Background and Objectives Ultrasound guidance reduces the required local anesthetic volume for successful peripheral nerve block, but it is unclear whether this influences block duration. We investigated the ropivacaine volume and concentration effect on interscalene block duration.

Methods One hundred eighty-five patients were randomized to 5 ropivacaine volume/concentration combinations (0.75% 5, 10, and 20 mL; 0.375% 20 and 40 mL) administered preoperatively via an interscalene catheter before shoulder surgery under general anesthesia. An elastomeric ropivacaine infusion commenced at the onset of pain. Patients were questioned at 24 hours primarily for the primary outcome: time to first pain. Group 5 mL was excluded post hoc because of an unacceptably high block failure rate. Multivariate Cox regression was used to assess the effect of volume and concentration (each corrected for the other) on the primary outcome.

Results Probability of pain as a function of time was associated with not only dose, but also volume corrected for concentration and concentration corrected for volume: hazard ratio (95% confidence interval) for dose = 0.992 (0.987–0.997) (P = 0.002), volume = 0.959 (0.937–0.982) (P = 0.001), concentration = 0.852 (0.743–0.976) (P = 0.021). Increasing the volume of ropivacaine 0.375% from 10 to 40 mL was estimated to increase median (quartiles) block duration from 10.0 (9.5–11.5) to 15.0 (10.75–21) hours. Similarly, increasing the concentration of 20 mL ropivacaine from 0.375% to 0.75% was estimated to increase median (quartiles) block duration from 10.75 (9.75–14.0) to 13.75 (10.5–21.0) hours.

Conclusions Block duration is influenced by both local anesthetic volume and concentration, a finding of increasing relevance with the current trend to lower volumes for ultrasound-guided regional anesthesia.