ELECTROMYOGRAPHIC COMPARISON OF OBTURATOR NERVE BLOCK TO 3-IN-ONE BLOCK

Obturator nerve block during spinal, epidural, or general anesthesia w ithout muscle relaxants has been recommended for transurethral surgery to prevent thigh adductor muscle contractions during operative electr ocautery. We investigated the effectiveness of direct obturator and 3- in-1 nerve motor blocks in 44 patients undergoing transurethral surger y during spinal anesthesia with isobaric bupivacaine. Patients were ra ndomly assigned to receive 3-in-1 block with 40 mL (n = 13) or 50 mL ( n = 11) of 1.5% lidocaine plus epinephrine, or direct obturator nerve block with 10 mL of 2% lidocaine plus epinephrine (n = 20). After both direct obturator and 3-in-1 blocks, compound muscle action potential (CMAP) testing of the obturator nerve was performed at 1-10-s interval s for 10 min. In patients given direct obturator nerve block (n = 20), CMAP amplitude decreased by 88.8 +/- 21% (mean +/- SD) from baseline. In contrast, 3-in-1 block reduced the evoked CMAP amplitude by 7.4 +/ - 19% (P < 0.05). Peak lidocaine plasma levels of 1.6 +/- 0.2 mu g/mL (range 1.0-2.8 mu g/mL) were reached 60-90 min after the block in thos e patients receiving 50 mL of local anesthetic. The 3-in-1 technique f ails to predictably result in effective motor block of the obturator n erve and thus may not prevent inadvertent thigh adductor muscle contra ctions during transurethral surgery. A direct approach to the obturato r nerve is significantly more effective in producing motor block, and even when given in larger than recommended dosages it results in subto xic peak plasma lidocaine concentrations.