Differential Neural Effects of Epidural Anesthetics

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Abstract
The purpose of this investigation was to clarify in a laboratory model the sites of action of epidurally administered local anesthetics. This report describes such a model, which is capable of monitoring sites of altered electrophysiologic activity induced by epidural anesthetic agents. Evoked potential response alterations measured from electrodes positioned along the conducing pathways were assessed in 6 monkeys [Macaca arctoides] following epidural injections of 0.5% bupivacaine, 3% chloroprocaine and 1% etidocaine. Bupivacaine in 10 studies caused its major effects at the dorsal root entry zone and the long tracts of the spinal cord white matter, associated with variable peripheral nerve alterations. Chloroprocaine effects in 6 studies were limited to alteration of responses recorded from the dorsal root entry zone and peripheral nerve. Etidocaine in 8 studies caused marked attenuation of the responses recorded from the long tracts of the spinal cord white matter, associated with only minimal corresponding changes at the dorsal entry zone or peripheral nerve levels. The capability of this experimental model to demonstrate relatively selective effects along the sensory and motor pathways for epidurally injected local anesthetic agents is illustrated.