Background. The precise sites of action of local anesthetics on the spinal cord remain speculative. Monitoring spinal cord conduction during spinal anesthesia may provide a more precise localization of anesthetic action.

Methods. Fifteen patients admitted for elective surgical implantation of spinal cord stimulators for treatment of chronic pain participated in the study. Lidocaine (mean dose, 90 mg) was used in 14 patients; one patient received 7.5 mg tetracaine. To assess spinal cord conduction, paresthesias elicited by spinal cord stimulation were monitored during onset and maintenance of spinal anesthesia. In eight patients, evoked potential monitoring was used to further assess cord conduction.

Results. Despite complete motor block of the lower extremities and sensory levels to upper thoracic dermatomes (T1-8), all patients felt trial stimulator-induced paresthesias during spinal anesthesia. In addition, cortical evoked potentials from cord stimulation were maintained during spinal anesthesia. However, attenuation of cortical evoked potential amplitudes and paresthesias occurred, which were restored by increasing stimulus intensity. In contrast, even at maximum stimulus intensity, paresthesias and somatosensory evoked potentials from tibial nerve stimulation were abolished during spinal anesthesia.

Conclusion. Based on these observations, the predominant site of action of subarachnoid lidocaine and tetracaine appeared to be at spinal rootlets, although partial block of afferent cord conduction also occurred.