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Impedance and Current Threshold in Neurostimulation

Title
Current threshold for nerve stimulation depends on electrical impedance of the tissue: a study of ultrasound-guided electrical nerve stimulation of the median nerve.
Authors
Sauter AR, Dodgson MS, Kalvey H et al.
Journal
Anesthesia & Analgesia 2009 Apr; 108(4): 1338-43.

 

Study Summary

 

Study design
  • Prospective volunteer study
Patient population
  • 29 healthy adult volunteers
Intervention
  • Insertion of a 25G insulated needle, primed with normal saline, to a distance of 0, 2.5, and 5 mm from the median nerve in the axilla and at the elbow.
  • Impedance was measured using specialized equipment.
  • The minimum current threshold to produce a motor response was sought.
Comparator(s)
  • None
Main findings
  • A significant inverse relationship between impedance and current threshold was found for the median nerve at the elbow when the needle was 2.5 or 5 mm from the nerve.
  • This was not seen in the axilla, nor with the needle in contact with the epineurium of the nerve (0mm distance).
  • Impedance was lower when the needle tip was in muscle, compared to fat / connective tissue..
  • Failure to obtain a motor response was frequently observed despite a current of 5 mA and impulse duration of 0.1 msec when the needle was 2.5-5mm away from the nerve. This was improved by use of 0.3 msec impulse duration.

 

Comments on the study

  • This is an important study to be aware of when using a combined ultrasound and neurostimulation-guided peripheral block technique.
  • It offers one possible explanation for why motor response may not be seen despite needle-nerve contact visualized on ultrasound. One area where we have frequently observed this is in the subgluteal sciatic region. Here the nerve is surrounded by muscle, which tends to have lower impedance according to this study, and hence may have a higher current threshold.
  • The findings validate our current practice of using neurostimulation only to confirm the identity of a motor nerve. We do not seek a minimum current threshold as the endpoint for needle insertion, but rather look for needle-nerve contact.
  • The results of this study also suggest that if a motor response is not obtained at high currents, one should try increasing the impulse duration from 0.1 to 0.3 msec. The authors reported a mean ratio of 1.82 for current thresholds obtained using 0.1 vs 0.3 msec impulse duration.