The old rule "no paresthesia - no anesthesia" loses its validity, because the need for reliable feedback from the patient concerning paresthesia is eliminated, and the danger of a mechanical nerve lesion is largely excluded.

The principle consists of triggering depolarizations with electrical pulses at, but not within, the nerve, causing muscular contractions at the effector muscle or sensitive sensations in the distribution area. Paresthesia due to direct contact of injection needles and the nerve is consciously avoided. PENS does not replace the anatomical knowledge required for regional anesthesia; rather it assumes accurate knowledge of the topography and the nerve distribution area.

For a more detailed explanation of the electrophysiology of peripheral nerve stimulation, please click here.

The various types of nerve fibers differ in regard to their sensitivity to electrical stimulation^1-3. The A-alpha motor fibers have the shortest chronaxia (50 - 100 µs). The fibers of pain sensation (A-delta and C-fibers) require a longer pulse (150 - 400 µs respectively) at minimum current. Mixed peripheral nerves can be localized with short pulses (0.1ms) without triggering pain sensations. For pure sensory nerves, a longer pulse (0.3 or 1.0 ms) is recommended^4,6.

Depolarization, Threshold level, Action Potential

When using unipolar needles (needles with an insulated shaft and a conductive tip), the current necessary to trigger muscular contractions (= pulse amplitude) correlates with the distance of the tip of the needle from the nerve: the lower the threshold current, the more accurately the nerve is localized. Time to onset of the block can be shortened and reliability of successful blockade can increase⁵.

Needle tip more distant from the nerve
Needle tip closer to the nerve
Needle tip closer to the nerve

The shorter the electrical pulse (= pulse width), the faster the rise in current to the nerve, resulting in clearer discrimination by the physician as to whether the needle tip is sufficiently close to the nerve. The stimulation needle should always be connected to the negative pole because higher currents are required if the polarity is reversed (needle positive)^2,4,6.

The geometry of the electrical current field is dependent on the geometry of the conductive tip of the stimulation needle. The smaller the emission site of the electrons at the tip of the needle, the higher the current density at this point and the lower the threshold current when the nerve is exactly localized.

The B. Braun peripheral nerve stimulators - Stimuplex® Dig RC and the new Stimuplex HNS11 - have been designed according to the most modern aspects and requirements which originate from the theory and practice of peripheral electrical nerve stimulation. They both come with alert systems necessary for the early detection of technical faults which could endanger the patient and place the success of the anesthesia in jeopardy. Click below to learn more about either device.