Continuous femoral nerve blocks: Decreasing local anesthetic concentration to minimize quadriceps femoris weakness

Background: Whether decreasing the local anesthetic concentration during a continuous femoral nerve block results in less quadriceps weakness remains unknown. Methods: Preoperatively, bilateral femoral perineural catheters were inserted in subjects undergoing bilateral knee arthroplasty (n = 36) at a single clinical center. Postoperatively, right-sided catheters were randomly assigned to receive perineural ropivacaine of either 0.1% (basal 12 ml/h; bolus 4 ml) or 0.4% (basal 3 ml/h; bolus 1 ml), with the left catheter receiving the alternative concentration/rate in an observer-and subject-masked fashion. The primary endpoint was the maximum voluntary isometric contraction of the quadriceps femoris muscles the morning of post-operative day 2. Equivalence of treatments would be concluded if the 95% CI for the difference fell within the interval-20%-20%. Secondary endpoints included active knee extension, passive knee flexion, tolerance to cutaneous electrical current applied over the distal quadriceps tendon, dynamic pain scores, opioid requirements, and ropivacaine consumption. Results: Quadriceps maximum voluntary isometric contraction for limbs receiving 0.1% ropivacaine was a mean (SD) of 13 (8) N · m, versus 12 (8) N · m for limbs receiving 0.4% [intrasubject difference of 3 (40) percentage points; 95% CI-10-17; P = 0.63]. Because the 95% CI fell within pre-specified tolerances, we conclude that the effect of the two concentrations were equivalent. Similarly, there were no statistically significant differences in secondary endpoints. Conclusions: For continuous femoral nerve blocks, we found no evidence that local anesthetic concentration and volume influence block characteristics, suggesting that local anesthetic dose (mass) is the primary determinant of perineural infusion effects.