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.
abstract: BACKGROUND
  Understanding the mechanisms causing variation in current thresholds for electrical nerve stimulation may improve the safety and success rate of peripheral nerve blocks. Electrical impedance of the tissue surrounding a nerve may affect the response to nerve stimulation. In this volunteer study, we investigated the relationship between impedance and current threshold needed to obtain a neuromuscular response.
  
  
  METHODS
  Electrical nerve stimulation and impedance measurements were performed for the median nerve in the axilla and at the elbow in 29 volunteers. The needletip was positioned at a distance of 5, 2.5, and 0 mm from the nerve as judged by ultrasound. Impulse widths of 0.1 and 0.3 ms were used for nerve stimulation.
  
  
  RESULTS
  A significant inverse relationship between impedance and current threshold was found at the elbow, at nerve-to-needle distances of 5 and 2.5 mm (P = 0.001 and P = 0.036). Impedance values were significantly lower in the axilla (mean 21.1, sd 9.7 kohm) than at the elbow (mean 36.6, sd 13.4 kohm) (P < 0.001). Conversely, current thresholds for nerve stimulation were significantly higher in the axilla than at the elbow (P < 0.001, P < 0.001, P = 0.024). A mean ratio of 1.82 was found for the measurements of current thresholds with 0.1 versus 0.3 ms impulse duration.
  
  
  CONCLUSIONS
  Our results demonstrate an inverse relationship between impedance measurements and current thresholds and suggest that current settings used for nerve stimulation may require adjustment based on the tissue type. Further studies should be performed to investigate the clinical impact of our findings.
authors.affiliation: Faculty of Medicine, University of Oslo, Division of Anesthesiology and Intensive Care Medicine, Rikshospitalet University Hospital, Oslo, Norway. sauter@start.no
authors.forename: Axel R
authors.initials: AR
authors.lastname: Sauter
authors.forename: Michael S
authors.initials: MS
authors.lastname: Dodgson
authors.forename: Håvard
authors.initials: H
authors.lastname: Kalvøy
authors.forename: Sverre
authors.initials: S
authors.lastname: Grimnes
authors.forename: Audun
authors.initials: A
authors.lastname: Stubhaug
authors.forename: Oivind
authors.initials: O
authors.lastname: Klaastad
mesh_terms: D000328:Adult; D001365:Axilla / Q000000981:diagnostic imaging / Q000294:innervation*; D004550:Elbow / Q000000981:diagnostic imaging / Q000294:innervation*; D017097:Electric Impedance; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000000981:diagnostic imaging*; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000294:innervation*; D009407:Nerve Block; D012684:Sensory Thresholds; D018084:Ultrasonography, Interventional; D055815:Young Adult