EFFECTS OF SUBARACHNOID BUPIVACAINE BLOCK ON ANESTHETIC REQUIREMENTS FOR THIOPENTAL IN RATS

Background: Subarachnoid bupivacaine blockade has been reported to red uce thiopental and midazolam hypnotic requirements in patients. The pu rpose of this study was to examine if local anesthetically induced lum bar intrathecal blockade would reduce thiopental requirements for bloc kade of motor responses to noxious and nonnoxious stimuli in rats. Met hods: After intrathecal and external jugular catheter placement, rats were assigned randomly to two groups in a crossover design study, with each rat to receive either 10 mu l of 0.75% bupivacaine or 10 mu l of normal saline intrathecally. The doses of intravenously administered thiopental required to ablate the eyelid reflex, to block the withdraw al reflex of a front Limb digit, and to block the corneal reflex were compared. In two separate groups of animals, hemodynamic parameters an d concentrations of thiopental in the brain were compared between intr athecally administered bupivacaine and saline. Results: The thiopental dose required to block the described responses was decreased with int rathecally administered bupivacaine versus intrathecally administered saline from (mean +/- SD) so +/- 5 to 24 +/- 4 mg/kg (P < 0.001) for t he eyelid reflex, from 51 +/- 6 to 29 +/- 6 mg/kg (P < 0.005) for fron t limb withdrawal and from 67 +/- 8 to 46 +/- 8 mg/kg (P < 0.01) for t he corneal reflex. The concentration of thiopental in the brain at the time of corneal reflex blockade for the group given bupivacaine was s ignificantly lower than in the group given saline (24.1 vs. 35.8 mu g/ g, P = 0.02). Conclusion: This study demonstrates that lumbar intrathe cally administered local anesthetic blockade decreases anesthetic requ irements for thiopental for a spectrum of end points tested. This effe ct Is due neither to altered pharmacokinetics nor to a direct action o f the local anesthetic on the brain; rather, it is most likely due to decreased afferent input.