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

Abstract

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 postoperative 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 prespecified 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.

Figure 1

Consort Flowchart.

Figure 2

Effects of continuous femoral nerve block ropivacaine concentration on quadriceps femoris strength following bilateral tricompartment knee arthroplasty. Muscle strength was evaluated using a dynamometer to measure maximum voluntary isometric contractions (MVIC). Data are expressed as mean (horizontal bar) with 95^th confidence interval of the mean (whiskers) for limbs randomly assigned to receive ropivacaine 0.1% (basal 12 mL/h, 4 mL bolus) or ropivacaine 0.4% (basal 3 mL/h, 1 mL bolus). There were no statistically significant differences between treatments. Furthermore, because the 95% confidence interval for the primary endpoint (intra-subject differences the morning of postoperative day 2) fell within prespecified tolerances, we found that the effect of the two concentrations on quadriceps MVIC were equivalent.