Epidural lidocaine decreases sevoflurane requirement for adequate depth ofanesthesia as measured by the Bispectral Index((R)) monitor

Background: Epidural anesthesia potentiates sedative drug effects and decre ases minimum alveolar concentration (MAC). The authors hypothesized that ep idural anesthesia also decreases the general anesthetic requirements for ad equate depth of anesthesia as measured by Bispectral Index (BIS). Methods: After premedication with 0.02 mg/kg midazolam and 1 mug/kg fentany l, 30 patients aged 20-65 yr were randomized in a double-blinded fashion to receive general anesthesia with either intravenous saline placebo or intra venous lidocaine control (1-mg/kg bolus dose; 25 mug . kg(-1) . min(-1)). A matched group was prospectively assigned o receive epidural Lidocaine (15 ml; 2%) with intravenous saline placebo. All patients received 4 mg/kg thio pental and 1 mg/kg rocuronium for tracheal intubation. After 10 min of a pr edetermined end-tidal sevoflurane concentration, BIS was measured. The ED50 of sevoflurane for each group was determined by up-down methodology based on BIS less than 50 (MAC(BIS50)). Plasma lidocaine concentrations were meas ured. Results: The MAC(BIS50) of sevoflurane (0.59% end tidal) was significantly decreased with lidocaine epidural anesthesia compared with general anesthes ia alone (0.92%) or with intravenous lidocaine (1 %; P < 0.0001). Plasma li docaine concentrations in the intravenous lidocaine group (1.9 <mu>g/ml) we re similar to those in the epidural lidocaine group (2.0 mug/ml). Conclusions: Epidural anesthesia reduced by 34% the sevoflurane required fo r adequate depth of anesthesia. This effect was not a result of systemic li docaine absorbtion, but may have been caused by deafferentation by epidural anesthesia or direct rostral spread of local anesthetic within the cerebro spinal fluid. Lower-than-expected concentrations of volatile agents may be sufficient during combined egidural-general anesthesia.