Effects of epidural bupivacaine or mepivacaine on somatosensory evoked potentials and skin resistance responses.

OBJECTIVE: The main purpose of this study was to compare the depth of neural block in lumbar epidural analgesia using either 20 mg/ml mepivacaine or 5 mg/ml bupivacaine. METHODS: Sixteen healthy patients were randomly divided to receive blindly either 17.5 ml mepivacaine (median, 15-20) or 17.5 ml bupivacaine (median, 14-20) for lumbar epidural analgesia. Afferent block was assessed by somatosensory evoked potentials (SEPs) during electrical stimulation of cutaneous sensory nerves at the T10, L1, L4, and L5 dermatomes. Efferent block was assessed by skin resistance responses (SRRs) recorded from the hand (C6), T12-L1, and foot (L5). RESULTS: Upper level of analgesia (pin-prick, cold) was T4 (T1-11) in the mepivacaine group and T4-5 (T2-9) in the bupivacaine group. In the mepivacaine group, SEPs were abolished in seven of eight cases at the T10 stimulation level and in five of eight cases at the L1 level. In the bupivacaine group, SEPs were abolished in two of eight cases at the T10 level and in three of eight cases at the L1 level. At the L4 and L5 levels, SEPs were only slightly changed in both groups. In the mepivacaine group, SRRs were completely or almost completely blocked (0-35% of control values) in all of eight cases at both the T12-L1 level and in the foot. Corresponding values for bupivacaine were six of seven and five of eight, respectively. Skin resistance responses in the foot and hand were significantly (p less than 0.05) lower in cases with abolished SEPs at the T10 stimulation level compared with those cases with preserved (although reduced) SEPs. CONCLUSIONS: Twenty mg/ml mepivacaine produces a more complete neural block than 5 mg/ml bupivacaine. The responsiveness of the afferent and efferent limbs of the nervous system was blocked in a similar manner as was shown by depression of the SEPs and SRRs.