COMPOUND MOTOR ACTION-POTENTIAL RECORDING DISTINGUISHES DIFFERENTIAL ONSET OF MOTOR BLOCK OF THE OBTURATOR NERVE IN RESPONSE TO ETIDOCAINE OR BUPIVACAINE

The purpose of this investigation was to establish an objective (quant itative) method for determining onset time of motor block induced by d ifferent local anesthetics. Twenty-four consenting patients undergoing transurethral surgery during spinal anesthesia were randomized to rec eive direct obturator nerve block with 10 mL of plain bupivacaine 0.5% (n = 12) or 10 ml of plain etidocaine 1% (n = 12). Another 14 patient s (control group) received obturator nerve ''block'' with saline. Afte r identification of the obturator nerve, patients underwent testing of nerve conduction by recording compound motor action potentials (CMAPs ) of thigh adductor muscles in response to stimulation provided by a n erve stimulator at 0.2 to 0.5-mA currents. Testing ended when CMAP amp litudes had returned to their baseline values (control group) or when motor blockade was 90% complete (local anesthetic groups). In all 38 p atients, the amplitude of the thigh CMAPs decreased immediately after injection of saline or local anesthetic. While CMAP amplitudes in the control group returned to their initial (baseline) values after 3-6 mi n, the patients receiving etidocaine or bupivacaine achieved greater t han or equal to 90% motor blockade after 6 and 13 min, respectively. I n the present report, the time to greater than or equal to 90% block w as significantly faster in patients given etidocaine compared with tho se given bupivacaine. We conclude that electromyographic recording of CMAPs call be used to compare the ability of different local anestheti cs to induce motor block.