Isoflurane requirements during combined general/epidural anesthesia for major abdominal surgery

Abstract

We evaluated the effects of bupivacaine on the requirements for thiopental and isoflurane during combined general/epidural anesthesia. Sixty patients scheduled for colon resection were randomly distributed into six groups that received, before the induction of anesthesia, an epidural (T9-10) bolus (8 mL) followed by an infusion (8 mL/h) of saline (Groups 1 and 4), bupivacaine 0.0625% plus fentanyl 2 microg/mL (Groups 2 and 5), or bupivacaine 0.125% plus fentanyl 2 microg/mL (Groups 3 and 6). We evaluated the amount of thiopental needed to abolish the eyelid reflex and the percentage of isoflurane required to maintain the bispectral index (BIS) between 50 and 60 (Groups 1-3) or the mean arterial blood pressure (MAP) within 20% of basal values (Groups 4-6). All groups required similar doses of thiopental (5 mg/kg); the time of evaluation, but not epidural treatment, had a significant effect (P < 0.0001) on BIS and MAP. After tracheal intubation, MAP and BIS increased by 18% and 49%, respectively (P < 0.05). In the bupivacaine groups, isoflurane requirements similarly decreased by 35% (P < 0.03). For BIS and MAP, the epidural treatment (P < 0.02) and type of evaluation (P < 0.03) had a significant effect; MAP was lower (P < 0.05) with 0.125% bupivacaine. We conclude that epidural bupivacaine does not alter the thiopental dose, but it decreases isoflurane requirements by 35%. This study demonstrates that both doses of bupivacaine and fentanyl induce similar isoflurane-sparing effects. However, patients receiving 0.125% bupivacaine showed lower values of MAP when compared with controls, and thus bupivacaine 0.0625% should be favored during combined anesthesia.

Implications: In patients undergoing colon resection under combined anesthesia, isoflurane requirements were assessed by changes in blood pressure or bispectral index. Epidural bupivacaine at concentrations of 0.125% or 0.0625% (each with 2 mg/mL of fentanyl) induced the same sparing of isoflurane (35%). The smaller dose produced less hypotension and should be favored.