What is the typical stimulation intensity used for peripheral nerve blocks?

Stimulation Intensity for Peripheral Nerve Blocks

For peripheral nerve blocks, the recommended stimulation intensity is 0.3-0.5 mA with a pulse width of 100 microseconds, which provides the optimal balance between block success and safety. 1

Principles of Nerve Stimulation for Peripheral Nerve Blocks

Optimal Current Settings

• Initial stimulation: Start at 1-2 mA to locate the general vicinity of the nerve
• Target stimulation: Reduce to 0.3-0.5 mA when approaching the nerve
• Minimum threshold:
  • Extraneural threshold: 0.34 ± 0.11 mA for cathodic stimulation
  • Intraneural threshold: 0.12 ± 0.03 mA for cathodic stimulation 2

Technical Parameters

• Pulse width: 100 microseconds is preferred over longer durations (e.g., 1000 microseconds) 3
  • At 100 microseconds, greater differences in current correspond to similar distances from the nerve
  • This provides superior control during nerve localization
• Polarity: Conventional approach uses cathodic stimulation (negative electrode at needle tip)
  • Cathodic stimulation produces a complex biphasic response that may cause conduction block at very close distances
  • Anodic stimulation produces more predictable responses but requires higher current (0.63 ± 0.12 mA) 2

Clinical Application

Monitoring Technique

• Use train-of-four (TOF) stimulation to assess depth of neuromuscular blockade
• Four stimuli delivered at 0.5-second intervals
• With increasing neuromuscular blockade, twitches disappear in order: T4→T3→T2→T1 1

Approach to Nerve Localization

1. Begin with higher current (1-2 mA) to identify approximate nerve location
2. Gradually decrease current while advancing needle
3. Optimal position achieved when muscle response is elicited at 0.3-0.5 mA
4. Caution: Stimulation at currents below 0.2 mA may indicate intraneural placement 3, 4

Common Pitfalls and Solutions

Technical Issues

• Nerve stimulator variability: Significant variation exists between devices in accuracy, especially at low current settings (0.1-0.5 mA) 5
  • Error rates increase from 2.4% at 0.5 mA to 10.4% at 0.1 mA
  • Always verify equipment calibration before use

Patient Factors Affecting Stimulation

• Monitoring site: Response differs between muscles (e.g., orbicularis oculi vs. adductor pollicis)
• Temperature: Hypothermia affects accuracy of peripheral nerve stimulation
• Peripheral edema: May obscure landmarks and affect current transmission
• Skin resistance: Diaphoresis can alter electrical conductivity 1

Safety Considerations

• Do not pursue very low stimulation thresholds: Similar block success rates are observed with stimulation thresholds ≤0.5 mA and >0.5 mA (96.3% vs 95.9%) 4
• Ultrasound guidance: Should be used alongside nerve stimulation to reduce risk of local anesthetic systemic toxicity 1

Special Considerations

Deep vs. Superficial Blocks

• Low-risk blocks (superficial, compressible areas):
  • Femoral block
  • Sciatic block in popliteal fossa
• High-risk blocks (deep, non-compressible areas):
  • Infraclavicular block
  • Parasacral sciatic block
  • Posterior lumbar plexus block 1

Block Testing

• Thorough testing for block success is essential to prevent need for emergency conversion to general anesthesia
• Allow extra onset time to reduce risk of conversion 1

By following these guidelines for stimulation intensity during peripheral nerve blocks, clinicians can optimize block success while minimizing the risk of complications such as intraneural injection and nerve injury.