from anesthesia, are decreased during early pregnancy. We also investigated whether blood progesterone was correlated with propofol dose and effect-site concentration for LOC. METHODS: We studied 57 ASA I-II women patients undergoing elective termination of pregnancy and 55 control patients undergoing transvaginal oocyte puncture for in vitro fertilization. Anesthesia was induced by administration of a 1% propofol infusion at 200 mL/min. Propofol dose and calculated effect-site concentration (Schnider model) were recorded at the time of LOC during induction. We also calculated effect-site concentration at the time of eye opening upon emergence from anesthesia. Blood progesterone was measured after surgery. RESULTS: Mean (±1 sd) propofol dose at LOC was significantly reduced in the pregnant patients compared with the nonpregnant control patients (108.57 ± 20.04 vs 117.59 ± 17.98 mg, respectively; P = 0.014). Similarly, the calculated propofol effect-site concentration at LOC was significantly lower in the pregnant patients than the nonpregnant control patients (4.59 ± 0.72 vs 5.01 ± 0.64 μg/mL, respectively; P = 0.0014). There was no difference in the calculated effect-site concentration on eye opening upon emergence. No significant relationship was observed between blood progesterone and propofol dose or calculated propofol effect-site concentration at LOC. CONCLUSION: Propofol dose and predicted propofol effect-site concentration at LOC are decreased during early pregnancy. Progesterone does not explain this result.