FRCA Notes

Nerve Stimulators for Regional Anaesthesia

The relevant curriculum item here is 'Recalls the relevant physics and clinical measurement related to the use of nerve stimulators in regional anaesthesia'.

Resources

  • Nerve stimulation emerged in the 1970s to provide a quantitative inference of the needle-nerve relationship
  • They helped reduce block failure rates compared to landmark techniques in a pre-ultrasound era
  • Since the advent of ultrasound, there utility has moved away from aiding nerve localisation and towards nerve safety

  • Electrical current applied to motor fibres within nerves causes depolarisation, resulting in contraction of the corresponding muscle(s) and alerting the anaesthetist to the proximity of the nerve

Electrical characteristics

  • A nerve stimulator is used to apply DC current (typically 0.2-0.5mA) through an insulated needle, where current density is focused at the needle tip
  • The current stimulus is of short duration (0.05 - 1ms) at a frequency of 1-2Hz

  • These short stimuli stimulate A⍺-motor neurons and mixed peripheral nerves, but don't stimulate Aδ- or C-fibres and therefore don't trigger pain sensations
  • Higher frequencies can cause patient discomfort; lower frequencies risks neural trauma as the needle is advanced between impulses
  • The needle outputs a constant, linear current despite changes in resistance of the surrounding tissues

  • The needle should be connected to the nerve stimulator via the negative lead (cathode), as this causes an area of depolarisation and requires less current
  • If the needle is connected to the positive electrode (anode) then it causes hyper-polarisation near the tip and depolarisation further afield, which requires a greater current to produce the same effect

Nerve localisation

  • The higher the current intensity (mA) the larger the 'sphere' of current present at the needle tip and the greater likelihood of causing depolarisation
  • The current intensity required to elicit a motor response is inversely proportional to the square of the distance to the nerve (Coulomb's Law)
  • As the needle tip is advanced towards the nerve, the current intensity required to cause a motor response reduces exponentially
  • Presence of motor response below a certain stimulation threshold (0.5mA) implies high proximity of needle tip to nerve

  • Start with higher (>1mA) current intensity and advance in anticipated direction of the nerve
  • When motor response obtained, current intensity is reduced until the motor response disappears/diminishes
  • The needle is then advanced until the response increases again, at which point current intensity is again reduced
  • This continues until a sustained motor response is obtained at an acceptably low threshold (0.2 - 0.5mA)
  • This implies sufficient proximity to the nerve such that injected local anaesthetic results in effective block

  • Provides a monitor against needle-nerve contact
  • Facilitates avoidance of nerves in the needle trajectory which are poorly visible under ultrasound

  • Protective nerve stimulation when used in addition to ultrasound
    • If motor response is obtained at <0.2mA implies high likelihood of needle-nerve contact or intra-neural location and therefore nerve stimulation is highly specific for needle-nerve contact
    • This technique is termed the 'dual guidance' approach; few data on rates of nerve injury vs. ultrasound alone

  • Teaching tool for novices
    • May reduce time to proficiency for blocks where there is high variability of nerve position e.g. axillary BPB
    • May help improve understanding of haptic feedback e.g. sudden appearance of motor response after 'pop' through a fascial plane
  • Can help differentiate hyperechoic artefact from nerve tissue (e.g. post-cystic enhancement)

  • If too high a current is used it may cause direct muscle fibre stimulus creating a false positive
  • Current may be channelled away from the nerve by a different path of lower (least) resistance leading to a false negative
  • Needle contact with nerves may fail to produce a motor response at low current intensity (<0.5mA) in up to 25% of cases
  • Absence of a motor response at 0.2 - 0.5mA does not exclude intraneural needle placement and may lead to unnecessary manipulation of the needle within the nerve tissue, increasing damage
  • In animal models, needle tips inserted into the nerve with high current intensities (>1mA) may still fail to produce a motor response
  • Some connective tissue layers may permit electrical conductivity but not local anaesthetic solution permeation, leading to false positives because the needle tip is in an adjacent tissue to the nerve
  • Once LA administration has commenced, the evoked motor response ceases due to conduction of electrical current to other locations by the LA solution; this limits use to pre-injection only
  • Increases rate of multiple skin puncture when used in conjunction with ultrasound

vs. ultrasound

  • Ultrasound guidance alone, compared to nerve stimulation alone, leads to:
    • Improved block success (92% vs. 83%)
    • Reduced need for rescue analgesia
    • Reduced procedural time
    • Reduced procedural pain
    • Faster block onset time
    • Lower rates of vascular puncture (84% relative risk reduction)
    • Longer block duration
  • Results applicable to catheter techniques too

  • When nerve stimulation is added to ultrasound, it does not improve block success rate for femoral nerve, various brachial plexus and popliteal sciatic nerve blocks
  • However, nerve stimulation may improve block success rate vs. ultrasound alone for nerves which are deep or difficult to visualise e.g. obturator nerve, sciatic nerve (parasacral, subgluteal) and posterior lumbar plexus block