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Popliteal Nerve Block

Title
A prospective randomized controlled trial of ultrasound guided versus nerve stimulation guided distal sciatic nerve block at the popliteal fossa
Authors
van Geffen GJ, van den Broek E, Braak GJJ et al.
Journal
Anaesthesia and Intensive Care 2009 Jan; 37(1): 32-37.

 

Study Summary

 

Study design
  • Randomized controlled clinical study
Patient population
  • 40 adult patients undergoing foot or ankle surgery under regional anesthesia
  • Patients received only a popliteal fossa sciatic nerve block.
Intervention
  • Ultrasound (US)-guided popliteal fossa sciatic nerve block in Sims’ position
  • Linear array 7-13 MHz transducer
  • Out-of-plane approach with a 50mm 22G block needle
  • Neurostimulation was not used
  • End-point for injection was circumferential spread of local anesthetic solution: up to 40mL of 1.5% lidocaine with 1:200,000 epinephrine no minimum volume specified)
Comparator(s)
  • Neurostimulation (NS)-guided posterior popliteal fossa sciatic nerve block in Sims’ position
  • 50mm 22 g block needle inserted 8-10cm above the popliteal skin crease at midpoint between tendons of biceps femoris and semitendinosus muscle
  • Endpoints for needle insertion was a minimum current threshold of 0.2-0.5mA that elicited plantar or dorsiflexion of foot / toes
  • 25-40mL of 1.5% lidocaine with 1:200,000 epinephrine (actual volume left to anesthesiologist’s discretion)
Main findings
  • The block success rate (defined as no need for intraoperative supplementation or conversion to general or spinal anesthesia) was 100% in the US-guided group and 75% in the NS-guided group.
  • Of the NS-guided group, the nerve could not be located in 2 patients and the block was not performed. Success rate in the patients who did receive a block was 83%.
  • The median local anesthetic volume used was 17 vs 37 mL in the US-guided and NS-guided groups respectively.
  • Block performance time, onset and duration were similar between the two groups
  • Significantly fewer needle insertion attempts were required in the US-guided group

 

Comments on the study

  • This well-designed randomized controlled trial adds to the growing literature that suggests the minimum effective local anesthetic volume is reduced when US-guidance is used instead of NS-guidance. Both block onset and duration were similar despite a twofold difference in the volume of local anesthetic used.
  • One of the strengths of this study is the lack of operator bias, as the NS-guided blocks were performed only by experienced anesthesiologists who had never used the US-guided approach.
  • In this study the success rate was also significantly higher in the US-guided approach to the sciatic nerve in the popliteal fossa ; however as the authors acknowledge, the study was not powered to detect this outcome and the difference may not have been apparent in a larger study.
  • Although block performance time was similar between groups, significantly fewer needle insertion attempts were required in the US-guided group. This resulted in lower block-associated pain scores (2.5 vs 4), although the difference was not statistically significant. These findings are in keeping with other comparative trials of US-guidance versus NS-guidance.