Effect of thoracic epidural bupivacaine 0.75% on somatosensory evoked potentials after dermatomal stimulation

The effect of thoracic epidural bupivacaine 0.75%, 9 ml, on early (less than 500 ms) somatosensory evoked potentials (SEPs) during electrical stimulation of the T10 and L1 dermatomes was examined in ten patients. Spread of analgesia (pinprick) was T3.7 +/- 0.5 to L2.5 +/- 1 (mean +/- SEM). Peak-to-peak amplitudes of the T10 SEPs were reduced (p less than 0.05) while an insignificant reduction was found in L1 stimulated potentials. SEPs were abolished in four and two patients at the T10 and L1 levels, respectively. The latency of the early SEP components (0, P1-3, N1-3) increased (p less than 0.05) at T10 but was not significant at L1 except for N2 and N3. Sensory threshold increased significantly in both stimulation areas during blockade. In conclusion, thoracic epidural administration of 9 ml of bupivacaine 0.75% does not provide total afferent somatic blockade despite sensory analgesia to pinprick.