Altered neural conduction with epidural bupivacaine.

The sites and magnitude of evoked potential response alterations induced by varying masses and concentrations of epidurally administered bupivacaine were assessed from electrodes positioned along the conducting pathways of the monkey. The mass of bupivacaine was the major factor in determining the level and degree of response alterations. At the lower levels of total drug mass, effects were limited to the dorsal root entry zone, whereas higher levels of mass not only increased the response attenuation at the gray matter level but resulted in additional changes in those responses recorded from the spinal cord white matter tracts. With all other factors stable, increasing concentration was associated with a greater degree of response attenuation, especially at the lower levels of total mass. These findings indicate that the mass of the drug is the major factor in determining the magnitude and level of bupivacaine-induced epidural analgesia. Increased concentration influences the local anesthetic's penetration at the dorsal root entry zone and, to a lesser degree, at the white tracts of the spinal cord.