Continuous femoral nerve blocks: the impact of catheter tip location relative to the femoral nerve (anterior versus posterior) on quadriceps weakness and cutaneous sensory block

Abstract

Background: During a continuous femoral nerve block, the influence of catheter tip position relative to the femoral nerve on infusion characteristics remains unknown.

Methods: We inserted bilateral femoral perineural catheters in volunteers (ultrasound-guided, needle in-plane). Subjects' dominant side was randomized to have the catheter tip placed either anterior or posterior to the femoral nerve. The contralateral limb received the alternative position. Ropivacaine 0.1% was administered through both catheters concurrently for 6 hours (4 mL/h). Outcome measures included the maximum voluntary isometric contraction (MVIC) of the quadriceps femoris muscle and tolerance to cutaneous electrical current over to the distal quadriceps tendon. Measurements were performed at hour 0 (baseline), and on the hour until hour 9, as well as hour 22. The primary end point was the MVIC of the quadriceps at hour 6.

Results: As a percentage of the baseline measurement, quadriceps MVIC for limbs with anterior (n = 16) and posterior (n = 16) catheter tip placement did not differ to a statistically significant degree at hour 6 (mean [SD] 29% [26] vs 30% [28], respectively; 95% confidence interval: -22% to 20%; P = 0.931), or at any other time point. However, the maximum tolerance to cutaneous electrical current was higher in limbs with anterior compared with posterior catheter tip placement at hour 6 (20 [23] mA vs 6 [4] mA, respectively; 95% confidence interval: 1-27 mA; P = 0.035), as well as at hours 1, 7, 8, and 9 (P < 0.04).

Conclusions: This study documents the significant (70%-80%) quadriceps femoris weakness induced by a continuous femoral nerve block infusion at a relatively low dose of ropivacaine (4 mg/h) delivered through a perineural catheter located both anterior and posterior to the femoral nerve. In contrast, an anterior placement increases cutaneous sensory block compared with a posterior insertion, without a concurrent relative increase in motor block.

Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1

Examples of posterior (Panel A) and anterior (Panel B) perineural catheter tip insertion. The yellow horizontal line indicates the approximate anterior surface of the femoral nerve.

Figure 2

Effects of perineural catheter tip position relative to the femoral nerve (anterior vs. posterior) on quadriceps femoris strength. Ropivacaine, 0.1%, was infused for 6 hours at 4 mL/h bilaterally. Mean (standard error) values are illustrated. There were no statistically significant differences at 95% confidence between treatments at any time point.

Figure 3

Effects of perineural catheter tip position relative to the femoral nerve (anterior vs. posterior) on tolerance of transcutaneous electrical current in the femoral nerve distribution. Ropivacaine, 0.1%, was infused for 6 hours at 4 mL/h bilaterally. Mean (standard error) values are illustrated. Statistically significant differences between treatments at 95% confidence are designated with an asterisk (§ denotes a statistically significant difference between treatments at 95% confidence exclusively with the mixed effects models).

Figure 4

Box plots of differences in quadriceps femoris strength and tolerance of transcutaneous electrical current between perineural catheter tip positions relative to the femoral nerve (anterior minus posterior). Ropivacaine, 0.1%, was infused for 6 hours at 4 mL/h bilaterally. The boxes, which depict quartiles, are superimposed over spaghetti plots of each subject’s trajectory.