EFFECTS OF SUBARACHNOID LIDOCAINE, MEPERIDINE AND FENTANYL ON SOMATOSENSORY AND MOTOR EVOKED-RESPONSES IN AWAKE HUMANS

Although the effects of local anaesthetics (LA) on motor and sensory t ransmission in the spinal cord have been described, the effects of opi oids are controversial. Our aim was to evaluate the action of clinical ly relevant doses of subarachnoid (SA) meperidine (MP) and fentanyl (F N), on somatosensory (SSEP) and cortical motor evoked responses (CMER) in awake subjects. Thirty ASA I-II patients scheduled for infra umbil ical surgery received SA (N=10/group): 1 mg/kg lidocaine (LD), 1 mg/kg MP or 25 mu g FN. SSEP elicited by stimulation of the posterior tibia l nerve at the ankle, and cortical motor evoked response at rest (r-CM ER) and during facilitation (f-CMER) were obtained prior and 30 min af ter treatment. Conduction at the proximal segment of the motor nerve ( F-wave) was evaluated by stimulation of the posterior tibial nerve at the popliteal fossa. Motor/sensory block and side effects were clinica lly assessed. LD completely abolished SSEP and CMER. At the same dose, MP abolished SSEP in 40% of the patients, while r-CMER and f-CMER wer e absent in 70% and 30%, respectively; in addition, the F-wave was abs ent in 50% of the patients. Fentanyl induced small changes in the late ncies of SSEP and F-wave; however, a 28% decrease in the amplitude of the f-CMER (P<0.05) was observed. Pruritus was present in 60% of patie nts in the FN group (P<0.006). Our results show that while LD and MP b lock sensory and motor conduction at the spinal roots, FN seems to dec rease the excitability of the spinal interneurons in the corticospinal tract.