Nerve Stimulation in Peripheral Nerve Blocks

Introduction

• Regional anesthetic techniques, such as PNBs, are effective for providing analgesia and even surgical anesthesia, potentially sparing patients from general anesthesia and its complications.¹
• Effective regional anesthesia (RA) requires injecting the local anesthetic (LA) close enough to the nerve to block nerve conduction without mechanically injuring the nerve or surrounding structures.²
• Historically, PNBs were performed using landmark-based techniques, haptic feedback, and the induction of paresthesias until the widespread adoption of electrical nerve stimulation (NS) in the 1970s; as a result, RA became more precise, with reduced failure rates and, theoretically, lower complication rates.³
• US has largely supplanted NS for peripheral nerve identification, but NS remains a useful adjunct for the localization and discrimination of nerves that may be difficult or impossible to visualize with US.⁴

Physics of Nerve Stimulation

• The direct application of electrical current to peripheral nerves most readily depolarizes efferent A-α motor neurons, allowing early identification of muscular contraction before eliciting unpleasant sensory stimulation.
• The intensity of current necessary to depolarize the nerve depends on the distance from the nerve, as determined by Coulomb’s law (Figure 1), which states that electrical current diminishes proportionally to the square of the distance from the nerve.
• As a result, the current intensity required to depolarize a nerve depends on the applied amperage, which decreases exponentially as the needle tip approaches the nerve.³

• Electrical current during NS is typically delivered via an insulated stimulating needle that concentrates the current at the tip, with amperages between 0.1 and 2.0 mA, in pulses of 0.1 ms to 0.3 ms, every 0.5 to 1 second.
• Shorter electrical pulses (0.1 ms) are generally sufficient to trigger muscular contraction, while longer impulses (0.3 ms) are often needed to stimulate sensory nerves.
• If a non-insulated needle is used, however, the shaft may depolarize the nerve even after the tip has passed beyond it.
• The use of electrically conductive solutions, such as LA or normal saline, will diminish or even terminate the motor response to NS by physical displacement of the nerve and dispersion of the current (Figure 2).⁵
• In practice, muscular contraction in response to 0.3 mA to 0.5 mA of current is generally accepted as sufficient proximity for LA administration, while muscular contraction below 0.2 mA suggests a juxtaneural or intraneural position.

Technique & Materials

Preparing for the Block

• NS is not a substitute for anatomical knowledge, and a comprehensive understanding of landmarks and anatomy is critical for proper needle placement.
• Care should be taken to ensure the correct patient and the correct body part are blocked for the correct procedure.
• A focused history and physical exam should be performed to exclude any relevant contraindications.
• Even under ultrasound guidance, inadvertent vascular injection or anaphylactic reactions may occur. The patient should be monitored for cardiovascular instability during the procedure, and resuscitation equipment should be readily available, especially if also using sedation.³

Materials

• An electrical nerve stimulator and a stimulating needle (Figure 2) are required.
  • Stimulating needles are commonly insulated, 22-24 gauge, short-beveled (i.e., blunt-tipped) that can be connected to the stimulator. The positive terminal, or anode, typically red, should connect to the skin by an electrode. The negative terminal, or cathode, should attach to the needle.⁵
• Select an appropriate local anesthetic based on nerve location, patient size, and desired block duration.
• After identifying the appropriate landmarks, clean and drape the site in a sterile fashion.

Performing the Block

• Insert the needle below the skin.
• Increase the current amplitude to 1 mA or as high as 3 mA for sufficiently deep structures, as shown in Figure 3.
• Advance the needle until the desired motor response is obtained. As distance between the needle and the nerve begin to decrease, the threshold current to evoke a response will decrease as well as seen in Figure 4.
• Continue to advance the needle while decreasing the current amplitude until a response is evoked at approximately 0.3 mA to 0.5 mA.^6,7
• A motor response at 0.1 mA to 0.2 mA may instead suggest intraneural placement.^6,7

• If motor response is lost during needle positioning, increase the amplitude again until the response is recovered before moving the needle further.
• Once a motor response is obtained at the desired amperages, inject 1-2 mL of LA, which should abolish it. At this point, increasing the current amplitude may no longer reliably evoke a motor response for further guidance.
• Aspirate to exclude intravascular placement and give the remainder of the local anesthetic.

Comparison to Ultrasound

• Several head-to-head studies and meta-analyses have demonstrated over time the superiority of US for needle placement compared to NS alone: US-guided blocks are more often successful, less painful to administer, less likely to require rescue analgesia, and less likely to result in inadvertent vascular puncture.²
• However, when combined with US techniques, the use of NS increases the success rate for deep or difficult-to-visualize structures, such as the obturator nerve or posterior lumbar plexus.
• Simultaneous US and NS also offers the theoretical advantage of reduced intraneural injection and nerve injury: experimental data suggest that in the absence of a motor response while < 0.2 mA during NS, intraneural placement is unlikely, and that successful discrimination between intraneural and extraneural needle placement with US alone is less than 80% even for experienced clinicians.²
• For this reason, a dual-guidance approach is standard practice at some institutions, but it has not yet been demonstrated to reduce the incidence of nerve injury.²

Summary

• When used appropriately, NS alone can be used to reliably identify peripheral nerves for regional anesthesia, but it is generally inferior to US in most performance characteristics
• NS may still serve as a useful adjunct to ultrasound for localizing deep or difficult to visualize nerves during peripheral nerve block procedures, and in particular for excluding intraneural positioning
• Whenever possible, insulated needles should be used for electrical nerve stimulation to ensure the charge is concentrated at the tip of the needle
• Higher amperages (1.0 mA or more) may be needed initially to guide needle placement.
• Motor contraction at 0.3-0.5 mA suggests appropriate needle placement for local anesthetic administration.
• Motor contraction at or below 0.2 mA suggests juxtaneural or intraneural placement, and the needle should be repositioned before injection.
• Injection of as little as 1-2 mL of a conducting solution, such as LA or saline, may abolish the motor response to electrical stimulation. Nonconducting solutions such as D5W will generally preserve this response.