Braun HNS 11 user manual Peripheral electrical nerve stimulation, Principles

Peripheral electrical nerve stimulation 20

Principles

By simplifying the accurate location of peripheral nerves, periph- eral electrical nerve stimulation (PNS) facilitates the performance of nerve and plexus blocks increasing their safety and reliability. The old rule ”no paraesthesia - no anaesthesia” (1) loses its valid- ity, because information from the patient concerning paraesthesia is eliminated, and the danger of a mechanical nerve lesion (8) is largely excluded. The principle consists of triggering depolariza- tions with electrical pulses at, but not within, the nerve, causing muscular contractions at the effector muscle or sensitive sensa- tions in the distribution area. Paraesthesia due to direct contact of injection needles and nerve is consciously avoided.

PNS does not replace the anatomical knowledge required for re- gional anaesthesia, rather it assumes accurate knowledge of the topography and the nerve distribution area.

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

When using unipolar needles (needles with an insulated needle shaft and a conductive tip), the current necessary to trigger mus- cular contractions (= pulse amplitude) correlates with the distance of the tip of the needle from the nerve: the lower the threshold current the more accurately is the nerve localized, and the shorter the onset and more reliable the success of the block (4). The short- er the electrical pulse (= pulse width), the faster is the rise in cur- rent to the nerve, and the clearer the discrimination as to whether the needle tip is sufficiently close to the nerve (2,5). The stimula- tion needle should always be connected to the negative pole be- cause higher currents are required if the polarity is reversed (nee- dle positive) (6).

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

The Stimuplex® HNS 11 stimulator has been designed according to the most modern aspects and the requirements which originate from the theory and practice of peripheral electrical nerve stimu- lation (3,6). It is provided with alarm systems necessary for the