Peripheral electrical nerve stimulation
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 µs). The fibres of pain sensation (A-delta
and C-fibres) require a longer pulse (150 and 400 µs 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
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