Abstract: AIM: To study the effects of hyperventilation on propofol anesthesia by measuring Bispectral index (BIS).METHODS: Twenty patients (ASA Ⅰ-Ⅱ) undergoing/Lower limbs surgeries were enrolled in this study.Anesthesia was maintained with target controlled infusion (TCI) propofol and remifentanyl intravenously. The target concentration of propofol and remifentanyl were 2-5 μg/mL and 2-6 ng/mL, respectively.During the period of anesthesia, BIS values were recorded when the end-tidal pressure of CO2 (PetCO2) was kept to 35 mm/Hg, 30 mm Hg and 25 mm Hg respectively for 30min by changing the ventilation rate and the tidal volume.Also, the BIS values and the time required were recorded following bolus intravenous injection of propofol at 1 mg/kg at above three PetCO2 levels.RESULTS: The BIS values kept stable when ventilation was adjusted to PetCO2 at 35 mm/Hg, 30mm Hg and 25mm Hg for 30 min respectively.Propofol injected at 1 mg/kg intravenously made the BIS values a significant decrease at the three PetCO2 levels. However, the decreasing extent of the BIS and the time when BIS began reducing were different between groups.The BIS values showed a less decrease and slower reacting time at the deep hyperventilation with PetCO2 descending (P <0.05).CONCLUSION: Hyperventilation or low PaCO2 can affect the anesthesia effects of propofol and slower action time when propofol injected at bolus.This effect may be related to the physiological reaction of CBF to CO2.

Key words: propofol, remifentanyl, target controlled infusion, hyperventilation, bispectral index