Maldistribution of local anesthetic administered through a subarachnoid catheter recently has been implicated as a possible cause of sacral root injury. To examine subarachnoid distribution of catheter-injected local anesthetic, we constructed a model of the subarachnoid space and administered solutions containing lidocaine and methylene blue through sacrally directed catheters. We studied three catheters: a 28-G endport, a 20-G endport, and a 20-G multiple sideport. To determine the injection rates to be used, ten clinicians were observed while they performed mock subarachnoid injections: the mean (+/- standard deviation) "normal" injection times for the 28-G and 20-G catheters were 52.6 +/- 17.2 and 11.9 +/- 7.2 s, respectively. The correlation coefficient for lidocaine concentration estimated by methylene blue spectrophotometric absorbance and measured by immunoassay was 0.977. Administration of hyperbaric local anesthetic through a sacrally directed catheter resulted in restricted distribution of anesthetic with a relatively high peak concentration. Rate of injection was a critical factor affecting distribution; faster injections tended to distribute solution more uniformly and to a higher segmental level, resulting in substantially lower peak concentrations. When catheters were injected at clinically relevant rates, the 28-G catheter produced the greatest degree of maldistribution; this difference appeared to be primarily a function of flow rate. Differences in peak lidocaine concentration between the two 20-G catheters were neither larger nor consistent. However, despite sacral placement, the multiple-sideport catheter distributed anesthetic toward "higher" spinal segments more consistently. Distribution was more favorable when the injected solution was less dense (closer to isobaric). We conclude that administration of hyperbaric local anesthetic through a sacrally directed catheter results in a restricted distribution and a high peak local anesthetic concentration. Several factors can affect distribution, including catheter size, tip configuration, tip position, injection rate, and baricity of local anesthetic solution.
