A randomized, triple-masked, active-controlled investigation of the relative effects of dose, concentration, and infusion rate for continuous popliteal-sciatic nerve blocks in volunteers

Abstract

Background: It remains unknown whether local anaesthetic dose is the only factor influencing continuous popliteal-sciatic nerve block effects, or whether concentration, volume, or both exert an influence as well.

Methods: Bilateral sciatic catheters were inserted in volunteers (n=24). Catheters were randomly assigned to ropivacaine of either 0.1% (8 ml h(-1)) or 0.4% (2 ml h(-1)) for 6 h. The primary endpoint was the tolerance to transcutaneous electrical stimulation within the tibial nerve distribution at hour 6. Secondary endpoints included current tolerance at other time points and plantar flexion maximum voluntary isometric contraction (22 h total).

Results: At hour 6, tolerance to cutaneous stimulation for limbs receiving 0.1% ropivacaine was [mean (standard deviation)] 27.0 (20.2) vs26.9 (20.4) mA for limbs receiving 0.4% [estimated mean difference 0.2 mA; 90% confidence interval (CI) -8.2 to 8.5; P=0.02 and 0.03 for lower and upper boundaries, respectively]. Because the 90% CI fell within the prespecified tolerance ±10 mA, we conclude that the effect of the two concentration/volume combinations were equivalent. Similar negative findings were found for the secondary outcomes.

Conclusions: For continuous popliteal-sciatic nerve blocks, we found no evidence that local anaesthetic concentration and volume influence block characteristics, suggesting that local anaesthetic dose (mass) is the primary determinant of perineural infusion effects in this anatomic location. These findings suggest that for ambulatory perineural local anaesthetic infusion-for which there is usually a finite local anaesthetic reservoir-decreasing the basal rate while increasing the local anaesthetic concentration may allow for increased infusion duration without compromising postoperative analgesia.

Clinical trial registration: NCT01898689.

Keywords: continuous peripheral nerve block; perineural infusion; perineural local anaesthetic infusion.

© The Author 2014. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Figures

Fig 1

Consolidated Standards of Reporting Trials (CONSORT) flow diagram.

Fig 2

Effects of continuous popliteal-sciatic nerve block ropivacaine concentration/volume combination on tolerance to cutaneous electrical current within the sciatic nerve distribution. Data are expressed as mean (solid circle) with standard error (whiskers) for limbs randomly assigned to receive ropivacaine 0.1% (basal 8 ml h−1=8 mg h−1) or 0.4% (basal 2 ml h−1=8 mg h−1). When assessed at individual time points, equivalence was concluded at all time points using raw P-values, and at most time points when adjusting for multiple comparisons (Table 2). Since the time-by-treatment interaction was not statistically significant (P>0.99), equivalence in the treatment effect was assessed marginally by collapsing over time: the mean difference (0.1−0.4%) was 0.2 (90% CI: −2.3 to 2.8) mA, which was well contained within the a priori equivalence region of −10 to 10 mA (P<0.001).

Fig 3

Effects of continuous popliteal-sciatic nerve block ropivacaine concentration/volume combination on MVIC during plantar flexion. Data are expressed as mean (solid circle) with standard error (whiskers) for limbs randomly assigned to receive ropivacaine 0.1% (basal 8 ml h−1=8 mg h−1) or 0.4% (basal 2 ml h−1=8 mg h−1). When assessed at individual time points, equivalence was concluded at most time points using raw P-values, but at only a few time points when adjusting for multiple comparisons (Table 3). Since the time-by-treatment interaction was not statistically significant (P=0.98), equivalence in the treatment effect was assessed marginally by collapsing over time: the mean difference (0.1−0.4%) was 0.7 (90% CI: −4.1 to 5.6)%, which was well contained within the a priori equivalence region of −20 to 20% of baseline (P<0.001).