Postoperative analgesia after knee surgery: a comparison of three different concentrations of ropivacaine for continuous femoral nerve blockade.

Background: The most effective ropivacaine concentration for femoral infusion after total knee arthroplasty is currently ill defined. We designed the present study to compare ropivacaine in three different concentrations (0.1, 0.2, and 0.3%) to evaluate analgesic quality, when administered as a continuous infusion with frequent infusion adjustments in patients receiving a combined femoral and sciatic nerve block. Secondary aims were to evaluate side effects such as motor blockade, rehabilitation indices, and ropivacaine plasma concentrations.

Methods: One hundred twenty-two patients undergoing total knee arthroplasty under combined general and regional anesthesia received femoral infusions of ropivacaine 0.1, 0.2, or 0.3%. Infusions were started after initial loading doses of 30 mL ropivacaine 0.5% into the femoral catheter and a sciatic catheter and were targeted to dynamic pain scores of 40 mm. Pain and side effects were assessed 1 h after tracheal extubation and on the first, second, third, fourth, and fifth postoperative days. Ropivacaine plasma concentrations were measured 24, 48, and 72 h after the start and 24 h after termination of femoral infusions in patients receiving ropivacaine 0.2% or 0.3%.

Results: Ropivacaine 0.1% provided ineffective analgesia. Ropivacaine 0.2% and 0.3% provided equivalent analgesia. Maximum infusion rates were 15.39 and 13.77 mL/h for ropivacaine 0.2% and 0.3%, respectively. There were no significant differences in motor blockade, mobilization, or ropivacaine plasma concentrations, which remained below toxic levels throughout the study period.

Conclusion: Ropivacaine 0.2% and 0.3% were similar in terms of analgesic quality. Initial infusion rates should be adjusted to 15 mL/h to obtain effective analgesia.