Background and Objectives. Cauda equina syndrome has been reported recently in patients receiving continuous spinal anesthesia using newly developed microcatheters (28 gauge). Failure of microcatheters to allow adequate mixing of local anesthetic was studied as a possible mechanism of the neurologic injury reported with these catheters.

Methods. A spinal canal model was developed and the distribution of hyperbaric lidocaine was measured after injection through catheters typically used for continuous spinal anesthesia (i.e., 20, 28, and 32 gauge).

Results. Lidocaine distribution was less uniform and lidocaine concentration and osmolarity in the dependent portions of the model were significantly higher after injection through microcatheters compared to the 20-gauge catheter.

Conclusions. Dependent drug concentrations were greater than those reported in the literature, capable of producing permanent neurologic injury. Directing the catheter tip in the nondependent direction and injecting lidocaine rapidly or through catheters with multiple end holes improved mixing and decreased dependent drug concentration.