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Technique of nerve stimulation

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HNS12Peripheral nerves are comprised of thousands of nerve fibres; they contain either sensory or motor fibres of the somatic and autonomic nervous systems, but sometimes both in combination. Electrical impulses reaching a nerve, when they exceed a specific threshold stimulus current (rheobase), trigger depolarisation of the neuron membrane, thereby inducing transmission of excitation along the nerve fibres.

General side effects and contraindications

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Side effects

  • Infection, haematoma or nerve lesion in the vicinity of the puncture site
  • Intoxication (central nervous and/or cardiovascular complications) secondary to intravasal injection
  • Methaemoglobinaemia when using prilocaine
  • Allergy (extremely rare)


Absolute contraindications

  • Infection or haematoma in the vicinity of the puncture site
  • Lesion of the nerves to be stimulated distal to the puncture site
  • Refusal of the procedure by the patient


Relative contraindications

  • Neurological deficit of the leg to be anaesthetised
  • Peripheral nerve block is possible upon careful diagnosis of the neurological status prior to the block

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The stimulator

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We use the new Stimuplex® HNS 12 nerve stimulator (B. Braun Melsungen) which features the following functions among others:

  • Exact amplitude ranges are selectable between 0 and 1 mA (or 5 mA). The device displays the actual current.
  • Pulse durations of 0.1, 0.3 or 1.0 msec are selectable.
  • The pulse frequency can be set to between 1 or 2 Hz.

At a given current, the electrical current required to trigger muscle contractions correlates with the distance of the tip of the needle to the nerve. That means that the closer the needle is to the nerve, the lower the electrical current that is required to induce contractions or sensory responses. In routine clinical practice, an 1.2 Electrical nerve stimulation Fundamentals Class Function Chronaxie Aa Motor 0.05–0.1 ms Ab Touch, pressure Ag Touch Ad Pain, temperature 0.150 ms B Sympathetic nervous system C Sympathetic nervous system, pain, temperature 0.4 ms 12 initial electrical current, called threshold current, of 1 mA is used to elicit a response. The stimulation needle has reached the desired position at the nerve when contractions of the effector muscle are induced at a threshold current of 0.2 - 0.3 (pulse duration of 0.1 ms). Lower pulse amplitudes may cause injury to the nerve. Therefore, as a general rule, the lowest stimulation current should be determined: This will allow the user to correct the needle position by retracting the needle if it gets too close to the nerve.

The needles

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We use stimulation needles that are completely insulated, except for a small area on the tip. They are fully coated and therefore have no sharp edges. This type of needle is call monopolar or unipolar. The electrical current has a very small exit opening. Because of this, the electrical field is bundled and generates higher current density at the tip of the needle. The higher the current density at the tip of the needle, the lower is the current required for stimulation. As the needle approaches the nerve, the current required for depolarisation drops. If the tip slips past the nerve, this value starts rising again just as rapidly. This method makes it possible to localise the nerve exactly while keeping the risk of injury at a minimum. Various opinions prevail with regard to the role the bevel plays in the risk of injury. The use of short-bevelled needles (45°) is supposed to reduce the danger of nerve lesions. It is easier to identify fascial structures, which is also important when a nerve stimulator is not used. We employ unipolar needles with a 15° bevel because they pass through tissue more easily and cause less trauma, which is helpful for localizing the nerves. In our opinion, the risk of nerve lesions is kept extremely low when a suitable nerve stimulator is used and nerve stimulation applied properly.

The procedure

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  • Test the function of the nerve stimulator. 
  • Disinfect the skin, create a skin weal and, if necessary, infiltrate puncture channel. When blocking superficial nerves (e.g. brachial plexus, femoral nerve), infiltrate the puncture site cautiously, as otherwise the effect of stimulation may be reduced by a premature partial block. 
  • Place an injection line with NaCl 0.9% syringe, rinse the injection line and needle, connect the current line and create a connection to the neutral electrode.

Transdermal nerve stimulation

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For locating nerves with a superficial location and in patients of normal weight, the Stimuplex® Pen can be used together with the stimulator to trigger a transdermal response from the target muscle. The pulse duration of the device must be set to 1 ms and the current range to the 5 mA. In this way, it is possible to get a better fix on the puncture site or even correct the puncture direction more precisely. Another important aspect is its use to demonstrate the proper stimulus of the target muscle when training inexperienced colleagues.

 

transdermal