No differences between 20, 30, or 40 mL ropivacaine 0.5% in continuous lateral popliteal sciatic-nerve block

The aim of this study was to compare the anesthetic and analgesic characteristics of the administration of an initial bolus of 20, 30, or 40 mL of ropivacaine 0.5% through a lateral popliteal sciatic catheter.

Sixty patients scheduled for foot surgery under continuous sciatic lateral popliteal nerve block were included in this double-blinded study. Patients were randomly assigned to 1 of 3 groups: 20 mL, 30 mL, or 40 mL of ropivacaine 0.5%. Time to obtain a complete sensory block and duration of analgesia were recorded. Failure of the continuous block and complications such as technical or neurologic problems were noted.

Data are presented as mean +/- SD, median and ranges, or percent of patients and were compared by application of analysis of variance or Kruskal-Wallis when required. Time to complete sensory block (31 [10-70] v 25 [5-50] v 25 [5-40] min), percentage of failure (5% v 5% v 0%), and duration of action (995 [278-1,800] v 967 [420-2,175] v 915 [190-1,900] min) were not statistically different between 20 mL, 30 mL, and 40 mL ropivacaine 0.5% groups, respectively. No complications were noted.

This dose-response study shows that 20 mL of ropivacaine 0.5% injected through an indwelling lateral popliteal catheter is the optimal dose for foot surgery. Higher doses of 30 mL or 40 mL did not provide any further advantage.