Spinal somatosensory evoked potentials after epidural isoproterenol in awake sheep

The use of 10-15 micrograms epinephrine as an epidural test-dose is controversial. Isoproterenol would be a better alternative. However, before 5 micrograms isoproterenol can be incorporated in an epidural test-dose, neurotoxicological studies have to be performed. The present study was designed to assess spinal somatosensory evoked potentials (spinal SSEP) before and after epidural isoproteronol. Spinal SSEPs were recorded before, 30 min after, and 72 hr after 50 micrograms isoproterenol were given epidurally (L3-4) to six chronically instrumented awake sheep. The spinal SSEPs after epidural (L3-4) administration of 15 ml lidocaine 2% were used to evaluate the model. The SSEPs were generated by transcutaneous stimulation of the sciatic nerve in the thigh. Spinal SSEPs were recorded directly from the spinal cord at vertebra T12 using a monopolar epidural electrode referenced to a subcutaneous needle electrode in the adjacent paraspinal area. Thirty minutes and 72 hr after epidural injection of 50 micrograms isoproterenol the latency and the amplitude of the SSEP waves were similar to baseline values. After lidocaine, no SSEPs could be generated in three sheep while in three sheep the latency of wave 2 (W2) was prolonged and the amplitude diminished. Administration of epidural isoproterenol did not affect spinal SSEPs in this study indicating an absence of neurotoxic side effects.