A comparison of two different concentrations and infusion rates of ropivacaine in perineural infusion administered at the same total dose for analgesia after foot and ankle surgery: a randomized, double blinded, controlled study.

Background: Continuous popliteal nerve block is utilized for postoperative analgesia after foot and ankle surgery. Whether only the total dose of local anesthetic or the combination of concentration and volume determine the characteristics of a continuous popliteal nerve infusion remains currently unknown. We hypothesized a reduction of the incidence of insensate extremity in patients given ropivacaine 0.4% at 4 mL/h compared to patients given ropivacaine 0.2% at 8 mL/h.

Methods: Sixty-four patients scheduled for major foot and ankle surgery requiring a continuous popliteal catheter infusion for postoperative analgesia were studied. Thirty-three patients were randomized to receive a continuous popliteal nerve block with 0.2% (basal 8 mL/h) and thirty-one with 0.4% (basal 4 mL/h) ropivacaine, reaching the same total dose (16 mg/h). The primary outcome was the incidence of persistent sensory block in the distal sciatic nerve distributions in the postoperative period. Secondary outcomes were the incidence of motor block, NRS pain scores at rest in the postoperative period up to 48 hours after surgery, opioid use and related side effects, patients' satisfaction.

Results: The incidence of persistent sensory block was similar in patients given 0.2% and in patients receiving 0.4% ropivacaine. The incidence of motor block, postoperative pain scores at rest, use of oral opioids, side effects and patients' satisfaction with the quality of recovery were also similar in both groups.

Conclusions: Our results suggest that local anesthetic total dose is the primary determinant of continuous popliteal perineural infusion effects.